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From Wikipedia, the free encyclopedia
This article is about crustaceans. For other uses, see Crab (disambiguation).
Crab
Temporal range: Early Jurassic – Present
PreꞒ
Ꞓ
O
S
D
C
P
T
J
K
Pg
N
Top
row, left to right: Dromia personata (Dromiidae), Dungeness crab
(Cancridae), Tasmanian giant crab (Menippidae); Middle row: Corystes
cassivelaunus (Corystidae), Liocarcinus vernalis (Portunidae), Carpilius
maculatus (Carpiliidae); Bottom row: Gecarcinus quadratus
(Gecarcinidae), Grapsus grapsus (Grapsidae), Ocypode ceratophthalmus
(Ocypodidae).
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Arthropoda
Class: Malacostraca
Order: Decapoda
Suborder: Pleocyemata
(unranked): Reptantia
Infraorder: Brachyura
Linnaeus, 1758
Sections and subsections[1]
Dromiacea
Raninoida
Cyclodorippoida
Eubrachyura
Heterotremata
Thoracotremata
Crabs
are decapod crustaceans of the infraorder Brachyura, which typically
have a very short projecting "tail" (abdomen), usually hidden entirely
under the thorax (brachyura means "short tail" in Greek[a]). They live
in all the world's oceans, in freshwater, and on land, are generally
covered with a thick exoskeleton, and have a single pair of pincers.
They first appeared during the Jurassic Period.
Description
Gecarcinus quadratus, a land crab from Central America
Crabs
are generally covered with a thick exoskeleton, composed primarily of
highly mineralized chitin,[4][5] and armed with a pair of chelae
(claws). Crabs vary in size from the pea crab, a few millimeters wide,
to the Japanese spider crab, with a leg span up to 4 m (13 ft).[6]
Several other groups of crustaceans with similar appearances – such as
king crabs and porcelain crabs – are not true crabs, but have evolved
features similar to true crabs through a process known as
carcinisation.[7][8][9][10]
Environment
Crabs are found in all
of the world's oceans, as well as in fresh water and on land,
particularly in tropical regions. About 850 species are freshwater
crabs.[11]
Sexual dimorphism
The underside of a male (top) and a
female (bottom) individual of Pachygrapsus marmoratus, showing the
difference in shape of the abdomen
Crabs often show marked sexual
dimorphism. Males often have larger claws,[12] a tendency that is
particularly pronounced in the fiddler crabs of the genus Uca
(Ocypodidae). In fiddler crabs, males have one greatly enlarged claw
used for communication, particularly for attracting a mate.[13] Another
conspicuous difference is the form of the pleon (abdomen); in most male
crabs, this is narrow and triangular in form, while females have a
broader, rounded abdomen.[14] This is because female crabs brood
fertilised eggs on their pleopods.
Reproduction and life cycle
Crab (Pachygrapsus marmoratus) on Istrian coast, Adriatic Sea
Crabs
attract a mate through chemical (pheromones), visual, acoustic, or
vibratory means. Pheromones are used by most fully aquatic crabs, while
terrestrial and semiterrestrial crabs often use visual signals, such as
fiddler crab males waving their large claws to attract females. The vast
number of brachyuran crabs have internal fertilisation and mate
belly-to-belly. For many aquatic species, mating takes place just after
the female has moulted and is still soft. Females can store the sperm
for a long time before using it to fertilise their eggs. When
fertilisation has taken place, the eggs are released onto the female's
abdomen, below the tail flap, secured with a sticky material. In this
location, they are protected during embryonic development. Females
carrying eggs are called "berried" since the eggs resemble round
berries.
When development is complete, the female releases the
newly hatched larvae into the water, where they are part of the
plankton. The release is often timed with the tidal and light/dark diel
cycle.[15][16] The free-swimming tiny zoea larvae can float and take
advantage of water currents. They have a spine, which probably reduces
the rate of predation by larger animals. The zoea of most species must
find food, but some crabs provide enough yolk in the eggs that the
larval stages can continue to live off the yolk.
Female crab Xantho poressa at spawning time in the Black Sea, carrying eggs under her abdomen
A Grapsus tenuicrustatus climbing up a rock in Hawaii
Each
species has a particular number of zoeal stages, separated by moults,
before they change into a megalopa stage, which resembles an adult crab,
except for having the abdomen (tail) sticking out behind. After one
more moult, the crab is a juvenile, living on the bottom rather than
floating in the water. This last moult, from megalopa to juvenile, is
critical, and it must take place in a habitat that is suitable for the
juvenile to survive.[17]: 63–77
Most species of terrestrial
crabs must migrate down to the ocean to release their larvae; in some
cases, this entails very extensive migrations. After living for a short
time as larvae in the ocean, the juveniles must do this migration in
reverse. In many tropical areas with land crabs, these migrations often
result in considerable roadkill of migrating crabs.[17]: 113–114
Once
crabs have become juveniles, they still have to keep moulting many more
times to become adults. They are covered with a hard shell, which would
otherwise prevent growth. The moult cycle is coordinated by hormones.
When preparing for moult, the old shell is softened and partly eroded
away, while the rudimentary beginnings of a new shell form under it. At
the time of moulting, the crab takes in a lot of water to expand and
crack open the old shell at a line of weakness along the back edge of
the carapace. The crab must then extract all of itself – including its
legs, mouthparts, eyestalks, and even the lining of the front and back
of the digestive tract – from the old shell. This is a difficult process
that takes many hours, and if a crab gets stuck, it will die. After
freeing itself from the old shell (now called an exuvia), the crab is
extremely soft and hides until its new shell has hardened. While the new
shell is still soft, the crab can expand it to make room for future
growth.[17]: 78–79
Behaviour
Carpilius convexus consuming Heterocentrotus trigonarius in Hawaii
Crabs
typically walk sideways[18] (hence the term crabwise), because of the
articulation of the legs which makes a sidelong gait more efficient.[19]
Some crabs walk forward or backward, including raninids,[20] Libinia
emarginata[21] and Mictyris platycheles.[18] Some crabs, like the
Portunidae and Matutidae, are also capable of swimming,[22] the
Portunidae especially so as their last pair of walking legs are
flattened into swimming paddles.[17]: 96
Crabs are mostly active
animals with complex behaviour patterns such as communicating by
drumming or waving their pincers. Crabs tend to be aggressive toward one
another, and males often fight to gain access to females.[23] On rocky
seashores, where nearly all caves and crevices are occupied, crabs may
also fight over hiding holes.[24] Fiddler crabs (genus Uca) dig burrows
in sand or mud, which they use for resting, hiding, and mating, and to
defend against intruders.[17]: 28–29, 99
Crabs are omnivores,
feeding primarily on algae,[25] and taking any other food, including
molluscs, worms, other crustaceans, fungi, bacteria, and detritus,
depending on their availability and the crab species. For many crabs, a
mixed diet of plant and animal matter results in the fastest growth and
greatest fitness.[26][27] Some species are more specialised in their
diets, based in plankton, clams or fish.[17]: 85
Crabs are known
to work together to provide food and protection for their family, and
during mating season to find a comfortable spot for the female to
release her eggs.[28]
Human consumption
Fisheries
A short video on catching and exporting shellfish in Wales.
Main article: Crab fisheries
Crabs
make up 20% of all marine crustaceans caught, farmed, and consumed
worldwide, amounting to 1.5 million tonnes annually. One species,
Portunus trituberculatus, accounts for one-fifth of that total. Other
commercially important taxa include Portunus pelagicus, several species
in the genus Chionoecetes, the blue crab (Callinectes sapidus),
Charybdis spp., Cancer pagurus, the Dungeness crab (Metacarcinus
magister), and Scylla serrata, each of which yields more than 20,000
tonnes annually.[29]
In some crab species, meat is harvested by
manually twisting and pulling off one or both claws and returning the
live crab to the water in the knowledge that the crab may survive and
regenerate the claws.[30][31][32]
Cookery
See also: Crab meat and List of crab dishes
Photo of cooked crab in bowl of soup
Crab masala from Karnataka, India
Crabs
are prepared and eaten as a dish in many different ways all over the
world. Some species are eaten whole, including the shell, such as
soft-shell crab; with other species, just the claws or legs are eaten.
The latter is particularly common for larger crabs, such as the snow
crab. In many cultures, the roe of the female crab is also eaten, which
usually appears orange or yellow in fertile crabs. This is popular in
Southeast Asian cultures, some Mediterranean and Northern European
cultures, and on the East, Chesapeake, and Gulf Coasts of the United
States.
In some regions, spices improve the culinary experience.
In Southeast Asia and the Indosphere, masala crab and chilli crab are
examples of heavily spiced dishes. In the Chesapeake Bay region, blue
crab is often steamed with Old Bay Seasoning. Alaskan king crab or snow
crab legs are usually simply boiled and served with garlic or lemon
butter.
For the British dish dressed crab, the crab meat is
extracted and placed inside the hard shell. One American way to prepare
crab meat is by extracting it and adding varying amounts of binders,
such as egg white, cracker meal, mayonnaise, or mustard, creating a crab
cake. Crabs can also be made into a bisque, a global dish of French
origin which in its authentic form includes in the broth the pulverized
shells of the shellfish from which it is made.
Imitation crab,
also called surimi, is made from minced fish meat that is crafted and
colored to resemble crab meat. While it is sometimes disdained among
some elements of the culinary industry as an unacceptably low-quality
substitute for real crab, this does not hinder its popularity,
especially as a sushi ingredient in Japan and South Korea, and in home
cooking, where cost is often a chief concern.[33] Indeed, surimi is an
important source of protein in most East and Southeast Asian cultures,
appearing in staple ingredients such as fish balls and fish cake.
Pain
Whether
crustaceans as a whole experience pain or not is a scientific debate
that has ethical implications for crab dish preparation. Crabs are very
often boiled alive as part of the cooking process.
This section is an excerpt from Pain in crustaceans § Opinions.[edit]
Advocates
for Animals, a Scottish animal welfare group, stated in 2005 that
"scientific evidence ... strongly suggests that there is a potential for
decapod crustaceans and cephalopods to experience pain and suffering".
This is primarily due to "The likelihood that decapod crustaceans can
feel pain [which] is supported by the fact that they have been shown to
have opioid receptors and to respond to opioids (analgesics such as
morphine) in a similar way to vertebrates." Similarities between decapod
and vertebrate stress systems and behavioral responses to noxious
stimuli were given as additional evidence for the capacity of decapods
to experience pain.[34]
In 2005 a review of the literature by the
Norwegian Scientific Committee for Food Safety tentatively concluded
that "it is unlikely that [lobsters] can feel pain," though they note
that "there is apparently a paucity of exact knowledge on sentience in
crustaceans, and more research is needed." This conclusion is based on
the lobster's simple nervous system. The report assumes that the violent
reaction of lobsters to boiling water is a reflex response (i.e. does
not involve conscious perception) to noxious stimuli.[35]
A European
Food Safety Authority (EFSA) 2005 publication[36] stated that the
largest of decapod crustaceans have complex behaviour, a pain system,
considerable learning abilities and appear to have some degree of
awareness. Based on this evidence, they placed all decapod crustaceans
into the same category of research-animal protection as vertebrates.
Evolution
Reconstruction of Eocarcinus, the earliest known crab
The
earliest unambiguous crab fossils date from the Early Jurassic, with
the oldest being Eocarcinus from the early Pliensbachian of Britain,
which likely represents a stem-group lineage, as it lacks several key
morphological features that define modern crabs.[37][38] Most Jurassic
crabs are only known from dorsal (top half of the body) carapaces,
making it difficult to determine their relationships.[39] Crabs radiated
in the Late Jurassic, corresponding with an increase in reef habitats,
though they would decline at the end of the Jurassic as the result of
the decline of reef ecosystems. Crabs increased in diversity through the
Cretaceous and represented the dominant group of decapods by the end of
the period.[40]
The crab infraorder Brachyura belongs to the
group Reptantia, which consists of the walking/crawling decapods
(lobsters and crabs). Brachyura is the sister clade to the infraorder
Anomura, which contains the hermit crabs and relatives. The cladogram
below shows Brachyura's placement within the larger order Decapoda, from
analysis by Wolfe et al., 2019.[41]
Decapoda
Dendrobranchiata (prawns)
Pleocyemata
Stenopodidea (boxer shrimp)
Procarididea
Caridea (true shrimp)
Reptantia
Achelata (spiny lobsters, slipper lobsters)
Polychelida (benthic crustaceans)
Astacidea (lobsters, crayfish)
Axiidea (mud shrimp, ghost shrimp, or burrowing shrimp)
Gebiidea (mud lobsters and mud shrimp)
Meiura
Anomura (hermit crabs and others)
Brachyura (crabs)
(crawling/walking decapods)
Brachyura
is separated into several sections, with the basal Dromiacea diverging
the earliest in the evolutionary history, around the Late Triassic or
Early Jurassic. The group consisting of Raninoida and Cyclodorippoida
split off next, during the Jurassic period. The remaining clade
Eubrachyura then divided during the Cretaceous period into Heterotremata
and Thoracotremata. A summary of the high-level internal relationships
within Brachyura can be shown in the cladogram below: [42] [41]
Brachyura
Dromiacea
Raninoida
Cyclodorippoida
Eubrachyura
Heterotremata
Thoracotremata
There
is a no consensus on the relationships of the subsequent superfamilies
and families. The proposed cladogram below is from analysis by Tsang et
al, 2014:[42]
Brachyura
Dromiacea
Dromioidea
Dromiidae (may be paraphyletic)
Dynomenidae
Homoloidea
Homolidae (paraphyletic)
Latreilliidae
Raninoida
Raninidae
Cyclodorippoida
Cyclodorippidae
Cymonomidae
Eubrachyura
Heterotremata
Freshwater crabs
Potamoidea
Potamonautidae
Potamidae
Gecarcinucidae
(Old World freshwater crabs)
Pseudothelphusoidea
Pseudothelphusidae
(New World freshwater crabs)
Trichodactylidae (freshwater crabs)
Orithyiidae
Belliidae
Chasmocarcinidae
Retroplumidae
Dorippoidea
Ethusidae
Dorippidae
Leucosiidae
Majoidea
Inachidae
Epialtidae (paraphyletic)
Majidae / Mithracidae
Corystidae
Euryplacidae
Matutidae
Calappidae
Parthenopidae
Cancridae
Carpiliidae
Aethridae
Pseudocarcinus of Menippidae
Menippe of Menippidae
Polybiidae
Portunidae
Pilumnoidea
Tanaochelidae
Galenidae
Pilumnidae
Mathildellidae
Eriphiidae
Oziidae
Vultocinidae
Trapeziidae
Goneplacidae
Scalopidiidae
Xanthoidea
Xanthidae (paraphyletic)
Panopeidae
Thoracotremata
Pinnotheridae
Dotillidae
Percnidae
Xenograpsidae
Cryptochiridae
Ocypodidae
Glyptograpsidae
Grapsidae
Plagusiidae
Gecarcinidae
Sesarmidae
Mictyridae
Varunidae
Macrophthalmidae
Classification
The
infraorder Brachyura contains approximately 7,000 species in 98
families,[42][22] as many as the remainder of the Decapoda.[43] The
evolution of crabs is characterised by an increasingly robust body, and a
reduction in the abdomen. Although many other groups have undergone
similar processes, carcinisation is most advanced in crabs. The telson
is no longer functional in crabs, and the uropods are absent, having
probably evolved into small devices for holding the reduced abdomen
tight against the sternum.
In most decapods, the gonopores
(sexual openings) are found on the legs. Since crabs use their first two
pairs of pleopods (abdominal appendages) for sperm transfer, this
arrangement has changed. As the male abdomen evolved into a slimmer
shape, the gonopores have moved toward the midline, away from the legs,
and onto the sternum.[44] A similar change occurred, independently, with
the female gonopores. The movement of the female gonopore to the
sternum defines the clade Eubrachyura, and the later change in the
position of the male gonopore defines the Thoracotremata. It is still a
subject of debate whether a monophyletic group is formed by those crabs
where the female, but not male, gonopores are situated on the
sternum.[43]
Superfamilies
Numbers of extant and extinct (†)
species are given in brackets.[1] The superfamily Eocarcinoidea,
containing Eocarcinus and Platykotta, was formerly thought to contain
the oldest crabs; it is now considered part of the Anomura.[45]
Examples of different crab sections
Dromia personata (Dromiacea: Dromiidae)
Ranina ranina (Raninoida: Raninidae)
Corystes cassivelaunus (Heterotremata: Corystidae)
Ocypode quadrata (Thoracotremata: Ocypodidae)
Goneplax rhomboides
Section †Callichimaeroida
†Callichimaeroidea (1†)[46]
Section Dromiacea
†Dakoticancroidea (6†)
Dromioidea (147, 85†)
Glaessneropsoidea (45†)
Homolodromioidea (24, 107†)
Homoloidea (73, 49†)
Section Raninoida (46, 196†)
Section Cyclodorippoida (99, 27†)
Section Eubrachyura
Subsection Heterotremata
Aethroidea (37, 44†)
Bellioidea (7)
Bythograeoidea (14)
Calappoidea (101, 71†)
Cancroidea (57, 81†)
Carpilioidea (4, 104†)
Cheiragonoidea (3, 13†)
Corystoidea (10, 5†)
†Componocancroidea (1†)
Dairoidea (4, 8†)
Dorippoidea (101, 73†)
Eriphioidea (67, 14†)
Gecarcinucoidea (349)
Goneplacoidea (182, 94†)
Hexapodoidea (21, 25†)
Leucosioidea (488, 113†)
Majoidea (980, 89†)
Orithyioidea (1)
Palicoidea (63, 6†)
Parthenopoidea (144, 36†)
Pilumnoidea (405, 47†)
Portunoidea (455, 200†)
Potamoidea (662, 8†)
Pseudothelphusoidea (276)
Pseudozioidea (22, 6†)
Retroplumoidea (10, 27†)
Trapezioidea (58, 10†)
Trichodactyloidea (50)
Xanthoidea (736, 134†) [47]
Subsection Thoracotremata [48]
Cryptochiroidea (46)
Grapsoidea (493, 28†)
Ocypodoidea (304, 14†)
Pinnotheroidea (304, 13†)
Recent
studies have found the following superfamilies and families to not be
monophyletic, but rather paraphyletic or polyphyletic:[42][41][48][47]
The Thoracotremata superfamily Grapsoidea is polyphyletic
The Thoracotremata superfamily Ocypodoidea is polyphyletic
The Heterotremata superfamily Calappoidea is polyphyletic
The Heterotremata superfamily Eriphioidea is polyphyletic
The Heterotremata superfamily Goneplacoidea is polyphyletic
The Heterotremata superfamily Potamoidea is paraphyletic with respect
to Gecarcinucoidea, which is resolved by placing Gecarcinucidae within
Potamoidea
The Majoidea families Epialtidae, Mithracidae and Majidae are polyphyletic with respect to each other
The Dromioidea family Dromiidae may be paraphyletic with respect to Dynomenidae
The Homoloidea family Homolidae is paraphyletic with respect to Latreilliidae
The Xanthoidea family Xanthidae is paraphyletic with respect to Panopeidae
Cultural influences
A crab divination pot in Kapsiki, North Cameroon.
Both
the constellation Cancer and the astrological sign Cancer are named
after the crab, and depicted as a crab. William Parsons, 3rd Earl of
Rosse drew the Crab Nebula in 1848 and noticed its similarity to the
animal; the Crab Pulsar lies at the centre of the nebula.[49] The Moche
people of ancient Peru worshipped nature, especially the sea,[50] and
often depicted crabs in their art.[51] In Greek mythology, Karkinos was a
crab that came to the aid of the Lernaean Hydra as it battled Heracles.
One of Rudyard Kipling's Just So Stories, The Crab that Played with the
Sea, tells the story of a gigantic crab who made the waters of the sea
go up and down, like the tides.[52] The auction for the crab quota in
2019, Russia is the largest revenue auction in the world except the
spectrum auctions. In Malay mythology (as related by Hugh Clifford to
Walter William Skeat), ocean tides are believed to be caused by water
rushing in and out of a hole in the Navel of the Seas (Pusat Tasek),
where "there sits a gigantic crab which twice a day gets out in order to
search for food".[53]: 7–8
The Kapsiki people of North Cameroon use the way crabs handle objects for divination.[citation needed]
The term crab mentality is derived from a type of detrimental social behavior observed in crabs.
Explanatory notes
Greek: βραχύς, romanized: brachys = short,[2] οὐρά / οura = tail[3]
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Robin Kennish (1996). "Diet
composition influences the fitness of the herbivorous crab Grapsus
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Tracy L. Buck;
Greg A. Breed; Steven C. Pennings; Margo E. Chase; Martin Zimmer;
Thomas H. Carefoot (2003). "Diet choice in an omnivorous salt-marsh
crab: different food types, body size, and habitat complexity". Journal
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doi:10.1016/S0022-0981(03)00146-1.
Danièle Guinot & J.–M.
Bouchard (1998). "Evolution of the abdominal holding systems of
brachyuran crabs (Crustacea, Decapoda, Brachyura)". Zoosystema. 20 (4):
613–694. Archived from the original (PDF) on 2006-11-18.
"Global
Capture Production 1950–2004". Food and Agriculture Organization.
Archived from the original on 2016-01-23. Retrieved 2006-08-26.
"Stone Crabs FAQs". Archived from the original on 2017-06-21. Retrieved 2012-09-23.
Lynsey
Patterson; Jaimie T.A. Dick; Robert W. Elwood (January 2009). "Claw
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116 (2): 302–305. doi:10.1016/j.applanim.2008.08.007.
Queen's University, Belfast (October 10, 2007). "Declawing crabs may lead to their death". Science Daily. Retrieved 2012-09-21.
Daniel P. Puzo (February 14, 1985) Imitation Crab Draws Criticisms. Los Angeles Times
Cephalopods and decapod crustaceans: their capacity to experience pain and suffering (PDF). Advocates for Animals. 2005.
Sømme,
L. (2005). "Sentience and pain in invertebrates: Report to Norwegian
Scientific Committee for Food Safety". Norwegian University of Life
Sciences, Oslo.
"Opinion on the aspects of the biology and welfare of
animals used for experimental and other scientific purposes". The EFSA
Journal. 292: 1–46. 2005.
Scholtz, Gerhard (November 2020).
"Eocarcinus praecursor Withers, 1932 (Malacostraca, Decapoda, Meiura) is
a stem group brachyuran". Arthropod Structure & Development. 59:
100991. doi:10.1016/j.asd.2020.100991. PMID 32891896.
Carrie E.
Schweitzer; Rodney M. Feldmann (2010). "The oldest Brachyura (Decapoda:
Homolodromioidea: Glaessneropsoidea) known to date (Jurassic)". Journal
of Crustacean Biology. 30 (2): 251–256. doi:10.1651/09-3231.1.
Guinot,
Danièle (2019-11-14). "New hypotheses concerning the earliest
brachyurans (Crustacea, Decapoda, Brachyura)". Geodiversitas. 41 (1):
747. doi:10.5252/geodiversitas2019v41a22. ISSN 1280-9659.
Klompmaker,
A. A.; Schweitzer, C. E.; Feldmann, R. M.; Kowalewski, M. (2013-11-01).
"The influence of reefs on the rise of Mesozoic marine crustaceans".
Geology. 41 (11): 1179–1182. Bibcode:2013Geo....41.1179K.
doi:10.1130/G34768.1. ISSN 0091-7613.
Wolfe, Joanna M.; Breinholt,
Jesse W.; Crandall, Keith A.; Lemmon, Alan R.; Lemmon, Emily Moriarty;
Timm, Laura E.; Siddall, Mark E.; Bracken-Grissom, Heather D. (24 April
2019). "A phylogenomic framework, evolutionary timeline and genomic
resources for comparative studies of decapod crustaceans". Proceedings
of the Royal Society B. 286 (1901). doi:10.1098/rspb.2019.0079. PMC
6501934. PMID 31014217.
Ling Ming Tsang; Christoph D. Schubart; Shane
T. Ahyong; Joelle C.Y. Lai; Eugene Y.C. Au; Tin-Yam Chan; Peter K.L.
Ng; Ka Hou Chu (2014). "Evolutionary History of True Crabs (Crustacea:
Decapoda: Brachyura) and the Origin of Freshwater Crabs". Molecular
Biology and Evolution. Oxford University Press . 31 (5): 1173–1187.
doi:10.1093/molbev/msu068. PMID 24520090.
Joel W. Martin; George E.
Davis (2001). An Updated Classification of the Recent Crustacea (PDF).
Natural History Museum of Los Angeles County. p. 132. Archived from the
original (PDF) on 2013-05-12. Retrieved 2009-12-14.
M. de Saint
Laurent (1980). "Sur la classification et la phylogénie des Crustacés
Décapodes Brachyoures. II. Heterotremata et Thoracotremata Guinot,
1977". Comptes rendus de l'Académie des sciences. t. 290: 1317–1320.
Jérôme
Chablais; Rodney M. Feldmann; Carrie E. Schweitzer (2011). "A new
Triassic decapod, Platykotta akaina, from the Arabian shelf of the
northern United Arab Emirates: earliest occurrence of the Anomura"
(PDF). Paläontologische Zeitschrift. 85: 93–102.
doi:10.1007/s12542-010-0080-y. S2CID 5612385. Archived (PDF) from the
original on 2012-03-19.
Luque, J.; Feldmann, R. M.; Vernygora, O.;
Schweitzer, C. E.; Cameron, C. B.; Kerr, K. A.; Vega, F. J.; Duque, A.;
Strange, M.; Palmer, A. R.; Jaramillo, C. (24 April 2019). "Exceptional
preservation of mid-Cretaceous marine arthropods and the evolution of
novel forms via heterochrony". Science Advances. 5 (4): eaav3875.
Bibcode:2019SciA....5.3875L. doi:10.1126/sciadv.aav3875. PMC 6482010.
PMID 31032408.
Jose C.E. Mendoza; Kin Onn Chan; Joelle C.Y. Lai;
Brent P. Thoma; Paul F. Clark; Danièle Guinot; Darryl L. Felder; Peter
K.L. Ng (2022). "A comprehensive molecular phylogeny of the brachyuran
crab superfamily Xanthoidea provides novel insights into its systematics
and evolutionary history". Molecular Phylogenetics and Evolution. 177:
107627. doi:10.1016/j.ympev.2022.107627. PMID 36096461.
Chandler T.T.
Tsang; Christoph D. Schubart; Ka Hou Chu; Peter K.L. Ng; Ling Ming
Tsang (2022). "Molecular phylogeny of Thoracotremata crabs (Decapoda,
Brachyura): Toward adopting monophyletic superfamilies, invasion history
into terrestrial habitats and multiple origins of symbiosis". Molecular
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B. B. Rossi (1969).
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Kipling, Rudyard (1902). "The Crab that Played with the Sea". Just So Stories. Macmillan.
Skeat, Walter William (1900). "Chapter 1: Nature". Malay Magic. London: Macmillan and Co., Limited. pp. 1–15.
External links
Decapoda at Curlie
vte
Subgroups of Order Decapoda
Kingdom: Animalia Phylum: Arthropoda Class: Malacostraca Subclass: Eumalacostraca Superorder: Eucarida
Dendrobranchiata
Penaeoidea Sergestoidea
Dendrobranchiata (prawns)
Stenopodidea
(boxer shrimp) Caridea (true shrimp) Achelata (spiny lobsters, slipper
lobsters) Astacidea (lobsters, crayfish) Anomura (hermit crabs and
others)
Brachyura (crabs)
Pleocyemata
Stenopodidea
Stenopodidae Macromaxillocarididae Spongicolidae
Procarididea
Procarididae
Caridea
Alpheoidea Atyoidea Bresilioidea Campylonotoidea Crangonoidea
Galatheacaridoidea Nematocarcinoidea Oplophoroidea Palaemonoidea
Pandaloidea Pasiphaeoidea Physetocaridoidea Processoidea Psalidopodoidea
Stylodactyloidea
Reptantia
Achelata
Palinuridae (inc. Synaxidae) Scyllaridae †Cancrinidae †Tricarinidae?
Polychelida
†Coleiidae †Eryonidae †Palaeopentachelidae Polychelidae †Tetrachelidae
Glypheidea
Glypheoidea †Erymoidea
Astacidea
Astacoidea Enoplometopoidea Nephropoidea †Palaeopalaemonoidea Parastacoidea
Axiidea
Axiidae Callianassidae Callianideidae Ctenochelidae Gourretiidae Micheleidae Strahlaxiidae
Gebiidea
Axianassidae Laomediidae Thalassinidae Upogebiidae
Anomura
Aegloidea Chirostyloidea †Eocarcinoidea Galatheoidea Hippoidea Lithodoidea Lomisoidea Paguroidea
Brachyura
see Brachyura
vte
Superfamilies of Infraorder Brachyura (true crabs)
Kingdom Animalia Phylum Arthropoda Class Malacostraca Order Decapoda Suborder Pleocyemata
Dromiacea
Dakoticancroidea† Dromioidea Glaessneropsoidea† Homolodromioidea Homoloidea
Raninoida
Raninoidea
Cyclodorippoida
Cyclodorippoidea
Eubrachyura
Heterotremata
Aethroidea Bellioidea Bythograeoidea Calappoidea Cancroidea
Carpilioidea Cheiragonoidea Componocancroidea† Corystoidea Dairoidea
Dorippoidea Eriphioidea Gecarcinucoidea Goneplacoidea Hexapodoidea
Leucosioidea Majoidea Orithyioidea Palicoidea Parthenopoidea Pilumnoidea
Portunoidea Potamoidea Pseudothelphusoidea Pseudozioidea Retroplumoidea
Trapezioidea Trichodactyloidea Xanthoidea
Thoracotremata
Cryptochiroidea Grapsoidea Ocypodoidea Pinnotheroidea
vte
Principal commercial fishery species groups
Wild
Large pelagic fish
Mackerel Salmon Shark Swordfish Tuna
albacore bigeye Atlantic bluefin Pacific bluefin southern bluefin skipjack yellowfin
Forage fish
Anchovy Capelin Herring Ilish Menhaden Sardines Saury Shad Sprat
european
Demersal fish
Catfish Cod
Atlantic Pacific Alaska pollock Flatfish
flounder halibut plaice sole turbot Haddock Mullet Orange roughy Pollock Rockfish Smelt-whitings Toothfish
Freshwater fish
Carp Sturgeon Tilapia Trout
Other wild fish
Eel Whitebait more...
Crustaceans
Crab Krill Lobster Shrimp more...
Molluscs
Abalone Mussels Octopus Oysters Scallops Squid more...
Echinoderms
Sea cucumbers Sea urchin more...
Atlantic cod
Lobster
Pacific oysters
Farmed
Carp
bighead common crucian grass silver Catfish Freshwater prawns Gilt-head bream Mussels Oysters Salmon
Atlantic salmon trout coho chinook Scallops Seaweed Shrimp Tilapia
Commercial fishing World fish production Commercial species Fishing topics Fisheries glossary
vte
Edible crustaceans
Shrimp/
prawns
Acetes Crangon crangon Cryphiops caementarius Dried shrimp Indian prawn
Litopenaeus setiferus Macrobrachium rosenbergii Palaemon serratus
Pandalus borealis Penaeus esculentus Penaeus monodon Processa edulis
Shrimp paste Whiteleg shrimp Xiphopenaeus kroyeri
Lobsters
(incl. slipper
& spiny)
American lobster Arctides guineensis California spiny lobster Homarus
gammarus Ibacus peronii Japanese spiny lobster Jasus Jasus edwardsii
Jasus lalandii Metanephrops challengeri Thenus orientalis Nephrops
norvegicus Palinurus elephas Panulirus argus Panulirus cygnus Panulirus
echinatus Panulirus guttatus Panulirus homarus Panulirus longipes
Panulirus ornatus Panulirus pascuensis Panulirus penicillatus Panulirus
versicolor Parribacus japonicus Sagmariasus Scyllarides herklotsii
Scyllarides latus Scyllarus arctus Thymops birsteini Tristan rock
lobster
Crabs
Callinectes sapidus Callinectes
similis Cancer irroratus Cancer bellianus Cancer pagurus Cancer
productus Chaceon fenneri Chaceon quinquedens Chinese mitten crab
Chionoecetes Declawing of crabs Dungeness crab Florida stone crab
Gecarcinus ruricola Horsehair crab Hypothalassia acerba Jonah crab Maja
squinado Menippe adina Orithyia sinica Ovalipes australiensis Pie crust
crab Portunus pelagicus Portunus trituberculatus Ranina ranina Scylla
paramamosain Scylla serrata
Crayfish
Acocil Astacus astacus Marron Cherax Paranephrops Procambarus clarkii Orconectes virilis Signal crayfish
Others
Austromegabalanus psittacus Coconut crab Galathea strigosa
Glyptolithodes Goose barnacle King crab Krill Langostino Lysiosquillina
maculata Mantis shrimp Oratosquilla oratoria Paralithodes camtschaticus
Red king crab Squat lobster Squilla mantis Tasmanian giant crab
Thalassina
Category
Portal:
icon Crustaceans
Taxon identifiers
Wikidata: Q40802 Wikispecies: Brachyura ADW: Brachyura AFD: Brachyura
BioLib: 19451 BugGuide: 354356 EoL: 46505748 Fauna Europaea: 13304 Fauna
Europaea (new): 8f16bf3d-fc20-4f48-bae0-6a8ba9a2108d Fossilworks: 58207
iNaturalist: 121639 ITIS: 98276 NBN: NHMSYS0021049609 NCBI: 6752 NZOR:
bcedb431-7823-4903-949b-c275b184c0dc WoRMS: 106673
Authority control databases Edit this at Wikidata
National
Germany Israel United States Japan Czech Republic
Other
NARA
Categories:
CrabsCommercial crustaceansEdible crustaceansExtant Jurassic first appearances
Marine life
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"Sealife" redirects here. For the aquarium attractions operated by Merlin Entertainments, see Sea Life.
General characteristics of a large marine ecosystem (Gulf of Alaska)
Killer
whales (orcas) are highly visible marine apex predators that hunt many
large species. But most biological activity in the ocean takes place
with microscopic marine organisms that cannot be seen individually with
the naked eye, such as marine bacteria and phytoplankton.[1]
Part of a series of overviews on
Marine life
Marine habitats
Marine microorganisms
Marine microbiomes
Marine viruses
Marine prokaryotes
Marine protists
Marine fungi
Marine invertebrates
Marine vertebrates
Marine primary production
Marine food web
Marine carbon pump
Marine biogeochemical cycles
Human impact on marine life
Marine conservation
Marine life portal
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Marine
life, sea life, or ocean life is the plants, animals, and other
organisms that live in the salt water of seas or oceans, or the brackish
water of coastal estuaries. At a fundamental level, marine life affects
the nature of the planet. Marine organisms, mostly microorganisms,
produce oxygen and sequester carbon. Marine life in part shape and
protect shorelines, and some marine organisms even help create new land
(e.g. coral building reefs).
Most life forms evolved initially in
marine habitats. By volume, oceans provide about 90% of the living
space on the planet.[2] The earliest vertebrates appeared in the form of
fish,[3] which live exclusively in water. Some of these evolved into
amphibians, which spend portions of their lives in water and portions on
land. One group of amphibians evolved into reptiles and mammals and a
few subsets of each returned to the ocean as sea snakes, sea turtles,
seals, manatees, and whales. Plant forms such as kelp and other algae
grow in the water and are the basis for some underwater ecosystems.
Plankton forms the general foundation of the ocean food chain,
particularly phytoplankton which are key primary producers.
Marine
invertebrates exhibit a wide range of modifications to survive in
poorly oxygenated waters, including breathing tubes as in mollusc
siphons. Fish have gills instead of lungs, although some species of
fish, such as the lungfish, have both. Marine mammals (e.g. dolphins,
whales, otters, and seals) need to surface periodically to breathe air.
As
of 2023, more than 242,000 marine species have been documented, and
perhaps two million marine species are yet to be documented. An average
of 2,332 new species per year are being described.[4][5]
Marine
species range in size from the microscopic like phytoplankton, which can
be as small as 0.02 micrometres, to huge cetaceans like the blue whale –
the largest known animal, reaching 33 m (108 ft) in length.[6][7]
Marine microorganisms, including protists and bacteria and their
associated viruses, have been variously estimated as constituting about
70%[8] or about 90%[9][1] of the total marine biomass. Marine life is
studied scientifically in both marine biology and in biological
oceanography. The term marine comes from the Latin mare, meaning "sea"
or "ocean".
Water
Elevation histogram showing the percentage of the Earth's surface above and below sea level
See also: Hydrosphere
There
is no life without water.[10] It has been described as the universal
solvent for its ability to dissolve many substances,[11][12] and as the
solvent of life.[13] Water is the only common substance to exist as a
solid, liquid, and gas under conditions normal to life on Earth.[14] The
Nobel Prize winner Albert Szent-Györgyi referred to water as the mater
und matrix: the mother and womb of life.[15]
Composition of seawater. Quantities in relation to 1 kg or 1 litre of sea water.
The
abundance of surface water on Earth is a unique feature in the Solar
System. Earth's hydrosphere consists chiefly of the oceans, but
technically includes all water surfaces in the world, including inland
seas, lakes, rivers, and underground waters down to a depth of 2,000
metres (6,600 ft) The deepest underwater location is Challenger Deep of
the Mariana Trench in the Pacific Ocean, having a depth of 10,900 metres
(6.8 mi).[note 1][16]
Conventionally the planet is divided into
five separate oceans, but these oceans all connect into a single world
ocean.[17] The mass of this world ocean is 1.35×1018 metric tons, or
about 1/4400 of Earth's total mass. The world ocean covers an area of
3.618×108 km2 with a mean depth of 3682 m, resulting in an estimated
volume of 1.332×109 km3.[18] If all of Earth's crustal surface was at
the same elevation as a smooth sphere, the depth of the resulting world
ocean would be about 2.7 kilometres (1.7 mi).[19][20]
The Earth's water cycle
About
97.5% of the water on Earth is saline; the remaining 2.5% is fresh
water. Most fresh water – about 69% – is present as ice in ice caps and
glaciers.[21] The average salinity of Earth's oceans is about 35 grams
(1.2 oz) of salt per kilogram of seawater (3.5% salt).[22] Most of the
salt in the ocean comes from the weathering and erosion of rocks on
land.[23] Some salts are released from volcanic activity or extracted
from cool igneous rocks.[24]
The oceans are also a reservoir of
dissolved atmospheric gases, which are essential for the survival of
many aquatic life forms.[25] Sea water has an important influence on the
world's climate, with the oceans acting as a large heat reservoir.[26]
Shifts in the oceanic temperature distribution can cause significant
weather shifts, such as the El Niño-Southern Oscillation.[27]
Jupiter's moon Europa may have an underground ocean which supports life.
Altogether
the ocean occupies 71 percent of the world surface,[2] averaging nearly
3.7 kilometres (2.3 mi) in depth.[28] By volume, the ocean provides
about 90 percent of the living space on the planet.[2] The science
fiction writer Arthur C. Clarke has pointed out it would be more
appropriate to refer to planet Earth as planet Ocean.[29][30]
However
water is found elsewhere in the solar system. Europa, one of the moons
orbiting Jupiter, is slightly smaller than the Earth's moon. There is a
strong possibility a large saltwater ocean exists beneath its ice
surface.[31] It has been estimated the outer crust of solid ice is about
10–30 km (6–19 mi) thick and the liquid ocean underneath is about 100
km (60 mi) deep.[32] This would make Europa's ocean over twice the
volume of the Earth's ocean. There has been speculation Europa's ocean
could support life,[33][34] and could be capable of supporting
multicellular microorganisms if hydrothermal vents are active on the
ocean floor.[35] Enceladus, a small icy moon of Saturn, also has what
appears to be an underground ocean which actively vents warm water from
the moon's surface.[36]
Evolution
Life timeline
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Single-celled life
Photosynthesis
Eukaryotes
Multicellular life
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Dinosaurs
Mammals
Birds
Primates
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a
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Earth formed
←
Earliest water
←
LUCA
←
Earliest fossils
←
LHB meteorites
←
Earliest oxygen
←
Pongola glaciation*
←
Atmospheric oxygen
←
Huronian glaciation*
←
Sexual reproduction
←
Earliest multicellular life
←
Earliest fungi
←
Earliest plants
←
Earliest animals
←
Cryogenian ice age*
←
Ediacaran biota
←
Cambrian explosion
←
Andean glaciation*
←
Earliest tetrapods
←
Karoo ice age*
←
Earliest apes / humans
←
Quaternary ice age*
(million years ago)
*Ice Ages
Further information: Evolutionary history of life and Timeline of evolutionary history of life
Historical development
The
Earth is about 4.54 billion years old.[37][38][39] The earliest
undisputed evidence of life on Earth dates from at least 3.5 billion
years ago,[40][41] during the Eoarchean era after a geological crust
started to solidify following the earlier molten Hadean Eon. Microbial
mat fossils have been found in 3.48 billion-year-old sandstone in
Western Australia.[42][43] Other early physical evidence of a biogenic
substance is graphite in 3.7 billion-year-old metasedimentary rocks
discovered in Western Greenland[44] as well as "remains of biotic life"
found in 4.1 billion-year-old rocks in Western Australia.[45][46]
According to one of the researchers, "If life arose relatively quickly
on Earth … then it could be common in the universe."[45]
All
organisms on Earth are descended from a common ancestor or ancestral
gene pool.[47][48] Highly energetic chemistry is thought to have
produced a self-replicating molecule around 4 billion years ago, and
half a billion years later the last common ancestor of all life
existed.[49] The current scientific consensus is that the complex
biochemistry that makes up life came from simpler chemical
reactions.[50] The beginning of life may have included self-replicating
molecules such as RNA[51] and the assembly of simple cells.[52] In 2016
scientists reported a set of 355 genes from the last universal common
ancestor (LUCA) of all life, including microorganisms, living on
Earth.[53]
Current species are a stage in the process of
evolution, with their diversity the product of a long series of
speciation and extinction events.[54] The common descent of organisms
was first deduced from four simple facts about organisms: First, they
have geographic distributions that cannot be explained by local
adaptation. Second, the diversity of life is not a set of unique
organisms, but organisms that share morphological similarities. Third,
vestigial traits with no clear purpose resemble functional ancestral
traits and finally, that organisms can be classified using these
similarities into a hierarchy of nested groups—similar to a family
tree.[55] However, modern research has suggested that, due to horizontal
gene transfer, this "tree of life" may be more complicated than a
simple branching tree since some genes have spread independently between
distantly related species.[56][57]
Past species have also left
records of their evolutionary history. Fossils, along with the
comparative anatomy of present-day organisms, constitute the
morphological, or anatomical, record.[58] By comparing the anatomies of
both modern and extinct species, paleontologists can infer the lineages
of those species. However, this approach is most successful for
organisms that had hard body parts, such as shells, bones or teeth.
Further, as prokaryotes such as bacteria and archaea share a limited set
of common morphologies, their fossils do not provide information on
their ancestry.
Evolutionary tree showing the divergence of modern
species from their common ancestor in the centre.[59] The three domains
are coloured, with bacteria blue, archaea green and eukaryotes red.
More
recently, evidence for common descent has come from the study of
biochemical similarities between organisms. For example, all living
cells use the same basic set of nucleotides and amino acids.[60] The
development of molecular genetics has revealed the record of evolution
left in organisms' genomes: dating when species diverged through the
molecular clock produced by mutations.[61] For example, these DNA
sequence comparisons have revealed that humans and chimpanzees share 98%
of their genomes and analysing the few areas where they differ helps
shed light on when the common ancestor of these species existed.[62]
Prokaryotes
inhabited the Earth from approximately 3–4 billion years ago.[63][64]
No obvious changes in morphology or cellular organisation occurred in
these organisms over the next few billion years.[65] The eukaryotic
cells emerged between 1.6 and 2.7 billion years ago. The next major
change in cell structure came when bacteria were engulfed by eukaryotic
cells, in a cooperative association called endosymbiosis.[66][67] The
engulfed bacteria and the host cell then underwent coevolution, with the
bacteria evolving into either mitochondria or hydrogenosomes.[68]
Another engulfment of cyanobacterial-like organisms led to the formation
of chloroplasts in algae and plants.[69]
Phylogenetic and symbiogenetic tree of living organisms, showing a view of the origins of eukaryotes and prokaryotes
The
history of life was that of the unicellular eukaryotes, prokaryotes and
archaea until about 610 million years ago when multicellular organisms
began to appear in the oceans in the Ediacaran period.[63][70] The
evolution of multicellularity occurred in multiple independent events,
in organisms as diverse as sponges, brown algae, cyanobacteria, slime
moulds and myxobacteria.[71] In 2016 scientists reported that, about 800
million years ago, a minor genetic change in a single molecule called
GK-PID may have allowed organisms to go from a single cell organism to
one of many cells.[72]
Soon after the emergence of these first
multicellular organisms, a remarkable amount of biological diversity
appeared over a span of about 10 million years, in an event called the
Cambrian explosion. Here, the majority of types of modern animals
appeared in the fossil record, as well as unique lineages that
subsequently became extinct.[73] Various triggers for the Cambrian
explosion have been proposed, including the accumulation of oxygen in
the atmosphere from photosynthesis.[74]
About 500 million years
ago, plants and fungi started colonising the land. Evidence for the
appearance of the first land plants occurs in the Ordovician, around 450
million years ago, in the form of fossil spores.[75] Land plants began
to diversify in the Late Silurian, from around 430 million years
ago.[76] The colonisation of the land by plants was soon followed by
arthropods and other animals.[77] Insects were particularly successful
and even today make up the majority of animal species.[78] Amphibians
first appeared around 364 million years ago, followed by early amniotes
and birds around 155 million years ago (both from "reptile"-like
lineages), mammals around 129 million years ago, homininae around 10
million years ago and modern humans around 250,000 years
ago.[79][80][81] However, despite the evolution of these large animals,
smaller organisms similar to the types that evolved early in this
process continue to be highly successful and dominate the Earth, with
the majority of both biomass and species being prokaryotes.[82]
Estimates
on the number of Earth's current species range from 10 million to 14
million,[83] of which about 1.2 million have been documented and over 86
percent have not yet been described.[84]
Microorganisms
microbial mats
Microbial
mats are the earliest form of life on Earth for which there is good
fossil evidence. The image shows a cyanobacterial-algal mat.
Stromatolites are formed from microbial mats as microbes slowly move upwards to avoid being smothered by sediment.
Main article: Marine microorganism
Microorganisms
make up about 70% of the marine biomass.[8] A microorganism, or
microbe, is a microscopic organism too small to be recognised with the
naked eye. It can be single-celled[85] or multicellular. Microorganisms
are diverse and include all bacteria and archaea, most protozoa such as
algae, fungi and certain microscopic animals such as rotifers.
Many
macroscopic animals and plants have microscopic juvenile stages. Some
microbiologists also classify viruses (and viroids) as microorganisms,
but others consider these as nonliving.[86][87]
Microorganisms
are crucial to nutrient recycling in ecosystems as they act as
decomposers. Some microorganisms are pathogenic, causing disease and
even death in plants and animals.[88] As inhabitants of the largest
environment on Earth, microbial marine systems drive changes in every
global system. Microbes are responsible for virtually all the
photosynthesis that occurs in the ocean, as well as the cycling of
carbon, nitrogen, phosphorus, other nutrients and trace elements.[89]
The range of sizes shown by prokaryotes (bacteria and archaea) and viruses relative to those of other organisms and biomolecules
Marine microorganisms
Viruses
Cellular life
Prokaryotes
Bacteria
Archaea
Eukaryotes
Protists
Microfungi
Microanimals
Marine microbial loop
Microscopic
life undersea is diverse and still poorly understood, such as for the
role of viruses in marine ecosystems.[90] Most marine viruses are
bacteriophages, which are harmless to plants and animals, but are
essential to the regulation of saltwater and freshwater
ecosystems.[91]: 5 They infect and destroy bacteria in aquatic
microbial communities, and are the most important mechanism of recycling
carbon in the marine environment. The organic molecules released from
the dead bacterial cells stimulate fresh bacterial and algal
growth.[91]: 593 Viral activity may also contribute to the biological
pump, the process whereby carbon is sequestered in the deep ocean.[92]
Sea
spray containing marine microorganisms can be swept high into the
atmosphere where they become aeroplankton, and can travel the globe
before falling back to earth.
Under a magnifier, a splash of seawater teems with life.
A
stream of airborne microorganisms circles the planet above weather
systems but below commercial air lanes.[93] Some peripatetic
microorganisms are swept up from terrestrial dust storms, but most
originate from marine microorganisms in sea spray. In 2018, scientists
reported that hundreds of millions of viruses and tens of millions of
bacteria are deposited daily on every square meter around the
planet.[94][95]
Microscopic organisms live throughout the
biosphere. The mass of prokaryote microorganisms — which includes
bacteria and archaea, but not the nucleated eukaryote microorganisms —
may be as much as 0.8 trillion tons of carbon (of the total biosphere
mass, estimated at between 1 and 4 trillion tons).[96] Single-celled
barophilic marine microbes have been found at a depth of 10,900 m
(35,800 ft) in the Mariana Trench, the deepest spot in the Earth's
oceans.[97][98] Microorganisms live inside rocks 580 m (1,900 ft) below
the sea floor under 2,590 m (8,500 ft) of ocean off the coast of the
northwestern United States,[97][99] as well as 2,400 m (7,900 ft; 1.5
mi) beneath the seabed off Japan.[100] The greatest known temperature at
which microbial life can exist is 122 °C (252 °F) (Methanopyrus
kandleri).[101] In 2014, scientists confirmed the existence of
microorganisms living 800 m (2,600 ft) below the ice of
Antarctica.[102][103] According to one researcher, "You can find
microbes everywhere — they're extremely adaptable to conditions, and
survive wherever they are."[97]
Marine viruses
Main article: Marine viruses
Viruses
are small infectious agents that do not have their own metabolism and
can replicate only inside the living cells of other organisms.[104]
Viruses can infect all types of life forms, from animals and plants to
microorganisms, including bacteria and archaea.[105] The linear size of
the average virus is about one one-hundredth that of the average
bacterium. Most viruses cannot be seen with an optical microscope so
electron microscopes are used instead.[106]
Viruses are found
wherever there is life and have probably existed since living cells
first evolved.[107] The origin of viruses is unclear because they do not
form fossils, so molecular techniques have been used to compare the DNA
or RNA of viruses and are a useful means of investigating how they
arise.[108]
Viruses are now recognised as ancient and as having
origins that pre-date the divergence of life into the three
domains.[109] But the origins of viruses in the evolutionary history of
life are unclear: some may have evolved from plasmids—pieces of DNA that
can move between cells—while others may have evolved from bacteria. In
evolution, viruses are an important means of horizontal gene transfer,
which increases genetic diversity.[110]
Bacteriophages (phages)
Multiple phages attached to a bacterial cell wall at 200,000× magnification
Diagram of a typical tailed phage
These are cyanophages, viruses that infect cyanobacteria (scale bars indicate 100 nm)
Opinions
differ on whether viruses are a form of life or organic structures that
interact with living organisms.[111] They are considered by some to be a
life form, because they carry genetic material, reproduce by creating
multiple copies of themselves through self-assembly, and evolve through
natural selection. However they lack key characteristics such as a
cellular structure generally considered necessary to count as life.
Because they possess some but not all such qualities, viruses have been
described as replicators[111] and as "organisms at the edge of
life".[112]
In terms of individual counts, tailed phage are the most abundant biological entities in the sea.
Bacteriophages,
often just called phages, are viruses that parasite bacteria and
archaea. Marine phages parasite marine bacteria and archaea, such as
cyanobacteria.[113] They are a common and diverse group of viruses and
are the most abundant biological entity in marine environments, because
their hosts, bacteria, are typically the numerically dominant cellular
life in the sea. Generally there are about 1 million to 10 million
viruses in each mL of seawater, or about ten times more double-stranded
DNA viruses than there are cellular organisms,[114][115] although
estimates of viral abundance in seawater can vary over a wide
range.[116][117] Tailed bacteriophages appear to dominate marine
ecosystems in number and diversity of organisms.[113] Bacteriophages
belonging to the families Corticoviridae,[118] Inoviridae[119] and
Microviridae[120] are also known to infect diverse marine bacteria.
Microorganisms
make up about 70% of the marine biomass.[8] It is estimated viruses
kill 20% of this biomass each day and that there are 15 times as many
viruses in the oceans as there are bacteria and archaea. Viruses are the
main agents responsible for the rapid destruction of harmful algal
blooms,[115] which often kill other marine life.[121] The number of
viruses in the oceans decreases further offshore and deeper into the
water, where there are fewer host organisms.[92]
There are also
archaeal viruses which replicate within archaea: these are
double-stranded DNA viruses with unusual and sometimes unique
shapes.[122][123] These viruses have been studied in most detail in the
thermophilic archaea, particularly the orders Sulfolobales and
Thermoproteales.[124]
Viruses are an important natural means of
transferring genes between different species, which increases genetic
diversity and drives evolution.[110] It is thought that viruses played a
central role in the early evolution, before the diversification of
bacteria, archaea and eukaryotes, at the time of the last universal
common ancestor of life on Earth.[125] Viruses are still one of the
largest reservoirs of unexplored genetic diversity on Earth.[92]
Marine bacteria
Vibrio vulnificus, a virulent bacterium found in estuaries and along coastal areas
Pelagibacter ubique, the most abundant bacteria in the ocean, plays a major role in the global carbon cycle.
Further information: Marine prokaryotes and Bacterioplankton
Bacteria
constitute a large domain of prokaryotic microorganisms. Typically a
few micrometres in length, bacteria have a number of shapes, ranging
from spheres to rods and spirals. Bacteria were among the first life
forms to appear on Earth, and are present in most of its habitats.
Bacteria inhabit soil, water, acidic hot springs, radioactive
waste,[126] and the deep portions of Earth's crust. Bacteria also live
in symbiotic and parasitic relationships with plants and animals.
Once
regarded as plants constituting the class Schizomycetes, bacteria are
now classified as prokaryotes. Unlike cells of animals and other
eukaryotes, bacterial cells do not contain a nucleus and rarely harbour
membrane-bound organelles. Although the term bacteria traditionally
included all prokaryotes, the scientific classification changed after
the discovery in the 1990s that prokaryotes consist of two very
different groups of organisms that evolved from an ancient common
ancestor. These evolutionary domains are called Bacteria and
Archaea.[127]
The ancestors of modern bacteria were unicellular
microorganisms that were the first forms of life to appear on Earth,
about 4 billion years ago. For about 3 billion years, most organisms
were microscopic, and bacteria and archaea were the dominant forms of
life.[65][128] Although bacterial fossils exist, such as stromatolites,
their lack of distinctive morphology prevents them from being used to
examine the history of bacterial evolution, or to date the time of
origin of a particular bacterial species. However, gene sequences can be
used to reconstruct the bacterial phylogeny, and these studies indicate
that bacteria diverged first from the archaeal/eukaryotic lineage.[129]
Bacteria were also involved in the second great evolutionary
divergence, that of the archaea and eukaryotes. Here, eukaryotes
resulted from the entering of ancient bacteria into endosymbiotic
associations with the ancestors of eukaryotic cells, which were
themselves possibly related to the Archaea.[67][66] This involved the
engulfment by proto-eukaryotic cells of alphaproteobacterial symbionts
to form either mitochondria or hydrogenosomes, which are still found in
all known Eukarya. Later on, some eukaryotes that already contained
mitochondria also engulfed cyanobacterial-like organisms. This led to
the formation of chloroplasts in algae and plants. There are also some
algae that originated from even later endosymbiotic events. Here,
eukaryotes engulfed a eukaryotic algae that developed into a
"second-generation" plastid.[130][131] This is known as secondary
endosymbiosis.
The marine Thiomargarita namibiensis, the largest known bacterium
The marine Thiomargarita namibiensis, the largest known bacterium
Cyanobacteria blooms can contain lethal cyanotoxins.
Cyanobacteria blooms can contain lethal cyanotoxins.
The chloroplasts of glaucophytes have a peptidoglycan layer, evidence
suggesting their endosymbiotic origin from cyanobacteria.[132]
The chloroplasts of glaucophytes have a peptidoglycan layer, evidence
suggesting their endosymbiotic origin from cyanobacteria.[132]
Bacteria can be beneficial. This Pompeii worm, an extremophile found
only at hydrothermal vents, has a protective cover of bacteria.
Bacteria can be beneficial. This Pompeii worm, an extremophile found
only at hydrothermal vents, has a protective cover of bacteria.
The
largest known bacterium, the marine Thiomargarita namibiensis, can be
visible to the naked eye and sometimes attains 0.75 mm (750
μm).[133][134]
Marine archaea
Archaea were initially viewed as
extremophiles living in harsh environments, such as the yellow archaea
pictured here in a hot spring, but they have since been found in a much
broader range of habitats.[135]
Further information: Marine prokaryotes
The
archaea (Greek for ancient[136]) constitute a domain and kingdom of
single-celled microorganisms. These microbes are prokaryotes, meaning
they have no cell nucleus or any other membrane-bound organelles in
their cells.
Archaea were initially classified as bacteria, but
this classification is outdated.[137] Archaeal cells have unique
properties separating them from the other two domains of life, Bacteria
and Eukaryota. The Archaea are further divided into multiple recognized
phyla. Classification is difficult because the majority have not been
isolated in the laboratory and have only been detected by analysis of
their nucleic acids in samples from their environment.
Archaea
and bacteria are generally similar in size and shape, although a few
archaea have very strange shapes, such as the flat and square-shaped
cells of Haloquadratum walsbyi.[138] Despite this morphological
similarity to bacteria, archaea possess genes and several metabolic
pathways that are more closely related to those of eukaryotes, notably
the enzymes involved in transcription and translation. Other aspects of
archaeal biochemistry are unique, such as their reliance on ether lipids
in their cell membranes, such as archaeols. Archaea use more energy
sources than eukaryotes: these range from organic compounds, such as
sugars, to ammonia, metal ions or even hydrogen gas. Salt-tolerant
archaea (the Haloarchaea) use sunlight as an energy source, and other
species of archaea fix carbon; however, unlike plants and cyanobacteria,
no known species of archaea does both. Archaea reproduce asexually by
binary fission, fragmentation, or budding; unlike bacteria and
eukaryotes, no known species forms spores.
Archaea are
particularly numerous in the oceans, and the archaea in plankton may be
one of the most abundant groups of organisms on the planet. Archaea are a
major part of Earth's life and may play roles in both the carbon cycle
and the nitrogen cycle.
Halobacteria, found in water near saturated with salt, are now recognised as archaea.
Halobacteria, found in water near saturated with salt, are now recognised as archaea.
Flat, square-shaped cells of the archaea Haloquadratum walsbyi
Flat, square-shaped cells of the archaea Haloquadratum walsbyi
Methanosarcina barkeri, a marine archaea that produces methane
Methanosarcina barkeri, a marine archaea that produces methane
Thermophiles, such as Pyrolobus fumarii, survive well over 100 °C.
Thermophiles, such as Pyrolobus fumarii, survive well over 100 °C.
Drawing of another marine thermophile, Pyrococcus furiosus
Drawing of another marine thermophile, Pyrococcus furiosus
Marine protists
Main article: Marine protists
Protists
are eukaryotes that cannot be classified as plants, fungi or animals.
They are usually single-celled and microscopic. Life originated as
single-celled prokaryotes (bacteria and archaea) and later evolved into
more complex eukaryotes. Eukaryotes are the more developed life forms
known as plants, animals, fungi and protists. The term protist came into
use historically as a term of convenience for eukaryotes that cannot be
strictly classified as plants, animals or fungi. They are not a part of
modern cladistics, because they are paraphyletic (lacking a common
ancestor). Protists can be broadly divided into four groups depending on
whether their nutrition is plant-like, animal-like, fungus-like,[139]
or a mixture of these.[140]
Protists according to how they get food
Type of protist Description Example Other examples
Plant-like
Algae
(see below)
Autotrophic protists that make their own food without needing to
consume other organisms, usually by using photosynthesis Red
algae, Cyanidium sp. Green algae, brown algae, diatoms and some
dinoflagellates. Plant-like protists are important components of
phytoplankton discussed below.
Animal-like
Protozoans
Heterotrophic protists that get their food consuming other organisms
Radiolarian protist as drawn by Haeckel Foraminiferans, and
some marine amoebae, ciliates and flagellates.
Fungus-like
Slime moulds
and
slime nets
Saprotrophic protists that get their food from the remains of
organisms that have broken down and decayed Marine slime nets
form labyrinthine networks of tubes in which amoeba without pseudopods
can travel Marine lichen
Mixotropes
Various
Mixotrophic and osmotrophic protists that get their food from a
combination of the above Euglena mutabilis, a photosynthetic
flagellate Many marine mixotropes are found among protists,
including among ciliates, Rhizaria and dinoflagellates [141]
micrograph
cell schematic
Choanoflagellates,
unicellular "collared" flagellate protists, are thought to be the
closest living relatives of the animals.[142] Getting to know our
single-celled ancestors - MicroCosmos
Protists are highly diverse
organisms currently organised into 18 phyla, but are not easy to
classify.[143][144] Studies have shown high protist diversity exists in
oceans, deep sea-vents and river sediments, suggesting a large number of
eukaryotic microbial communities have yet to be discovered.[145][146]
There has been little research on mixotrophic protists, but recent
studies in marine environments found mixotrophic protests contribute a
significant part of the protist biomass.[141]
Single-celled and microscopic protists
Diatoms are a major algae group generating about 20% of world oxygen production.[147]
Diatoms are a major algae group generating about 20% of world oxygen production.[147]
Diatoms have glass like cell walls made of silica and called frustules.[148]
Diatoms have glass like cell walls made of silica and called frustules.[148]
Fossil diatom frustule from 32 to 40 mya
Fossil diatom frustule from 32 to 40 mya
Radiolarian
Radiolarian
Single-celled alga, Gephyrocapsa oceanica
Single-celled alga, Gephyrocapsa oceanica
Two dinoflagellates
Two dinoflagellates
Zooxanthellae is a photosynthetic algae that lives inside hosts like coral.
Zooxanthellae is a photosynthetic algae that lives inside hosts like coral.
A single-celled ciliate with green zoochlorellae living inside endosymbiotically.
A single-celled ciliate with green zoochlorellae living inside endosymbiotically.
Euglenoid
Euglenoid
This ciliate is digesting cyanobacteria. The cytostome or mouth is at the bottom right.
This ciliate is digesting cyanobacteria. The cytostome or mouth is at the bottom right.
0:20
Video of a ciliate ingesting a diatom
In
contrast to the cells of prokaryotes, the cells of eukaryotes are
highly organised. Plants, animals and fungi are usually multi-celled and
are typically macroscopic. Most protists are single-celled and
microscopic. But there are exceptions. Some single-celled marine
protists are macroscopic. Some marine slime molds have unique life
cycles that involve switching between unicellular, colonial, and
multicellular forms.[149] Other marine protist are neither single-celled
nor microscopic, such as seaweed.
Macroscopic protists (see also unicellular macroalgae →)
The single-celled giant amoeba has up to 1000 nuclei and reaches lengths of 5 mm (0.20 in).
The single-celled giant amoeba has up to 1000 nuclei and reaches lengths of 5 mm (0.20 in).
Gromia sphaerica is a large spherical testate amoeba which makes mud trails. Its diameter is up to 3.8 cm (1.5 in).[150]
Gromia sphaerica is a large spherical testate amoeba which makes mud trails. Its diameter is up to 3.8 cm (1.5 in).[150]
Spiculosiphon oceana, a unicellular foraminiferan with an appearance and lifestyle that mimics a sponge, grows to 5 cm long.
Spiculosiphon oceana, a unicellular foraminiferan with an appearance and lifestyle that mimics a sponge, grows to 5 cm long.
The xenophyophore, another single-celled foraminiferan, lives in
abyssal zones. It has a giant shell up to 20 cm (7.9 in) across.[151]
The xenophyophore, another single-celled foraminiferan, lives in
abyssal zones. It has a giant shell up to 20 cm (7.9 in) across.[151]
Giant kelp, a brown algae, is not a true plant, yet it is multicellular and can grow to 50m.
Giant kelp, a brown algae, is not a true plant, yet it is multicellular and can grow to 50m.
Protists
have been described as a taxonomic grab bag where anything that doesn't
fit into one of the main biological kingdoms can be placed.[152] Some
modern authors prefer to exclude multicellular organisms from the
traditional definition of a protist, restricting protists to unicellular
organisms.[153][154] This more constrained definition excludes seaweeds
and slime molds.[155]
Marine microanimals
See also: Microanimal and Ichthyoplankton
External video
video icon Copepods: The Diatom-Devouring King of Plankton - Journey to the Microcosmos
As
juveniles, animals develop from microscopic stages, which can include
spores, eggs and larvae. At least one microscopic animal group, the
parasitic cnidarian Myxozoa, is unicellular in its adult form, and
includes marine species. Other adult marine microanimals are
multicellular. Microscopic adult arthropods are more commonly found
inland in freshwater, but there are marine species as well. Microscopic
adult marine crustaceans include some copepods, cladocera and
tardigrades (water bears). Some marine nematodes and rotifers are also
too small to be recognised with the naked eye, as are many loricifera,
including the recently discovered anaerobic species that spend their
lives in an anoxic environment.[156][157] Copepods contribute more to
the secondary productivity and carbon sink of the world oceans than any
other group of organisms.[158][159] While mites are not normally thought
of as marine organisms, most species of the family Halacaridae live in
the sea.[160]
Marine microanimals
Over 10,000 marine species are copepods, small, often microscopic crustaceans
Over 10,000 marine species are copepods, small, often microscopic crustaceans
Darkfield photo of a gastrotrich, a worm-like animal living between sediment particles
Darkfield photo of a gastrotrich, a worm-like animal living between sediment particles
Armoured Pliciloricus enigmaticus, about 0.2 mm long, live in spaces between marine gravel.
Armoured Pliciloricus enigmaticus, about 0.2 mm long, live in spaces between marine gravel.
Drawing of a tardigrade (water bear) on a grain of sand
Drawing of a tardigrade (water bear) on a grain of sand
Rotifers, usually 0.1–0.5 mm long, may look like protists but have many cells and belongs to the Animalia.
Rotifers, usually 0.1–0.5 mm long, may look like protists but have many cells and belongs to the Animalia.
Fungi
Lichen on a rock in a marine splash zone. Lichens are mutualistic associations between a fungus and an alga or cyanobacterium.
A sea snail, Littoraria irrorata, covered in lichen. This snail farms intertidal ascomycetous fungi.
See also: Marine fungi, Mycoplankton, and Evolution of fungi
Over
1500 species of fungi are known from marine environments.[161] These
are parasitic on marine algae or animals, or are saprobes feeding on
dead organic matter from algae, corals, protozoan cysts, sea grasses,
wood and other substrata.[162] Spores of many species have special
appendages which facilitate attachment to the substratum.[163] Marine
fungi can also be found in sea foam and around hydrothermal areas of the
ocean.[164] A diverse range of unusual secondary metabolites is
produced by marine fungi.[165]
Mycoplankton are saprotropic
members of the plankton communities of marine and freshwater
ecosystems.[166][167] They are composed of filamentous free-living fungi
and yeasts associated with planktonic particles or phytoplankton.[168]
Similar to bacterioplankton, these aquatic fungi play a significant role
in heterotrophic mineralization and nutrient cycling.[169] Mycoplankton
can be up to 20 mm in diameter and over 50 mm in length.[170]
A
typical milliliter of seawater contains about 103 to 104 fungal
cells.[171] This number is greater in coastal ecosystems and estuaries
due to nutritional runoff from terrestrial communities. A higher
diversity of mycoplankton is found around coasts and in surface waters
down to 1000 metres, with a vertical profile that depends on how
abundant phytoplankton is.[172][173] This profile changes between
seasons due to changes in nutrient availability.[174] Marine fungi
survive in a constant oxygen deficient environment, and therefore depend
on oxygen diffusion by turbulence and oxygen generated by
photosynthetic organisms.[175]
Marine fungi can be classified as:[175]
Lower fungi - adapted to marine habitats (zoosporic fungi, including mastigomycetes: oomycetes and chytridiomycetes)
Higher fungi - filamentous, modified to planktonic lifestyle
(hyphomycetes, ascomycetes, basidiomycetes). Most mycoplankton species
are higher fungi.[172]
Lichens are mutualistic associations
between a fungus, usually an ascomycete, and an alga or a
cyanobacterium. Several lichens are found in marine environments.[176]
Many more occur in the splash zone, where they occupy different vertical
zones depending on how tolerant they are to submersion.[177] Some
lichens live a long time; one species has been dated at 8,600
years.[178] However their lifespan is difficult to measure because what
defines the same lichen is not precise.[179] Lichens grow by
vegetatively breaking off a piece, which may or may not be defined as
the same lichen, and two lichens of different ages can merge, raising
the issue of whether it is the same lichen.[179] The sea snail
Littoraria irrorata damages plants of Spartina in the sea marshes where
it lives, which enables spores of intertidal ascomycetous fungi to
colonise the plant. The snail then eats the fungal growth in preference
to the grass itself.[180]
According to fossil records, fungi date
back to the late Proterozoic era 900-570 million years ago. Fossil
marine lichens 600 million years old have been discovered in China.[181]
It has been hypothesized that mycoplankton evolved from terrestrial
fungi, likely in the Paleozoic era (390 million years ago).[182]
Origin of animals
Dickinsonia may be the earliest animal. They appear in the fossil record 571 million to 541 million years ago.
Further
information: Marine invertebrates, Origin of eukaryotes, Evolutionary
origin of animals, Avalon explosion, and Cambrian explosion
The
earliest animals were marine invertebrates, that is, vertebrates came
later. Animals are multicellular eukaryotes,[note 2] and are
distinguished from plants, algae, and fungi by lacking cell walls.[183]
Marine invertebrates are animals that inhabit a marine environment apart
from the vertebrate members of the chordate phylum; invertebrates lack a
vertebral column. Some have evolved a shell or a hard exoskeleton.
The
earliest animal fossils may belong to the genus Dickinsonia,[184] 571
million to 541 million years ago.[185] Individual Dickinsonia typically
resemble a bilaterally symmetrical ribbed oval. They kept growing until
they were covered with sediment or otherwise killed,[186] and spent most
of their lives with their bodies firmly anchored to the sediment.[187]
Their taxonomic affinities are presently unknown, but their mode of
growth is consistent with a bilaterian affinity.[188]
Apart from
Dickinsonia, the earliest widely accepted animal fossils are the rather
modern-looking cnidarians (the group that includes coral, jellyfish, sea
anemones and Hydra), possibly from around 580 Ma[189] The Ediacara
biota, which flourished for the last 40 million years before the start
of the Cambrian,[190] were the first animals more than a very few
centimetres long. Like Dickinsonia, many were flat with a "quilted"
appearance, and seemed so strange that there was a proposal to classify
them as a separate kingdom, Vendozoa.[191] Others, however, have been
interpreted as early molluscs (Kimberella[192][193]), echinoderms
(Arkarua[194]), and arthropods (Spriggina,[195] Parvancorina[196]).
There is still debate about the classification of these specimens,
mainly because the diagnostic features which allow taxonomists to
classify more recent organisms, such as similarities to living
organisms, are generally absent in the Ediacarans. However, there seems
little doubt that Kimberella was at least a triploblastic bilaterian
animal, in other words, an animal significantly more complex than the
cnidarians.[197]
Small shelly fauna are a very mixed collection
of fossils found between the Late Ediacaran and Middle Cambrian periods.
The earliest, Cloudina, shows signs of successful defense against
predation and may indicate the start of an evolutionary arms race. Some
tiny Early Cambrian shells almost certainly belonged to molluscs, while
the owners of some "armor plates," Halkieria and Microdictyon, were
eventually identified when more complete specimens were found in
Cambrian lagerstätten that preserved soft-bodied animals.[198]
Body plans and phyla
Kimberella,
an early mollusc important for understanding the Cambrian explosion.
Invertebrates are grouped into different phyla (body plans).
Invertebrates
are grouped into different phyla. Informally phyla can be thought of as
a way of grouping organisms according to their body
plan.[199][200]: 33 A body plan refers to a blueprint which describes
the shape or morphology of an organism, such as its symmetry,
segmentation and the disposition of its appendages. The idea of body
plans originated with vertebrates, which were grouped into one phylum.
But the vertebrate body plan is only one of many, and invertebrates
consist of many phyla or body plans. The history of the discovery of
body plans can be seen as a movement from a worldview centred on
vertebrates, to seeing the vertebrates as one body plan among many.
Among the pioneering zoologists, Linnaeus identified two body plans
outside the vertebrates; Cuvier identified three; and Haeckel had four,
as well as the Protista with eight more, for a total of twelve. For
comparison, the number of phyla recognised by modern zoologists has
risen to 35.[200]
Taxonomic biodiversity of accepted marine species, according to WoRMS, 18 October 2019.[201][202]
Opabinia, an extinct stem group arthropod appeared in the Middle Cambrian.[203]: 124–136
Historically
body plans were thought of as having evolved rapidly during the
Cambrian explosion,[204] but a more nuanced understanding of animal
evolution suggests a gradual development of body plans throughout the
early Palaeozoic and beyond.[205] More generally a phylum can be defined
in two ways: as described above, as a group of organisms with a certain
degree of morphological or developmental similarity (the phenetic
definition), or a group of organisms with a certain degree of
evolutionary relatedness (the phylogenetic definition).[205]
In
the 1970s there was already a debate about whether the emergence of the
modern phyla was "explosive" or gradual but hidden by the shortage of
Precambrian animal fossils.[198] A re-analysis of fossils from the
Burgess Shale lagerstätte increased interest in the issue when it
revealed animals, such as Opabinia, which did not fit into any known
phylum. At the time these were interpreted as evidence that the modern
phyla had evolved very rapidly in the Cambrian explosion and that the
Burgess Shale's "weird wonders" showed that the Early Cambrian was a
uniquely experimental period of animal evolution.[206] Later discoveries
of similar animals and the development of new theoretical approaches
led to the conclusion that many of the "weird wonders" were evolutionary
"aunts" or "cousins" of modern groups[207]—for example that Opabinia
was a member of the lobopods, a group which includes the ancestors of
the arthropods, and that it may have been closely related to the modern
tardigrades.[208] Nevertheless, there is still much debate about whether
the Cambrian explosion was really explosive and, if so, how and why it
happened and why it appears unique in the history of animals.[209]
Earliest animals
Further information: Animal § Phylogeny
The
deepest-branching animals — the earliest animals that appeared during
evolution — are marine non-vertebrate organisms. The earliest animal
phyla are the Porifera, Ctenophora, Placozoa and Cnidaria. No member of
these clades exhibit body plans with bilateral symmetry.
Choanoflagellata unicellular protists thought to be the closest living relatives of animals
950 mya
Animals
Porifera sponges – asymmetric
Ctenophora comb jellies – biradial symmetry
Placozoa simplest animals – asymmetric
Cnidaria have tentacles with stingers – radial symmetry
bilaterians all remaining animals – bilateral symmetry →
760 mya
There
has been much controversy over which invertebrate phyla, sponges or
comb jellies, is the most basal.[210] Currently, sponges are more widely
considered to be the most basal.[211][212]
Marine sponges
Sponges are perhaps the most basal animals. They have no nervous, digestive or circulatory system.
Sponges
are animals of the phylum Porifera (from Modern Latin for bearing
pores[213]). They are multicellular organisms that have bodies full of
pores and channels allowing water to circulate through them, consisting
of jelly-like mesohyl sandwiched between two thin layers of cells. They
have unspecialized cells that can transform into other types and that
often migrate between the main cell layers and the mesohyl in the
process. Sponges do not have nervous, digestive or circulatory systems.
Instead, most rely on maintaining a constant water flow through their
bodies to obtain food and oxygen and to remove wastes.
Sponges
are similar to other animals in that they are multicellular,
heterotrophic, lack cell walls and produce sperm cells. Unlike other
animals, they lack true tissues and organs, and have no body symmetry.
The shapes of their bodies are adapted for maximal efficiency of water
flow through the central cavity, where it deposits nutrients, and leaves
through a hole called the osculum. Many sponges have internal skeletons
of spongin and/or spicules of calcium carbonate or silicon dioxide. All
sponges are sessile aquatic animals. Although there are freshwater
species, the great majority are marine (salt water) species, ranging
from tidal zones to depths exceeding 8,800 m (5.5 mi). Some sponges live
to great ages; there is evidence of the deep-sea glass sponge
Monorhaphis chuni living about 11,000 years.[214][215]
While most
of the approximately 5,000–10,000 known species feed on bacteria and
other food particles in the water, some host photosynthesizing
micro-organisms as endosymbionts and these alliances often produce more
food and oxygen than they consume. A few species of sponge that live in
food-poor environments have become carnivores that prey mainly on small
crustaceans.[216]
Sponge biodiversity. There are four sponge species in this photo.
Sponge biodiversity. There are four sponge species in this photo.
Branching vase sponge
Branching vase sponge
Venus' flower basket at a depth of 2572 meters
Venus' flower basket at a depth of 2572 meters
Barrel sponge
Barrel sponge
The long-living Monorhaphis chuni
The long-living Monorhaphis chuni
Linnaeus
mistakenly identified sponges as plants in the order Algae.[217] For a
long time thereafter sponges were assigned to a separate subkingdom,
Parazoa (meaning beside the animals).[218] They are now classified as a
paraphyletic phylum from which the higher animals have evolved.[219]
Ctenophores
Together with sponges, brilliantly bioluminescent ctenophores (comb jellies) are the most basal animals.
Ctenophores
(from Greek for carrying a comb), commonly known as comb jellies, are a
phylum that live worldwide in marine waters. They are the largest
non-colonial animals to swim with the help of cilia (hairs or
combs).[220] Coastal species need to be tough enough to withstand waves
and swirling sediment, but some oceanic species are so fragile and
transparent that it is very difficult to capture them intact for
study.[221] In the past ctenophores were thought to have only a modest
presence in the ocean, but it is now known they are often significant
and even dominant parts of the planktonic biomass.[222]: 269
The
phylum has about 150 known species with a wide range of body forms.
Sizes range from a few millimeters to 1.5 m (4 ft 11 in). Cydippids are
egg-shaped with their cilia arranged in eight radial comb rows, and
deploy retractable tentacles for capturing prey. The benthic
platyctenids are generally combless and flat. The coastal beroids have
gaping mouths and lack tentacles. Most adult ctenophores prey on
microscopic larvae and rotifers and small crustaceans but beroids prey
on other ctenophores.
Light diffracting along the comb rows of a cydippid, left tentacle deployed, right retracted
Light diffracting along the comb rows of a cydippid, left tentacle deployed, right retracted
Deep-sea ctenophore trailing tentacles studded with tentilla (sub-tentacles)
Deep-sea ctenophore trailing tentacles studded with tentilla (sub-tentacles)
Egg-shaped cydippid ctenophore
Egg-shaped cydippid ctenophore
Group of small benthic creeping comb jellies streaming tentacles and living symbiotically on a starfish.
Group of small benthic creeping comb jellies streaming tentacles and living symbiotically on a starfish.
Lobata sp. with paired thick lobes
Lobata sp. with paired thick lobes
The sea walnut has a transient anus which forms only when it needs to defecate.[223]
The sea walnut has a transient anus which forms only when it needs to defecate.[223]
The beroid ctenophore, mouth gaping, preys on other ctenophores.
Early
writers combined ctenophores with cnidarians. Ctenophores resemble
cnidarians in relying on water flow through the body cavity for both
digestion and respiration, as well as in having a decentralized nerve
net rather than a brain. Also like cnidarians, the bodies of ctenophores
consist of a mass of jelly, with one layer of cells on the outside and
another lining the internal cavity. In ctenophores, however, these
layers are two cells deep, while those in cnidarians are only a single
cell deep. While cnidarians exhibit radial symmetry, ctenophores have
two anal canals which exhibit biradial symmetry (half-turn rotational
symmetry).[224] The position of the ctenophores in the evolutionary
family tree of animals has long been debated, and the majority view at
present, based on molecular phylogenetics, is that cnidarians and
bilaterians are more closely related to each other than either is to
ctenophores.[222]: 222
External video
video icon Iridescent red ctenophore — EVNautilus
Placozoa
Placozoa
(from Greek for flat animals) have the simplest structure of all
animals. They are a basal form of free-living (non-parasitic)
multicellular organism[225] that do not yet have a common name.[226]
They live in marine environments and form a phylum containing sofar only
three described species, of which the first, the classical Trichoplax
adhaerens, was discovered in 1883.[227] Two more species have been
discovered since 2017,[228][229] and genetic methods indicate this
phylum has a further 100 to 200 undescribed species.[230]
Placozoa have the simplest structure of all animals.
Crawling motility and food uptake by T. adhaerens
Trichoplax
is a small, flattened, animal about one mm across and usually about 25
µm thick. Like the amoebae they superficially resemble, they continually
change their external shape. In addition, spherical phases occasionally
form which may facilitate movement. Trichoplax lacks tissues and
organs. There is no manifest body symmetry, so it is not possible to
distinguish anterior from posterior or left from right. It is made up of
a few thousand cells of six types in three distinct layers.[231] The
outer layer of simple epithelial cells bear cilia which the animal uses
to help it creep along the seafloor.[232] Trichoplax feed by engulfing
and absorbing food particles – mainly microbes and organic detritus –
with their underside.
Marine cnidarians
Cnidarians, like this
starlet sea anemone, are the simplest animals to organise cells into
tissue. Yet they have the same genes that form the vertebrate (including
human) head.
Cnidarians (from Greek for nettle) are
distinguished by the presence of stinging cells, specialized cells that
they use mainly for capturing prey. Cnidarians include corals, sea
anemones, jellyfish and hydrozoans. They form a phylum containing over
10,000[233] species of animals found exclusively in aquatic (mainly
marine) environments. Their bodies consist of mesoglea, a non-living
jelly-like substance, sandwiched between two layers of epithelium that
are mostly one cell thick. They have two basic body forms: swimming
medusae and sessile polyps, both of which are radially symmetrical with
mouths surrounded by tentacles that bear cnidocytes. Both forms have a
single orifice and body cavity that are used for digestion and
respiration.
Fossil cnidarians have been found in rocks formed
about 580 million years ago. Fossils of cnidarians that do not build
mineralized structures are rare. Scientists currently think cnidarians,
ctenophores and bilaterians are more closely related to calcareous
sponges than these are to other sponges, and that anthozoans are the
evolutionary "aunts" or "sisters" of other cnidarians, and the most
closely related to bilaterians.
Cnidarians are the simplest
animals in which the cells are organised into tissues.[234] The starlet
sea anemone is used as a model organism in research.[235] It is easy to
care for in the laboratory and a protocol has been developed which can
yield large numbers of embryos on a daily basis.[236] There is a
remarkable degree of similarity in the gene sequence conservation and
complexity between the sea anemone and vertebrates.[236] In particular,
genes concerned in the formation of the head in vertebrates are also
present in the anemone.[237][238]
Sea anemones are common in tidepools.
Sea anemones are common in tidepools.
Their tentacles sting and paralyse small fish.
Their tentacles sting and paralyse small fish.
Close up of polyps on the surface of a coral, waving their tentacles.
Close up of polyps on the surface of a coral, waving their tentacles.
If an island sinks below the sea, coral growth can keep up with rising water and form an atoll.
If an island sinks below the sea, coral growth can keep up with rising water and form an atoll.
The mantle of the red paper lantern jellyfish crumples and expands like a paper lantern.[239]
The mantle of the red paper lantern jellyfish crumples and expands like a paper lantern.[239]
The Portuguese man o' war is a colonial siphonophore
The Portuguese man o' war is a colonial siphonophore
Marrus orthocanna another colonial siphonophore, assembled from two types of zooids.
Marrus orthocanna another colonial siphonophore, assembled from two types of zooids.
Porpita porpita consists of a colony of hydroids[240]
Porpita porpita consists of a colony of hydroids[240]
Lion's mane jellyfish, largest known jellyfish[241]
Lion's mane jellyfish, largest known jellyfish[241]
Turritopsis dohrnii achieves biological immortality by transferring its cells back to childhood.[242][243]
Turritopsis dohrnii achieves biological immortality by transferring its cells back to childhood.[242][243]
The sea wasp is the most lethal jellyfish in the world.[244]
The sea wasp is the most lethal jellyfish in the world.[244]
Bilateral invertebrate animals
Idealised
wormlike bilaterian body plan. With a cylindrical body and a direction
of movement the animal has head and tail ends. Sense organs and mouth
form the basis of the head. Opposed circular and longitudinal muscles
enable peristaltic motion.
Some of the earliest bilaterians were
wormlike, and the original bilaterian may have been a bottom dwelling
worm with a single body opening.[245] A bilaterian body can be
conceptualized as a cylinder with a gut running between two openings,
the mouth and the anus. Around the gut it has an internal body cavity, a
coelom or pseudocoelom.[a] Animals with this bilaterally symmetric body
plan have a head (anterior) end and a tail (posterior) end as well as a
back (dorsal) and a belly (ventral); therefore they also have a left
side and a right side.[246][247]
Having a front end means that
this part of the body encounters stimuli, such as food, favouring
cephalisation, the development of a head with sense organs and a
mouth.[248] The body stretches back from the head, and many bilaterians
have a combination of circular muscles that constrict the body, making
it longer, and an opposing set of longitudinal muscles, that shorten the
body;[247] these enable soft-bodied animals with a hydrostatic skeleton
to move by peristalsis.[249] They also have a gut that extends through
the basically cylindrical body from mouth to anus. Many bilaterian phyla
have primary larvae which swim with cilia and have an apical organ
containing sensory cells. However, there are exceptions to each of these
characteristics; for example, adult echinoderms are radially symmetric
(unlike their larvae), and certain parasitic worms have extremely
simplified body structures.[246][247]
Ikaria wariootia, an early bilaterian[250]
← bilaterians
Xenacoelomorpha
basal bilaterians (lack a true gut)[245]
Nephrozoa
protostomes
develops mouth first →
610 mya
deuterostomes
develops anus first →
650 mya
Protostomes
See also: Embryological origins of the mouth and anus
Protostomes
(from Greek for first mouth) are a superphylum of animals. It is a
sister clade of the deuterostomes (from Greek for second mouth), with
which it forms the Nephrozoa clade. Protostomes are distinguished from
deuterostomes by the way their embryos develop. In protostomes the first
opening that develops becomes the mouth, while in deuterostomes it
becomes the anus.[251][252]
← Protostomes
Ecdysozoa
Scalidophora penis worms and mud dragons
arthropods mainly crustaceans
nematodes roundworms
>529 mya
Spiralia
Gnathifera
rotifers
arrow worms
Platytrochozoa
flatworms
Lophotrochozoa
molluscs gastropods, bivalves and cephalopods
ringed worms
550 mya
580 mya
(extant)
Marine worms
Further information: Marine worm and Sea worm
Many
marine worms are related only distantly, so they form a number of
different phyla. The worm shown is an arrow worm, found worldwide as a
predatory component of plankton.
Worms (Old English for serpents)
form a number of phyla. Different groups of marine worms are related
only distantly, so they are found in several different phyla such as the
Annelida (segmented worms), Chaetognatha (arrow worms), Phoronida
(horseshoe worms), and Hemichordata. All worms, apart from the
Hemichordata, are protostomes. The Hemichordata are deuterostomes and
are discussed in their own section below.
The typical body plan
of a worm involves long cylindrical tube-like bodies and no limbs.
Marine worms vary in size from microscopic to over 1 metre (3.3 ft) in
length for some marine polychaete worms (bristle worms)[253] and up to
58 metres (190 ft) for the marine nemertean worm (bootlace worm).[254]
Some marine worms occupy a small variety of parasitic niches, living
inside the bodies of other animals, while others live more freely in the
marine environment or by burrowing underground. Many of these worms
have specialized tentacles used for exchanging oxygen and carbon dioxide
and also may be used for reproduction. Some marine worms are tube
worms, such as the giant tube worm which lives in waters near underwater
volcanoes and can withstand temperatures up to 90 degrees Celsius.
Platyhelminthes (flatworms) form another worm phylum which includes a
class of parasitic tapeworms. The marine tapeworm Polygonoporus
giganticus, found in the gut of sperm whales, can grow to over 30 m (100
ft).[255][256]
Nematodes (roundworms) constitute a further worm
phylum with tubular digestive systems and an opening at both
ends.[257][258] Over 25,000 nematode species have been
described,[259][260] of which more than half are parasitic. It has been
estimated another million remain undescribed.[261] They are ubiquitous
in marine, freshwater and terrestrial environments, where they often
outnumber other animals in both individual and species counts. They are
found in every part of the earth's lithosphere, from the top of
mountains to the bottom of oceanic trenches.[262] By count they
represent 90% of all animals on the ocean floor.[263] Their numerical
dominance, often exceeding a million individuals per square meter and
accounting for about 80% of all individual animals on earth, their
diversity of life cycles, and their presence at various trophic levels
point at an important role in many ecosystems.[264]
Giant tube worms cluster around hydrothermal vents.
Giant tube worms cluster around hydrothermal vents.
Nematodes are ubiquitous pseudocoelomates which can parasite marine plants and animals.
Nematodes are ubiquitous pseudocoelomates which can parasite marine plants and animals.
Bloodworms are typically found on the bottom of shallow marine waters.
Bloodworms are typically found on the bottom of shallow marine waters.
Marine molluscs
Bigfin reef squid displaying vivid iridescence at night. Cephalopods are the most neurologically advanced invertebrates.[265]
Blue dragon, a pelagic sea slug
Bolinus brandaris, a sea snail from which the Phoenicians extracted royal Tyrian purple dye colour code: #66023C _____[266]
Hypothetical ancestral mollusc
See also: Evolution of molluscs and Evolution of cephalopods
Molluscs
(Latin for soft) form a phylum with about 85,000 extant recognized
species.[267] They are the largest marine phylum in terms of species
count, containing about 23% of all the named marine organisms.[268]
Molluscs have more varied forms than other invertebrate phyla. They are
highly diverse, not just in size and in anatomical structure, but also
in behaviour and in habitat.
Drawing of a giant clam (NOAA)
The
mollusc phylum is divided into 9 or 10 taxonomic classes. These classes
include gastropods, bivalves and cephalopods, as well as other
lesser-known but distinctive classes. Gastropods with protective shells
are referred to as snails, whereas gastropods without protective shells
are referred to as slugs. Gastropods are by far the most numerous
molluscs in terms of species.[269] Bivalves include clams, oysters,
cockles, mussels, scallops, and numerous other families. There are about
8,000 marine bivalves species (including brackish water and estuarine
species). A deep sea ocean quahog clam has been reported as having lived
507 years[270] making it the longest recorded life of all animals apart
from colonial animals, or near-colonial animals like sponges.[214]
Gastropods and bivalves
Marine gastropods are sea snails or sea slugs. This nudibranch is a sea slug.
Marine gastropods are sea snails or sea slugs. This nudibranch is a sea slug.
The sea snail Syrinx aruanus has a shell up to 91 cm long, the largest of any living gastropod.
The sea snail Syrinx aruanus has a shell up to 91 cm long, the largest of any living gastropod.
Molluscs usually have eyes. Bordering the edge of the mantle of a scallop, a bivalve mollusc, can be over 100 simple eyes.
Molluscs usually have eyes. Bordering the edge of the mantle of a scallop, a bivalve mollusc, can be over 100 simple eyes.
Common mussel, another bivalve
Common mussel, another bivalve
Cephalopods
include octopus, squid and cuttlefish. About 800 living species of
marine cephalopods have been identified,[271] and an estimated 11,000
extinct taxa have been described.[272] They are found in all oceans, but
there are no fully freshwater cephalopods.[273]
Cephalopods
The nautilus is a living fossil little changed since it evolved 500
million years ago as one of the first cephalopods.[274][275][276]
The nautilus is a living fossil little changed since it evolved 500
million years ago as one of the first cephalopods.[274][275][276]
Reconstruction of an ammonite, a highly successful early cephalopod that appeared 400 mya.
Reconstruction of an ammonite, a highly successful early cephalopod that appeared 400 mya.
Cephalopods, like this cuttlefish, use their mantle cavity for jet propulsion.
Cephalopods, like this cuttlefish, use their mantle cavity for jet propulsion.
Colossal squid, the largest of all invertebrates[277]
Colossal squid, the largest of all invertebrates[277]
Molluscs
have such diverse shapes that many textbooks base their descriptions of
molluscan anatomy on a generalized or hypothetical ancestral mollusc.
This generalized mollusc is unsegmented and bilaterally symmetrical with
an underside consisting of a single muscular foot. Beyond that it has
three further key features. Firstly, it has a muscular cloak called a
mantle covering its viscera and containing a significant cavity used for
breathing and excretion. A shell secreted by the mantle covers the
upper surface. Secondly (apart from bivalves) it has a rasping tongue
called a radula used for feeding. Thirdly, it has a nervous system
including a complex digestive system using microscopic, muscle-powered
hairs called cilia to exude mucus. The generalized mollusc has two
paired nerve cords (three in bivalves). The brain, in species that have
one, encircles the esophagus. Most molluscs have eyes and all have
sensors detecting chemicals, vibrations, and touch.[278][279]
Good
evidence exists for the appearance of marine gastropods, cephalopods
and bivalves in the Cambrian period 538.8 to 485.4 million years ago.
Marine arthropods
Head
___________
Thorax
___________
Abdomen
___________
Segments
and tagmata of an arthropod[278]: 518–52 The thorax bears the main
locomotory appendages. The head and thorax are fused in some arthropods,
such as crabs and lobsters.
First known air-breathing animal to colonise land, the millipede Pneumodesmus newmani,[280] lived in the Early Devonian.[281]
Arthropods
(Greek for jointed feet) have an exoskeleton (external skeleton), a
segmented body, and jointed appendages (paired appendages). They form a
phylum which includes insects, arachnids, myriapods, and crustaceans.
Arthropods are characterized by their jointed limbs and cuticle made of
chitin, often mineralised with calcium carbonate. The arthropod body
plan consists of segments, each with a pair of appendages. The rigid
cuticle inhibits growth, so arthropods replace it periodically by
moulting. Their versatility has enabled them to become the most
species-rich members of all ecological guilds in most environments.
The
evolutionary ancestry of arthropods dates back to the Cambrian period
and is generally regarded as monophyletic. However, basal relationships
of arthropods with extinct phyla such as lobopodians have recently been
debated.[282][283]
Panarthropoda
tardigrades water bears
Lobopodia
velvet worms (terrestrial)
arthropods mainly crustaceans
Some palaeontologists think Lobopodia represents a basal grade which lead to an arthropod body plan.[284]
Tardigrades (water bears) are a phylum of eight-legged, segmented microanimals able to survive in extreme conditions.
Arthropod fossils and living fossils
Fossil trilobite. Trilobites first appeared about 521 Ma. They were
highly successful and were found everywhere in the ocean for 270
Ma.[285]
Fossil trilobite. Trilobites first appeared about
521 Ma. They were highly successful and were found everywhere in the
ocean for 270 Ma.[285]
The Anomalocaris ("abnormal shrimp") was one of the first apex predators and first appeared about 515 Ma.
The Anomalocaris ("abnormal shrimp") was one of the first apex predators and first appeared about 515 Ma.
The largest known arthropod, the sea scorpion Jaekelopterus rhenaniae,
has been found in estuarine strata from about 390 Ma. It was up to 2.5 m
(8.2 ft) long.[286][287]
The largest known arthropod, the
sea scorpion Jaekelopterus rhenaniae, has been found in estuarine strata
from about 390 Ma. It was up to 2.5 m (8.2 ft) long.[286][287]
Xiphosurans, the group including modern Horseshoe crabs appeared around 480 Ma.[288]
Xiphosurans, the group including modern Horseshoe crabs appeared around 480 Ma.[288]
Extant
marine arthropods range in size from the microscopic crustacean
Stygotantulus to the Japanese spider crab. Arthropods' primary internal
cavity is a hemocoel, which accommodates their internal organs, and
through which their haemolymph - analogue of blood - circulates; they
have open circulatory systems. Like their exteriors, the internal organs
of arthropods are generally built of repeated segments. Their nervous
system is "ladder-like", with paired ventral nerve cords running through
all segments and forming paired ganglia in each segment. Their heads
are formed by fusion of varying numbers of segments, and their brains
are formed by fusion of the ganglia of these segments and encircle the
esophagus. The respiratory and excretory systems of arthropods vary,
depending as much on their environment as on the subphylum to which they
belong.
Modern crustaceans
Many crustaceans are very small, like this tiny amphipod, and make up a significant part of the ocean's zooplankton.
Many crustaceans are very small, like this tiny amphipod, and make up a significant part of the ocean's zooplankton.
The Japanese spider crab has the longest leg span of any arthropod, reaching 5.5 metres (18 ft) from claw to claw.[289]
The Japanese spider crab has the longest leg span of any arthropod, reaching 5.5 metres (18 ft) from claw to claw.[289]
The Tasmanian giant crab is long-lived and slow-growing, making it vulnerable to overfishing.[290]
The Tasmanian giant crab is long-lived and slow-growing, making it vulnerable to overfishing.[290]
Mantis shrimp have the most advanced eyes in the animal kingdom,[291]
and smash prey by swinging their club-like raptorial claws.[292]
Mantis shrimp have the most advanced eyes in the animal kingdom,[291]
and smash prey by swinging their club-like raptorial claws.[292]
Arthropod
vision relies on various combinations of compound eyes and pigment-pit
ocelli: in most species the ocelli can only detect the direction from
which light is coming, and the compound eyes are the main source of
information. Arthropods also have a wide range of chemical and
mechanical sensors, mostly based on modifications of the many setae
(bristles) that project through their cuticles. Arthropod methods of
reproduction are diverse: terrestrial species use some form of internal
fertilization while marine species lay eggs using either internal or
external fertilization. Arthropod hatchlings vary from miniature adults
to grubs that lack jointed limbs and eventually undergo a total
metamorphosis to produce the adult form.
Deuterostomes
See also: Evolution of brachiopods
In
deuterostomes the first opening that develops in the growing embryo
becomes the anus, while in protostomes it becomes the mouth.
Deuterostomes form a superphylum of animals and are the sister clade of
the protostomes.[251][252] It is once considered that the earliest known
deuterostomes are Saccorhytus fossils from about 540 million years
ago.[293] However, another study considered that Saccorhytus is more
likely to be an ecdysozoan.[294]
← deuterostomes
ambulacrarians
echinoderms
hemichordates
chordates
cephalochordates
tunicates
vertebrates →
(extant)
Echinoderms
Adult echinoderms have fivefold symmetry but as larvae have bilateral symmetry. This is why they are in the Bilateria.
Echinoderms
(Greek for spiny skin) is a phylum which contains only marine
invertebrates. The phylum contains about 7000 living species,[295]
making it the second-largest grouping of deuterostomes, after the
chordates.
Adult echinoderms are recognizable by their radial
symmetry (usually five-point) and include starfish, sea urchins, sand
dollars, and sea cucumbers, as well as the sea lilies.[296] Echinoderms
are found at every ocean depth, from the intertidal zone to the abyssal
zone. They are unique among animals in having bilateral symmetry at the
larval stage, but fivefold symmetry (pentamerism, a special type of
radial symmetry) as adults.[297]
Echinoderms are important both
biologically and geologically. Biologically, there are few other
groupings so abundant in the biotic desert of the deep sea, as well as
shallower oceans. Most echinoderms are able to regenerate tissue,
organs, limbs, and reproduce asexually; in some cases, they can undergo
complete regeneration from a single limb. Geologically, the value of
echinoderms is in their ossified skeletons, which are major contributors
to many limestone formations, and can provide valuable clues as to the
geological environment. They were the most used species in regenerative
research in the 19th and 20th centuries.
Echinoderm literally means "spiny skin", as this water melon sea urchin illustrates.
Echinoderm literally means "spiny skin", as this water melon sea urchin illustrates.
The ochre sea star was the first keystone predator to be studied. They
limit mussels which can overwhelm intertidal communities.[298]
The ochre sea star was the first keystone predator to be studied. They
limit mussels which can overwhelm intertidal communities.[298]
Colorful sea lilies in shallow waters
Colorful sea lilies in shallow waters
Sea cucumbers filter feed on plankton and suspended solids.
Sea cucumbers filter feed on plankton and suspended solids.
The sea pig, a deep water sea cucumber, is the only echinoderm that uses legged locomotion.
The sea pig, a deep water sea cucumber, is the only echinoderm that uses legged locomotion.
A benthopelagic and bioluminescent swimming sea cucumber, 3200 metres deep
A benthopelagic and bioluminescent swimming sea cucumber, 3200 metres deep
It
is held by some scientists that the radiation of echinoderms was
responsible for the Mesozoic Marine Revolution. Aside from the
hard-to-classify Arkarua (a Precambrian animal with echinoderm-like
pentamerous radial symmetry), the first definitive members of the phylum
appeared near the start of the Cambrian.
Hemichordates
Gill (pharyngeal) slits
The acorn worm is associated with the development of gill slits.
Gill slits in an acorn worm (left) and tunicate (right)
Gill
slits have been described as "the foremost morphological innovation of
early deuterostomes".[299][300] In aquatic organisms, gill slits allow
water that enters the mouth during feeding to exit. Some invertebrate
chordates also use the slits to filter food from the water.[301]
Hemichordates
form a sister phylum to the echinoderms. They are solitary worm-shaped
organisms rarely seen by humans because of their lifestyle. They include
two main groups, the acorn worms and the Pterobranchia. Pterobranchia
form a class containing about 30 species of small worm-shaped animals
that live in secreted tubes on the ocean floor. Acorn worms form a class
containing about 111 species that generally live in U-shaped burrows on
the seabed, from the shoreline to a depth of 3000 metres. The worms lie
there with the proboscis sticking out of one opening in the burrow,
subsisting as deposit feeders or suspension feeders. It is supposed the
ancestors of acorn worms used to live in tubes like their relatives, the
Pterobranchia, but eventually started to live a safer and more
sheltered existence in sediment burrows.[302] Some of these worms may
grow to be very long; one particular species may reach a length of 2.5
metres (8 ft 2 in), although most acorn worms are much smaller.
Acorn
worms are more highly specialised and advanced than other worm-like
organisms. They have a circulatory system with a heart that also
functions as a kidney. Acorn worms have gill-like structures they use
for breathing, similar to the gills of fish. Therefore, acorn worms are
sometimes said to be a link between classical invertebrates and
vertebrates. Acorn worms continually form new gill slits as they grow in
size, and some older individuals have more than a hundred on each side.
Each slit consists of a branchial chamber opening to the pharynx
through a U-shaped cleft. Cilia push water through the slits,
maintaining a constant flow, just as in fish.[303] Some acorn worms also
have a postanal tail which may be homologous to the post-anal tail of
vertebrates.
The three-section body plan of the acorn worm is no
longer present in the vertebrates, except in the anatomy of the frontal
neural tube, later developed into a brain divided into three parts. This
means some of the original anatomy of the early chordate ancestors is
still present in vertebrates even if it is not always visible. One
theory is the three-part body originated from an early common ancestor
of the deuterostomes, and maybe even from a common bilateral ancestor of
both deuterostomes and protostomes. Studies have shown the gene
expression in the embryo share three of the same signaling centers that
shape the brains of all vertebrates, but instead of taking part in the
formation of their neural system,[304] they are controlling the
development of the different body regions.[305]
Marine chordates
The
lancelet, like all cephalochordates, has a head. Adult lancelets retain
the four key features of chordates: a notochord, a dorsal hollow nerve
cord, pharyngeal slits, and a post-anal tail. Water from the mouth
enters the pharyngeal slits, which filter out food particles. The
filtered water then collects in the atrium and exits through the
atriopore.[306]
The chordate phylum has three subphyla, one of
which is the vertebrates (see below). The other two subphyla are marine
invertebrates: the tunicates (salps and sea squirts) and the
cephalochordates (such as lancelets). Invertebrate chordates are close
relatives to vertebrates. In particular, there has been discussion about
how closely some extinct marine species, such as Pikaiidae,
Palaeospondylus, Zhongxiniscus and Vetulicolia, might relate ancestrally
to vertebrates.
Invertebrate chordates are close relatives of vertebrates
The lancelet, a small translucent fish-like cephalochordate, is one of
the closest living invertebrate relative of the vertebrates.[307][308]
The lancelet, a small translucent fish-like cephalochordate, is one of
the closest living invertebrate relative of the vertebrates.[307][308]
Tunicates, like these fluorescent-colored sea squirts, may provide clues to vertebrate and therefore human ancestry.[309]
Tunicates, like these fluorescent-colored sea squirts, may provide clues to vertebrate and therefore human ancestry.[309]
Pyrosomes are free-floating bioluminescent tunicates made up of hundreds of individuals.
Pyrosomes are free-floating bioluminescent tunicates made up of hundreds of individuals.
Salp chain
Salp chain
In chordates, the four above labelled common features appear at some point during development.[301]
The
larval stage of the tunicate possesses all of the features
characteristic of chordates: a notochord, a dorsal hollow nerve cord,
pharyngeal slits, and a post-anal tail.[301]
In the adult stage of the tunicate the notochord, nerve cord, and tail disappear.[301]
Vertebrate animals
Ray-finned fish
Marine tetrapod (sperm whale)
Skeletal structures showing the vertebral column and internal skeleton running from the head to the tail.
Main article: Marine vertebrate
Vertebrates
(Latin for joints of the spine) are a subphylum of chordates. They are
chordates that have a vertebral column (backbone). The vertebral column
provides the central support structure for an internal skeleton which
gives shape, support, and protection to the body and can provide a means
of anchoring fins or limbs to the body. The vertebral column also
serves to house and protect the spinal cord that lies within the
vertebral column.
Marine vertebrates can be divided into marine fish and marine tetrapods.
Marine fish
Further information: Fish, diversity of fish, and evolution of fish
Fish
typically breathe by extracting oxygen from water through gills and
have a skin protected by scales and mucous. They use fins to propel and
stabilise themselves in the water, and usually have a two-chambered
heart and eyes well adapted to seeing underwater, as well as other
sensory systems. Over 33,000 species of fish have been described as of
2017,[310] of which about 20,000 are marine fish.[311]
← vertebrates
jawless fish
hagfish
lampreys
jawed fish
cartilaginous fish
bony fish →
(extant)
Jawless fish
The
Tully monster, a strange looking extinct animal with eyes like a
hammerhead protruding from its back, may be an early jawless fish.
Early
fish had no jaws. Most went extinct when they were outcompeted by jawed
fish (below), but two groups survived: hagfish and lampreys. Hagfish
form a class of about 20 species of eel-shaped, slime-producing marine
fish. They are the only known living animals that have a skull but no
vertebral column. Lampreys form a superclass containing 38 known extant
species of jawless fish.[312] The adult lamprey is characterized by a
toothed, funnel-like sucking mouth. Although they are well known for
boring into the flesh of other fish to suck their blood,[313] only 18
species of lampreys are actually parasitic.[314] Together hagfish and
lampreys are the sister group to vertebrates. Living hagfish remain
similar to hagfish from around 300 million years ago.[315] The lampreys
are a very ancient lineage of vertebrates, though their exact
relationship to hagfishes and jawed vertebrates is still a matter of
dispute.[316] Molecular analysis since 1992 has suggested that hagfish
are most closely related to lampreys,[317] and so also are vertebrates
in a monophyletic sense. Others consider them a sister group of
vertebrates in the common taxon of craniata.[318]
The Tully
monster is an extinct genus of soft-bodied bilaterians that lived in
tropical estuaries about 300 million years ago. Since 2016 there has
been controversy over whether this animal was a vertebrate or an
invertebrate.[319][320] In 2020 researchers found "strong evidence" that
the Tully monster was a vertebrate, and was a jawless fish in the
lineage of the lamprey,[321][322] while in 2023 other researchers found
3D fossils scans did not support those conclusions.[323]
Hagfish are the only known living animals with a skull but no vertebral column.
Hagfish are the only known living animals with a skull but no vertebral column.
Lampreys are often parasitic and have a toothed, funnel-like sucking mouth.
Lampreys are often parasitic and have a toothed, funnel-like sucking mouth.
The extinct Pteraspidomorphi, ancestral to jawed vertebrates
The extinct Pteraspidomorphi, ancestral to jawed vertebrates
Pteraspidomorphi
is an extinct class of early jawless fish ancestral to jawed
vertebrates. The few characteristics they share with the latter are now
considered as primitive for all vertebrates.
Around the start of
the Devonian, fish started appearing with a deep remodelling of the
vertebrate skull that resulted in a jaw.[324] All vertebrate jaws,
including the human jaw, have evolved from these early fish jaws. The
appearance of the early vertebrate jaw has been described as "perhaps
the most profound and radical evolutionary step in vertebrate
history".[325][326] Jaws make it possible to capture, hold, and chew
prey. Fish without jaws had more difficulty surviving than fish with
jaws, and most jawless fish became extinct during the Triassic period.
Cartilaginous fish
Main article: Cartilaginous fish
Jawed
fish fall into two main groups: fish with bony internal skeletons and
fish with cartilaginous internal skeletons. Cartilaginous fish, such as
sharks and rays, have jaws and skeletons made of cartilage rather than
bone. Megalodon is an extinct species of shark that lived about 28 to
1.5 Ma. It may looked much like a stocky version of the great white
shark, but was much larger with estimated lengths reaching 20.3 metres
(67 ft).[327] Found in all oceans[328] it was one of the largest and
most powerful predators in vertebrate history,[327] and probably had a
profound impact on marine life.[329] The Greenland shark has the longest
known lifespan of all vertebrates, about 400 years.[330] Some sharks
such as the great white are partially warm blooded and give live birth.
The manta ray, largest ray in the world, has been targeted by fisheries
and is now vulnerable.[331]
Cartilaginous fishes
Cartilaginous fishes may have evolved from spiny sharks.
Cartilaginous fishes may have evolved from spiny sharks.
Stingray
Stingray
Manta ray, the largest ray
Manta ray, the largest ray
Sawfish, rays with long rostrums resembling a saw. All species are now endangered.[332]
Sawfish, rays with long rostrums resembling a saw. All species are now endangered.[332]
The extinct megalodon resembled a giant great white shark.
The extinct megalodon resembled a giant great white shark.
The Greenland shark lives longer than any other vertebrate.
The Greenland shark lives longer than any other vertebrate.
The largest extant fish, the whale shark, is now a vulnerable species.
The largest extant fish, the whale shark, is now a vulnerable species.
Bony fish
Guiyu oneiros, the earliest-known bony fish lived during the Late Silurian 419 million years ago.
Lobe fins are bedded into the body by bony stalks. They evolved into the legs of the first tetrapod land vertebrates.
Ray fins have spines (rays) which can be erected to stiffen the fin for better control of swimming performance.
Further information: Bony fish
Bony
fish have jaws and skeletons made of bone rather than cartilage. Bony
fish also have hard, bony plates called operculum which help them
respire and protect their gills, and they often possess a swim bladder
which they use for better control of their buoyancy. Bony fish can be
further divided into those with lobe fins and those with ray fins. The
approximate dates in the phylogenetic tree are from Near et al.,
2012[333] and Zhu et al., 2009.[334]
← bony fish
lobe fins
coelacanths
lungfish
tetrapods →
419 mya
ray fins
chondrosteans
(sturgeon, paddlefish, bichir, reedfish)
neopterygians
holosteans
(bowfin, gars)
275 mya
teleosts
all remaining fish (about 14,000 marine species)
310 mya
360 mya
400 mya
(extant)
Lobe
fins have the form of fleshy lobes supported by bony stalks which
extend from the body.[335] Guiyu oneiros, the earliest-known bony fish,
lived during the Late Silurian 419 million years ago. It has the
combination of both ray-finned and lobe-finned features, although
analysis of the totality of its features place it closer to lobe-finned
fish.[334] Lobe fins evolved into the legs of the first tetrapod land
vertebrates, so by extension an early ancestor of humans was a
lobe-finned fish. Apart from the coelacanths and the lungfishes,
lobe-finned fishes are now extinct.
The remaining bony fish have
ray fins. These are made of webs of skin supported by bony or horny
spines (rays) which can be erected to control the fin stiffness.
The main distinguishing feature of the chondrosteans (sturgeon,
paddlefish, bichir and reedfish) is the cartilaginous nature of their
skeletons. The ancestors of the chondrosteans are thought to be bony
fish, but the characteristic of an ossified skeleton was lost in later
evolutionary development, resulting in a lightening of the frame.[336]
Neopterygians (from Greek for new fins) appeared sometime in the Late
Permian, before dinosaurs. They were a very successful group of fish,
because they could move more rapidly than their ancestors. Their scales
and skeletons began to lighten during their evolution, and their jaws
became more powerful and efficient.[337]
Teleosts
Teleosts have homocercal tails.
Main article: Teleost
About
96% of all modern fish species are teleosts,[338] of which about 14,000
are marine species.[339] Teleosts can be distinguished from other bony
fish by their possession of a homocercal tail, a tail where the upper
half mirrors the lower half.[340] Another difference lies in their jaw
bones – teleosts have modifications in the jaw musculature which make it
possible for them to protrude their jaws. This enables them to grab
prey and draw it into their mouth.[340] In general, teleosts tend to be
quicker and more flexible than more basal bony fishes. Their skeletal
structure has evolved towards greater lightness. While teleost bones are
well calcified, they are constructed from a scaffolding of struts,
rather than the dense cancellous bones of holostean fish.[341]
Teleosts
are found in almost all marine habitats.[342] They have enormous
diversity, and range in size from adult gobies 8mm long [343] to ocean
sunfish weighing over 2,000 kg.[344] The following images show something
of the diversity in the shape and colour of modern marine teleosts...
Sailfish
Sailfish
Mahi-mahi
Mahi-mahi
Eel
Eel
Seahorse
Seahorse
Ocean sunfish
Ocean sunfish
Anglerfish
Anglerfish
Pufferfish
Pufferfish
Clown triggerfish
Clown triggerfish
Mandarin dragonet
Mandarin dragonet
Nearly half of all extant vertebrate species are teleosts.[345]
Marine tetrapods
See also: Tetrapods and evolution of tetrapods
Tiktaalik, an extinct lobe-finned fish, developed limb-like fins that could take it onto land.
A
tetrapod (Greek for four feet) is a vertebrate with limbs (feet).
Tetrapods evolved from ancient lobe-finned fishes about 400 million
years ago during the Devonian Period when their earliest ancestors
emerged from the sea and adapted to living on land.[346] This change
from a body plan for breathing and navigating in gravity-neutral water
to a body plan with mechanisms enabling the animal to breath in air
without dehydrating and move on land is one of the most profound
evolutionary changes known.[347][348] Tetrapods can be divided into four
classes: amphibians, reptiles, birds and mammals.
← tetrapods
amphibians (there are no true marine amphibians)
amniotes
mammals
sauropsids
lepidosaurs (lizards, including snakes)
archosaurs (turtles, crocodiles & birds)
Marine
tetrapods are tetrapods that returned from land back to the sea again.
The first returns to the ocean may have occurred as early as the
Carboniferous Period[349] whereas other returns occurred as recently as
the Cenozoic, as in cetaceans, pinnipeds,[350] and several modern
amphibians.[351] Amphibians (from Greek for both kinds of life) live
part of their life in water and part on land. They mostly require fresh
water to reproduce. A few inhabit brackish water, but there are no true
marine amphibians.[352] There have been reports, however, of amphibians
invading marine waters, such as a Black Sea invasion by the natural
hybrid Pelophylax esculentus reported in 2010.[353]
Reptiles
Main article: Marine reptile
See also: Evolution of reptiles
Reptiles
(Late Latin for creeping or crawling) do not have an aquatic larval
stage, and in this way are unlike amphibians. Most reptiles are
oviparous, although several species of squamates are viviparous, as were
some extinct aquatic clades[354] — the fetus develops within the
mother, contained in a placenta rather than an eggshell. As amniotes,
reptile eggs are surrounded by membranes for protection and transport,
which adapt them to reproduction on dry land. Many of the viviparous
species feed their fetuses through various forms of placenta analogous
to those of mammals, with some providing initial care for their
hatchlings.
Some reptiles are more closely related to birds than
other reptiles, and many scientists prefer to make Reptilia a
monophyletic group which includes the birds.[355][356][357][358] Extant
non-avian reptiles which inhabit or frequent the sea include sea
turtles, sea snakes, terrapins, the marine iguana, and the saltwater
crocodile. Currently, of the approximately 12,000 extant reptile species
and sub-species, only about 100 of are classed as marine reptiles.[359]
Except
for some sea snakes, most extant marine reptiles are oviparous and need
to return to land to lay their eggs. Apart from sea turtles, the
species usually spend most of their lives on or near land rather than in
the ocean. Sea snakes generally prefer shallow waters nearby land,
around islands, especially waters that are somewhat sheltered, as well
as near estuaries.[360][361] Unlike land snakes, sea snakes have evolved
flattened tails which help them swim.[362]
Marine iguana
Marine iguana
Leatherback sea turtle
Leatherback sea turtle
Saltwater crocodile
Saltwater crocodile
Marine snakes have flattened tails.
Marine snakes have flattened tails.
The ancient Ichthyosaurus communis independently evolved flippers similar to dolphins.
The ancient Ichthyosaurus communis independently evolved flippers similar to dolphins.
Some
extinct marine reptiles, such as ichthyosaurs, evolved to be viviparous
and had no requirement to return to land. Ichthyosaurs resembled
dolphins. They first appeared about 245 million years ago and
disappeared about 90 million years ago. The terrestrial ancestor of the
ichthyosaur had no features already on its back or tail that might have
helped along the evolutionary process. Yet the ichthyosaur developed a
dorsal and tail fin which improved its ability to swim.[363] The
biologist Stephen Jay Gould said the ichthyosaur was his favourite
example of convergent evolution.[364] The earliest marine reptiles arose
in the Permian. During the Mesozoic many groups of reptiles became
adapted to life in the seas, including ichthyosaurs, plesiosaurs,
mosasaurs, nothosaurs, placodonts, sea turtles, thalattosaurs and
thalattosuchians. Marine reptiles were less numerous after mass
extinction at the end of the Cretaceous.
Birds
Main article: Seabird
Waterbird food web in Chesapeake Bay
Marine
birds are adapted to life within the marine environment. They are often
called seabirds. While marine birds vary greatly in lifestyle,
behaviour and physiology, they often exhibit striking convergent
evolution, as the same environmental problems and feeding niches have
resulted in similar adaptations. Examples include albatross, penguins,
gannets, and auks.
In general, marine birds live longer, breed
later and have fewer young than terrestrial birds do, but they invest a
great deal of time in their young. Most species nest in colonies, which
can vary in size from a few dozen birds to millions. Many species are
famous for undertaking long annual migrations, crossing the equator or
circumnavigating the Earth in some cases. They feed both at the ocean's
surface and below it, and even feed on each other. Marine birds can be
highly pelagic, coastal, or in some cases spend a part of the year away
from the sea entirely. Some marine birds plummet from heights, plunging
through the water leaving vapour-like trails, similar to that of fighter
planes.[365] Gannets plunge into the water at up to 100 kilometres per
hour (60 mph). They have air sacs under their skin in their face and
chest which act like bubble-wrap, cushioning the impact with the water.
European herring gull attack herring schools from above.
European herring gull attack herring schools from above.
Gentoo penguin swimming underwater
Gentoo penguin swimming underwater
Albatrosses range over huge areas of ocean and some even circle the globe.
Albatrosses range over huge areas of ocean and some even circle the globe.
Gannets "divebomb" at high speed
Gannets "divebomb" at high speed
The first marine birds evolved in the Cretaceous period, and modern marine bird families emerged in the Paleogene.
Mammals
Sea otter, a classic keystone species which controls sea urchin numbers
Main article: Marine mammal
See also: Evolution of cetaceans, Evolution of sirenians, and List of marine mammal species
Mammals
(from Latin for breast) are characterised by the presence of mammary
glands which in females produce milk for feeding (nursing) their young.
There are about 130 living and recently extinct marine mammal species
such as seals, dolphins, whales, manatees, sea otters and polar
bears.[366] They do not represent a distinct taxon or systematic
grouping, but are instead unified by their reliance on the marine
environment for feeding. Both cetaceans and sirenians are fully aquatic
and therefore are obligate water dwellers. Seals and sea-lions are
semiaquatic; they spend the majority of their time in the water, but
need to return to land for important activities such as mating, breeding
and molting. In contrast, both otters and the polar bear are much less
adapted to aquatic living. Their diet varies considerably as well: some
may eat zooplankton; others may eat fish, squid, shellfish, and
sea-grass; and a few may eat other mammals.
In a process of
convergent evolution, marine mammals, especially cetaceans such as
dolphins and whales, redeveloped their body plan to parallel the
streamlined fusiform body plan of pelagic fish. Front legs became
flippers and back legs disappeared, a dorsal fin reappeared and the tail
morphed into a powerful horizontal fluke. This body plan is an
adaptation to being an active predator in a high drag environment. A
parallel convergence occurred with the now extinct marine reptile
ichthyosaur.[367]
Endangered blue whale, the largest living animal[368]
Endangered blue whale, the largest living animal[368]
The bottlenose dolphin has the highest encephalization of any animal after humans[369]
The bottlenose dolphin has the highest encephalization of any animal after humans[369]
Beluga whale
Beluga whale
Dugong grazing on seagrass
Dugong grazing on seagrass
Walrus
Walrus
Polar bear
Polar bear
Primary producers
Composite
image showing the global distribution of photosynthesis, including both
oceanic phytoplankton and terrestrial vegetation. Dark red and
blue-green indicate regions of high photosynthetic activity in the ocean
and on land, respectively.
Main article: marine primary production
See also: evolution of photosynthesis
Primary
producers are the autotroph organisms that make their own food instead
of eating other organisms. This means primary producers become the
starting point in the food chain for heterotroph organisms that do eat
other organisms. Some marine primary producers are specialised bacteria
and archaea which are chemotrophs, making their own food by gathering
around hydrothermal vents and cold seeps and using chemosynthesis.
However most marine primary production comes from organisms which use
photosynthesis on the carbon dioxide dissolved in the water. This
process uses energy from sunlight to convert water and carbon
dioxide[370]: 186–187 into sugars that can be used both as a source of
chemical energy and of organic molecules that are used in the structural
components of cells.[370]: 1242 Marine primary producers are important
because they underpin almost all marine animal life by generating most
of the oxygen and food that provide other organisms with the chemical
energy they need to exist.
The principal marine primary producers
are cyanobacteria, algae and marine plants. The oxygen released as a
by-product of photosynthesis is needed by nearly all living things to
carry out cellular respiration. In addition, primary producers are
influential in the global carbon and water cycles. They stabilize
coastal areas and can provide habitats for marine animals. The term
division has been traditionally used instead of phylum when discussing
primary producers, but the International Code of Nomenclature for algae,
fungi, and plants now accepts both terms as equivalents.[371]
Cyanobacteria
Cyanobacteria
Cyanobacteria from a microbial mat. Cyanobacteria were the first organisms to release oxygen via photosynthesis.
The cyanobacterium genus Prochlorococcus is a major contributor to atmospheric oxygen.
Cyanobacteria
were the first organisms to evolve an ability to turn sunlight into
chemical energy. They form a phylum (division) of bacteria which range
from unicellular to filamentous and include colonial species. They are
found almost everywhere on earth: in damp soil, in both freshwater and
marine environments, and even on Antarctic rocks.[372] In particular,
some species occur as drifting cells floating in the ocean, and as such
were amongst the first of the phytoplankton.
The first primary
producers that used photosynthesis were oceanic cyanobacteria about 2.3
billion years ago.[373][374] The release of molecular oxygen by
cyanobacteria as a by-product of photosynthesis induced global changes
in the Earth's environment. Because oxygen was toxic to most life on
Earth at the time, this led to the near-extinction of oxygen-intolerant
organisms, a dramatic change which redirected the evolution of the major
animal and plant species.[375]
The tiny marine cyanobacterium
Prochlorococcus, discovered in 1986, forms today part of the base of the
ocean food chain and accounts for much of the photosynthesis of the
open ocean[376] and an estimated 20% of the oxygen in the Earth's
atmosphere.[377] It is possibly the most plentiful genus on Earth: a
single millilitre of surface seawater may contain 100,000 cells or
more.[378]
Originally, biologists classified cyanobacteria as
algae, and referred to it as "blue-green algae". The more recent view is
that cyanobacteria are bacteria, and hence are not even in the same
Kingdom as algae. Most authorities today exclude all prokaryotes, and
hence cyanobacteria from the definition of algae.[379][380]
Algae
Diatoms
Centric
Pennate
Diatoms have a silica shell (frustule) with radial (centric) or bilateral (pennate) symmetry.
Dinoflagellates
Armoured
Unarmoured
Traditionally dinoflagellates have been presented as armoured or unarmoured.
Algae
is an informal term for a widespread and diverse group of
photosynthetic protists which are not necessarily closely related and
are thus polyphyletic. Marine algae can be divided into six groups:
green algae, an informal group containing about 8,000 recognised
species.[381] Many species live most of their lives as single cells or
are filamentous, while others form colonies made up from long chains of
cells, or are highly differentiated macroscopic seaweeds.
red
algae, a (disputed) phylum containing about 7,000 recognised
species,[382] mostly multicellular and including many notable
seaweeds.[382][383]
brown algae, a class containing about 2,000
recognised species,[384] mostly multicellular and including many
seaweeds, including kelp
diatoms, a (disputed) phylum containing
about 100,000 recognised species of mainly unicellular algae. Diatoms
generate about 20 percent of the oxygen produced on the planet each
year,[147] take in over 6.7 billion metric tons of silicon each year
from the waters in which they live,[385] and contribute nearly half of
the organic material found in the oceans. The shells (frustules) of dead
diatoms can reach as much as half a mile deep on the ocean floor.[386]
dinoflagellates, a phylum of unicellular flagellates with about 2,000
marine species.[387] Many dinoflagellates are known to be
photosynthetic, but a large fraction of these are in fact mixotrophic,
combining photosynthesis with ingestion of prey (phagotrophy).[388] Some
species are endosymbionts of marine animals and play an important part
in the biology of coral reefs. Others predate other protozoa, and a few
forms are parasitic.
euglenophytes, a phylum of unicellular flagellates with only a few marine members
Unlike higher plants, algae lack roots, stems, or leaves. They can be classified by size as microalgae or macroalgae.
Microalgae
are the microscopic types of algae, not visible to the naked eye. They
are mostly unicellular species which exist as individuals or in chains
or groups, though some are multicellular. Microalgae are important
components of the marine protists (discussed above), as well as the
phytoplankton (discussed below). They are very diverse. It has been
estimated there are 200,000-800,000 species of which about 50,000
species have been described.[389] Depending on the species, their sizes
range from a few micrometers (µm) to a few hundred micrometers. They are
specially adapted to an environment dominated by viscous forces.
Chlamydomonas globosa, a unicellular green alga with two flagella just visible at bottom left
Chlamydomonas globosa, a unicellular green alga with two flagella just visible at bottom left
Chlorella vulgaris, a common green microalgae, in endosymbiosis with a ciliate[390]
Chlorella vulgaris, a common green microalgae, in endosymbiosis with a ciliate[390]
Centric diatom
Centric diatom
Dinoflagellates
Dinoflagellates
Macroalgae
are the larger, multicellular and more visible types of algae, commonly
called seaweeds. Seaweeds usually grow in shallow coastal waters where
they are anchored to the seafloor by a holdfast. Seaweed that becomes
adrift can wash up on beaches. Kelp is a large brown seaweed that forms
large underwater forests covering about 25% of the world
coastlines.[391] They are among the most productive and dynamic
ecosystems on Earth.[392] Some Sargassum seaweeds are planktonic
(free-floating). Like microalgae, macroalgae (seaweeds) are technically
marine protists since they are not true plants.
A seaweed is a macroscopic form of red or brown or green algae.
A seaweed is a macroscopic form of red or brown or green algae.
Sargassum seaweed is a planktonic brown alga with air bladders that help it float.
Sargassum seaweed is a planktonic brown alga with air bladders that help it float.
Sargassum fish are camouflaged to live among drifting Sargassum seaweed.
Sargassum fish are camouflaged to live among drifting Sargassum seaweed.
Kelp forests are among the most productive ecosystems on the planet.
Unicellular macroalgae (see also macroscopic protists ←)
The unicellular bubble algae lives in tidal zones. It can have a 4 cm diameter.[393]
The unicellular bubble algae lives in tidal zones. It can have a 4 cm diameter.[393]
The unicellular mermaid's wineglass are mushroom-shaped algae that grow up to 10 cm high.
The unicellular mermaid's wineglass are mushroom-shaped algae that grow up to 10 cm high.
Killer algae are single-celled organisms, but look like ferns and grow stalks up to 80 cm long.[394]
Killer algae are single-celled organisms, but look like ferns and grow stalks up to 80 cm long.[394]
Unicellular
organisms are usually microscopic, less than one tenth of a millimeter
long. There are exceptions. Mermaid's wineglass, a genus of subtropical
green algae, is single-celled but remarkably large and complex in form
with a single large nucleus, making it a model organism for studying
cell biology.[395] Another single celled algae, Caulerpa taxifolia, has
the appearance of a vascular plant including "leaves" arranged neatly up
stalks like a fern. Selective breeding in aquariums to produce hardier
strains resulted in an accidental release into the Mediterranean where
it has become an invasive species known colloquially as killer
algae.[396]
Origin of plants
Evolution of mangroves and seagrasses
Back
in the Silurian, some phytoplankton evolved into red, brown and green
algae. These algae then invaded the land and started evolving into the
land plants we know today. Later, in the Cretaceous, some of these land
plants returned to the sea as marine plants, such as mangroves and
seagrasses.[397]
Marine plants can be found in intertidal zones
and shallow waters, such as seagrasses like eelgrass and turtle grass,
Thalassia. These plants have adapted to the high salinity of the ocean
environment. Plant life can also flourish in the brackish waters of
estuaries, where mangroves or cordgrass or beach grass beach grass might
grow.
Mangroves
Mangroves
Seagrass meadow
Seagrass meadow
Sea dragons camouflaged to look like floating seaweed live in kelp forests and seagrass meadows.[398]
Sea dragons camouflaged to look like floating seaweed live in kelp forests and seagrass meadows.[398]
The
total world area of mangrove forests was estimated in 2010 as 134,257
square kilometres (51,837 sq mi) (based on satellite data).[399][400]
The total world area of seagrass meadows is more difficult to determine,
but was conservatively estimated in 2003 as 177,000 square kilometres
(68,000 sq mi).[401]
Mangroves and seagrasses provide important
nursery habitats for marine life, acting as hiding and foraging places
for larval and juvenile forms of larger fish and invertebrates.[402]
Plankton and trophic interactions
Six
relatively large variously shaped organisms with dozens of small
light-colored dots all against a dark background. Some of the organisms
have antennae that are longer than their bodies.
Plankton are
drifting or floating organisms that cannot swim against a current, and
include organisms from most areas of life: bacteria, archaea, algae,
protozoa and animals.
Further information: Plankton, Bacterioplankton, Ichthyoplankton, and Mycoplankton
Plankton
(from Greek for wanderers) are a diverse group of organisms that live
in the water column of large bodies of water but cannot swim against a
current. As a result, they wander or drift with the currents.[403]
Plankton are defined by their ecological niche, not by any phylogenetic
or taxonomic classification. They are a crucial source of food for many
marine animals, from forage fish to whales. Plankton can be divided into
a plant-like component and an animal component.
Phytoplankton
Phytoplankton
are the plant-like components of the plankton community ("phyto" comes
from the Greek for plant). They are autotrophic (self-feeding), meaning
they generate their own food and do not need to consume other organisms.
Phytoplankton
consist mainly of microscopic photosynthetic eukaryotes which inhabit
the upper sunlit layer in all oceans. They need sunlight so they can
photosynthesize. Most phytoplankton are single-celled algae, but other
phytoplankton are bacteria and some are protists.[404] Phytoplankton
groups include cyanobacteria (above), diatoms, various other types of
algae (red, green, brown, and yellow-green), dinoflagellates,
euglenoids, coccolithophorids, cryptomonads, chrysophytes, chlorophytes,
prasinophytes, and silicoflagellates. They form the base of the primary
production that drives the ocean food web, and account for half of the
current global primary production, more than the terrestrial
forests.[405]
Coccolithophores
...have plates called coccoliths
...extinct fossil
Coccolithophores build calcite skeletons important to the marine carbon cycle.[406]
Phytoplankton
Phytoplankton are the foundation of the ocean food chain.
Phytoplankton are the foundation of the ocean food chain.
Phytoplankton come in many shapes and sizes.
Phytoplankton come in many shapes and sizes.
Diatoms are one of the most common types of phytoplankton.
Diatoms are one of the most common types of phytoplankton.
Colonial phytoplankton
Colonial phytoplankton
The cyanobacterium Prochlorococcus accounts for much of the ocean's primary production.
The cyanobacterium Prochlorococcus accounts for much of the ocean's primary production.
Green cyanobacteria scum washed up on a rock in California
Green cyanobacteria scum washed up on a rock in California
Gyrodinium, one of the few naked dinoflagellates which lack armour
Gyrodinium, one of the few naked dinoflagellates which lack armour
Zoochlorellae (green) living inside the ciliate Stichotricha secunda
Zoochlorellae (green) living inside the ciliate Stichotricha secunda
There are over 100,000 species of diatoms which account for 50% of the ocean's primary production.
Red,
orange, yellow and green represent areas where algal blooms abound.
Blue areas represent nutrient-poor zones where phytoplankton exist in
lower concentrations.
Coccolithophores named after the BBC documentary series. The Blue Planet
Coccolithophores named after the BBC documentary series. The Blue Planet
The coccolithophore Emiliania huxleyi
The coccolithophore Emiliania huxleyi
Algae bloom of Emiliania huxleyi off the southern coast of England
Algae bloom of Emiliania huxleyi off the southern coast of England
Guinardia delicatula, a diatom responsible for algal blooms in the North Sea and the English Channel[407]
Guinardia delicatula, a diatom responsible for algal blooms in the North Sea and the English Channel[407]
Zooplankton
Radiolarians
Drawings by Haeckel 1904
Zooplankton
are the animal component of the planktonic community ("zoo" comes from
the Greek for animal). They are heterotrophic (other-feeding), meaning
they cannot produce their own food and must consume instead other plants
or animals as food. In particular, this means they eat phytoplankton.
Foraminiferans
...can have more than one nucleus
...and defensive spines
Foraminiferans are important unicellular zooplankton protists, with calcium shells.
Turing and radiolarian morphology
Shell of a spherical radiolarian
Shell micrographs
Computer simulations of Turing patterns on a sphere closely replicate some radiolarian shell patterns.[408]
Zooplankton
are generally larger than phytoplankton, mostly still microscopic but
some can be seen with the naked eye. Many protozoans (single-celled
protists that prey on other microscopic life) are zooplankton, including
zooflagellates, foraminiferans, radiolarians and some dinoflagellates.
Other dinoflagellates are mixotrophic and could also be classified as
phytoplankton; the distinction between plants and animals often breaks
down in very small organisms. Other zooplankton include pelagic
cnidarians, ctenophores, molluscs, arthropods and tunicates, as well as
planktonic arrow worms and bristle worms.
Radiolarians are unicellular protists with elaborate silica shells
Microzooplankton: major grazers of the plankton
Radiolarians come in many shapes.
Radiolarians come in many shapes.
Group of planktic foraminiferans
Group of planktic foraminiferans
Copepods eat phytoplankton. This one is carrying eggs.
Copepods eat phytoplankton. This one is carrying eggs.
The dinoflagellate, Protoperidinium extrudes a large feeding veil to capture prey.
The dinoflagellate, Protoperidinium extrudes a large feeding veil to capture prey.
Larger zooplankton can be predatory on smaller zooplankton.
Macrozooplankton
Moon jellyfish
Moon jellyfish
Venus girdle, a ctenophore
Venus girdle, a ctenophore
Arrow worm
Arrow worm
Tomopteris, a planktonic segmented worm with unusual yellow bioluminescence[409]
Tomopteris, a planktonic segmented worm with unusual yellow bioluminescence[409]
Marine amphipod
Marine amphipod
Krill
Krill
Pelagic sea cucumber
Pelagic sea cucumber
External video
video icon Venus Girdle - Youtube
Many
marine animals begin life as zooplankton in the form of eggs or larvae,
before they develop into adults. These are meroplanktic, that is, they
are planktonic for only part of their life.
Larvae and juveniles
Salmon larva hatching from its egg
Salmon larva hatching from its egg
Ocean sunfish larva
Ocean sunfish larva
Juvenile planktonic squid
Juvenile planktonic squid
Larva stage of a spiny lobster
Larva stage of a spiny lobster
Mixotrophic plankton
A
surf wave at night sparkles with blue light due to the presence of a
bioluminescent dinoflagellate, such as Lingulodinium polyedrum
A suggested explanation for glowing seas[410]
See also: Mixotrophic dinoflagellate
Dinoflagellates are often mixotrophic or live in symbiosis with other organisms.
Mixoplankton
Tintinnid ciliate Favella
Tintinnid ciliate Favella
Euglena mutabilis, a photosynthetic flagellate
Euglena mutabilis, a photosynthetic flagellate
Noctiluca scintillans, a bioluminescence dinoflagellate
Noctiluca scintillans, a bioluminescence dinoflagellate
Some
dinoflagellates are bioluminescent. At night, ocean water can light up
internally and sparkle with blue light because of these
dinoflagellates.[411][412] Bioluminescent dinoflagellates possess
scintillons, individual cytoplasmic bodies which contain dinoflagellate
luciferase, the main enzyme involved in the luminescence. The
luminescence, sometimes called the phosphorescence of the sea, occurs as
brief (0.1 sec) blue flashes or sparks when individual scintillons are
stimulated, usually by mechanical disturbances from, for example, a boat
or a swimmer or surf.[413]
Marine food web
See also: Marine food web
Pelagic food web
Compared
to terrestrial environments, marine environments have biomass pyramids
which are inverted at the base. In particular, the biomass of consumers
(copepods, krill, shrimp, forage fish) is larger than the biomass of
primary producers. This happens because the ocean's primary producers
are tiny phytoplankton which tend to be r-strategists that grow and
reproduce rapidly, so a small mass can have a fast rate of primary
production. In contrast, terrestrial primary producers, such as mature
forests, are often K-strategists that grow and reproduce slowly, so a
much larger mass is needed to achieve the same rate of primary
production.
Because of this inversion, it is the zooplankton that
make up most of the marine animal biomass. As primary consumers, they
are the crucial link between the primary producers (mainly
phytoplankton) and the rest of the marine food web (secondary
consumers).[414]
If phytoplankton dies before it is eaten, it
descends through the euphotic zone as part of the marine snow and
settles into the depths of sea. In this way, phytoplankton sequester
about 2 billion tons of carbon dioxide into the ocean each year, causing
the ocean to become a sink of carbon dioxide holding about 90% of all
sequestered carbon.[415]
In 2010 researchers found whales carry
nutrients from the depths of the ocean back to the surface using a
process they called the whale pump.[416] Whales feed at deeper levels in
the ocean where krill is found, but return regularly to the surface to
breathe. There whales defecate a liquid rich in nitrogen and iron.
Instead of sinking, the liquid stays at the surface where phytoplankton
consume it. In the Gulf of Maine the whale pump provides more nitrogen
than the rivers.[417]
Other interactions
Biogeochemical cycles
Marine biogeochemical cycles
The
dominant feature of the planet viewed from space is water – oceans of
liquid water flood most of the surface while water vapour swirls in
atmospheric clouds and the poles are capped with ice.
Further information: Marine biogeochemical cycles, biological pump, and blue carbon
Taken
as a whole, the oceans form a single marine system where water – the
"universal solvent" [418] – dissolves nutrients and substances
containing elements such as oxygen, carbon, nitrogen and phosphorus.
These substances are endlessly cycled and recycled, chemically combined
and then broken down again, dissolved and then precipitated or
evaporated, imported from and exported back to the land and the
atmosphere and the ocean floor. Powered both by the biological activity
of marine organisms and by the natural actions of the sun and tides and
movements within the Earth's crust, these are the marine biogeochemical
cycles.[419][420]
Marine carbon cycle[421]
Marine carbon cycle[421]
Oxygen cycle
Oxygen cycle
Marine nitrogen cycle
Marine nitrogen cycle
Marine phosphorus cycle
Marine phosphorus cycle
Sediments and biogenic ooze
Thickness of marine sediments
See also: Marine sediment and Protist shells
Sediments
at the bottom of the ocean have two main origins, terrigenous and
biogenous. Terrigenous sediments account for about 45% of the total
marine sediment, and originate in the erosion of rocks on land,
transported by rivers and land runoff, windborne dust, volcanoes, or
grinding by glaciers.
Biogenous sediments account for the other
55% of the total sediment, and originate in the skeletal remains of
marine protists (single-celled plankton and benthos organisms). Much
smaller amounts of precipitated minerals and meteoric dust can also be
present. Ooze, in the context of a marine sediment, does not refer to
the consistency of the sediment but to its biological origin. The term
ooze was originally used by John Murray, the "father of modern
oceanography", who proposed the term radiolarian ooze for the silica
deposits of radiolarian shells brought to the surface during the
Challenger Expedition.[422] A biogenic ooze is a pelagic sediment
containing at least 30 percent from the skeletal remains of marine
organisms.
Main types of biogenic ooze
type mineral
forms protist
responsible name of
skeleton description
Siliceous ooze SiO2
quartz
glass
opal
chert diatoms frustule Individual diatoms range in size from 0.002 to 0.2 mm.[423]
radiolarians
skeleton Radiolarians are protozoa with diameters typically
between 0.1 and 0.2 mm that produce intricate mineral skeletons,
usually made of silica
Calcareous ooze CaCO3
calcite
aragonite
limestone
chalk
foraminiferans test There are about 10,000 living
species of foraminiferans,[424] usually under 1 mm in size.
coccolithophores
coccolith Coccolithophores are spherical cells usually less
than 0.1 mm across, enclosed by calcareous plates called
coccoliths.[425] Coccoliths are important microfossils. They are the
largest global source of biogenic calcium carbonate, and make
significant contributions to the global carbon cycle.[426] They are the
main constituent of chalk deposits such as the white cliffs of Dover.
An elaborate mineral skeleton of a radiolarian made of silica.
An elaborate mineral skeleton of a radiolarian made of silica.
Diatoms, major components of marine plankton, also have silica skeletons called frustules.
Diatoms, major components of marine plankton, also have silica skeletons called frustules.
Coccolithophores have plates or scales made with calcium carbonate called coccoliths
Coccolithophores have plates or scales made with calcium carbonate called coccoliths
Calcified test of a planktic foraminiferan
Calcified test of a planktic foraminiferan
A diatom microfossil from 40 million years ago
A diatom microfossil from 40 million years ago
Diatomaceous earth is a soft, siliceous, sedimentary rock made up of
microfossils in the form of the frustules (shells) of single cell
diatoms (click to magnify).
Diatomaceous earth is a soft,
siliceous, sedimentary rock made up of microfossils in the form of the
frustules (shells) of single cell diatoms (click to magnify).
Illustration of a Globigerina ooze
Illustration of a Globigerina ooze
Shells (tests), usually made of calcium carbonate, from a foraminiferal ooze on the deep ocean floor
Shells (tests), usually made of calcium carbonate, from a foraminiferal ooze on the deep ocean floor
Land interactions
The
drainage basins of the principal oceans and seas of the world are
marked by continental divides. The grey areas are endorheic basins that
do not drain to the ocean.
See also: Freshwater ecosystem and Continental shelf pump
Land
interactions impact marine life in many ways. Coastlines typically have
continental shelves extending some way from the shore. These provide
extensive shallows sunlit down to the seafloor, allowing for
photosynthesis and enabling habitats for seagrass meadows, coral reefs,
kelp forests and other benthic life. Further from shore the continental
shelf slopes towards deep water. Wind blowing at the ocean surface or
deep ocean currents can result in cold and nutrient rich waters from
abyssal depths moving up the continental slopes. This can result in
upwellings along the outer edges of continental shelves, providing
conditions for phytoplankton blooms.
Water evaporated by the sun
from the surface of the ocean can precipitate on land and eventually
return to the ocean as runoff or discharge from rivers, enriched with
nutrients as well as pollutants. As rivers discharge into estuaries,
freshwater mixes with saltwater and becomes brackish. This provides
another shallow water habitat where mangrove forests and estuarine fish
thrive. Overall, life in inland lakes can evolve with greater diversity
than happens in the sea, because freshwater habitats are themselves
diverse and compartmentalised in a way marine habitats are not. Some
aquatic life, such as salmon and eels, migrate back and forth between
freshwater and marine habitats. These migrations can result in exchanges
of pathogens and have impacts on the way life evolves in the ocean.
Anthropogenic impacts
Global cumulative human impact on the ocean[427]
Main article: Human impact on marine life
Human
activities affect marine life and marine habitats through overfishing,
pollution, acidification and the introduction of invasive species. These
impact marine ecosystems and food webs and may result in consequences
as yet unrecognised for the biodiversity and continuation of marine life
forms.[428]
Biodiversity and extinction events
Apparent marine fossil diversity during the Phanerozoic[429]
Marine extinction intensity during the Phanerozoic
%
Millions of years ago
(H)
K–Pg
Tr–J
P–Tr
Cap
Late D
O–S
Apparent
extinction intensity, i.e. the fraction of genera going extinct at any
given time as reconstructed from the fossil record (excluding the
current Holocene extinction event)
Biodiversity is the result of
over three billion years of evolution. Until approximately 600 million
years ago, all life consisted of archaea, bacteria, protozoans and
similar single-celled organisms. The history of biodiversity during the
Phanerozoic (the last 540 million years), starts with rapid growth
during the Cambrian explosion – a period during which nearly every
phylum of multicellular organisms first appeared. Over the next 400
million years or so, invertebrate diversity showed little overall trend
and vertebrate diversity shows an overall exponential trend.[430]
However,
more than 99 percent of all species that ever lived on Earth, amounting
to over five billion species,[431] are estimated to be
extinct.[432][433] These extinctions occur at an uneven rate. The
dramatic rise in diversity has been marked by periodic, massive losses
of diversity classified as mass extinction events.[430] Mass extinction
events occur when life undergoes precipitous global declines. Most
diversity and biomass on earth is found among the microorganisms, which
are difficult to measure. Recorded extinction events are therefore based
on the more easily observed changes in the diversity and abundance of
larger multicellular organisms, rather than the total diversity and
abundance of life.[434] Marine fossils are mostly used to measure
extinction rates because of their superior fossil record and
stratigraphic range compared to land organisms.
Based on the
fossil record, the background rate of extinctions on Earth is about two
to five taxonomic families of marine animals every million years. The
Great Oxygenation Event was perhaps the first major extinction event.
Since the Cambrian explosion five major mass extinctions have
significantly exceeded the background extinction rate.[435] The worst
was the Permian-Triassic extinction event, 251 million years ago. One
generally estimates that the Big Five mass extinctions of the
Phanerozoic (the last 540 million years) wiped out more than 40% of
marine genera and probably more than 70% of marine species.[436] The
current Holocene extinction caused by human activity, and now referred
to as the "sixth extinction", may prove ultimately more devastating.
See also
Marine life portaliconOceans portal
Blue Planet – 2001 British nature documentary television series - David Attenborough
Blue Planet II – 2017 British nature documentary television series
Census of Marine Life – 10 year international marine biological program
Colonization of land – Processes by which organisms evolved on Earth
Marine larval ecology
Taxonomy of invertebrates – System of classification of animals with emphasis on the invertebrates
Notes
This
is the measurement taken by the vessel Kaikō in March 1995 and is
considered the most accurate measurement to date. See the Challenger
Deep article for more details.
Myxozoa were thought to be an
exception, but are now thought to be heavily modified members of the
Cnidaria. Jiménez-Guri E, Philippe H, Okamura B, Holland PW (July 2007).
"Buddenbrockia is a cnidarian worm". Science. 317 (5834): 116–8.
Bibcode:2007Sci...317..116J. doi:10.1126/science.1142024. PMID 17615357.
S2CID 5170702.
The earliest Bilateria may have had only a single opening, and no coelom.[245]
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Further reading
Halpern BS, Walbridge S, Selkoe KA, Kappel CV, Micheli F, D'Agrosa C,
et al. (February 2008). "A global map of human impact on marine
ecosystems". Science. 319 (5865): 948–52. Bibcode:2008Sci...319..948H.
doi:10.1126/science.1149345. PMID 18276889. S2CID 26206024.
Paleczny M, Hammill E, Karpouzi V, Pauly D (2015). "Population Trend of
the World's Monitored Seabirds, 1950-2010". PLOS ONE. 10 (6): e0129342.
Bibcode:2015PLoSO..1029342P. doi:10.1371/journal.pone.0129342. PMC
4461279. PMID 26058068.
Ruppert EE, Fox RS, Barnes RD (2004). Invertebrate Zoology (7th ed.). Brooks / Cole. ISBN 978-0-03-025982-1.
"After 60 million years of extreme living, seabirds are crashing". The Guardian. 22 September 2015.
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100+ Great Ideas for Hermit Crab Names (From Bubbles to Wave)
Kristy Callan
Mar 31, 2023 3:12 PM EDT
Take inspiration from this list and find the name that fits your hermit crab best.
Take inspiration from this list and find the name that fits your hermit crab best.
Photo by Mohamed Haris on Unsplash
Hermit Crab Names: Cute, Cool, Sea-Themed, and Unique Name Ideas
Hermit
crabs can make great pets. They are much lower maintenance than a cat
or a dog and, unlike a fish, you can play with them and don't need to
worry about things like the pH level of the water. Once the initial
setup is out of the way, hermit crabs are cheap to look after, too!
The Comprehensive Guide to Chow Chow Dogs: History,
Care, and Training
Read More
The hardest thing is finding the perfect name. Here are a bunch to choose from!
What name do you think fits this little guy?
What name do you think fits this little guy?
Ahmed Sobah via Unsplash
Cool Names for Your Pet Hermit Crab
Bubbles
Coral or Coraline
Homer
Captain Hook
Coconut
Jacques
Clawdia
Groucho
Jaws
Krusty
Moe
Oscar
Pearl
Puddle
Sashimi
Scuttle
Seychelles
Sushi
Cute Hermit Crab Name Ideas
Scuttle
Squirt
Squeezy
Poops
Pinchy
Jabby
Stabby
Little Creep
Sideways
Hors d'oeuvre
Snip
Mouthfull
Dwebble
Pokemon
Kraken
Famous Crab Names
If
it strikes your fancy, you could name your crab after a famous one. I
honestly couldn't think of very many, so if you can think of others,
leave a comment below!
Mr. Krabs from SpongeBob SquarePants
Sebastian from The Little Mermaid
Cancer, after the star sign and constellation
Tamatoa, the giant crab from Moana
Ms. Krabappel (okay, she's not a crab in the literal sense of the word.
But she's got the word 'crab' in her name, and being Bart Simpson's
teacher has made her very well-known. And she's crabby. So she's on the
list!)
Sea-Themed Hermit Crab Names
Starla
Shell-by
Shell-don
Shelly
Crabby
Salty
Scrab (sea crab)
Rocky
Anemone
Fish-ish
Jaws
Claws
Sushi
Sea-more
Sand
Sandy
Gill
Wave
Make sure to choose a name that fits your hermit crab's personality.
Make sure to choose a name that fits your hermit crab's personality.
Jessica Diamond via Flickr Commons
Crabby Names for Your Hermit Crab
Cantankerous
Gordon Ramsay
Huffy
Churl
Groucho
Kvetch
Crabby
Grumpy
Mumpish
Peeves
Prickle
Snappy
Sulky
Surly
Tetchy
Popular Hermit Crab Names
Bob
Bruce
Rover
Henry
Molly
Pincer
Scuttle
Bait
Bubbles
Kirby
Abby
Gabby
Hermit Crab Translated
For
an unoriginal name that sounds original, why not translate the word
"hermit" or the word "crab" into different languages? Play around with
Google Translate until you find a translation you like. I've translated
it into some languages for you already, which you can find below.
"Hermit" in Different Languages "Crab" in Different Languages
Eremita (Italian)
Granchio (Italian)
Ermitano (Spanish)
Cangrejo (Spanish)
Pertapa (Indonesian)
Krabba (Swedish)
Pustelnik (Polish)
Krebs (German)
Name Ideas That Play on the Term "Hermit Crab"
Hermit
Hermione
Hermann
Herbert
Crabby
Craberella
How Long Will a Pet Hermit Crab Live?
In the wild, in their natural habitat, they can live up to 30 years.
In captivity, no matter how comfortable you make them, they will have a
much shorter life, typically about two years. Depending on food, care,
and environmental factors, they could live anywhere from less than a
year to up to 20 years in the best circumstances.
The oldest
hermit crab on record belongs to Carol Ann Ormes in Florida, US.
Jonathan Livingston Crab is almost 40 years old. Carol Ann Ormes created
a roomy, humid tank for him but also let's him roam wherever he wants.
Recommended For You
what-to-do-when-your-hermit-crab-is-molting
How to Care for a Molting Hermit Crab
Do Hermit Crabs Need Water?
Hermit
crabs use water for drinking, bathing, and replenishing the moisture
levels in their shells. But don't plop a hermit crab in a tank: Simply
provide deep bowls of both fresh and salt water and let the hermit crab
decide what it needs.
Can Water Hurt a Hermit Crab?
The chlorine in tap water can be harmful to hermit crabs, so if your tap water is harsh, consider using bottled water.
Can They Swim?
Not
really, although land hermit crabs can go underwater without drowning.
They do want to submerge under the water, but they need to be able to
crawl out when they're done. Make sure your bowls have tilted sides so
they can crawl in and out when they want to.
How Do You Know if a Hermit Crab Is a Boy or a Girl?
It's
very hard to know for sure what the gender of a hermit crab is. Some
say that hermit crabs might even be able to change their gender during
their lifetimes. Even experts have a hard time figuring this out, but
below, you'll find some possible clues. (Note: Never force a crab out of
its shell, even if you just want to examine its body to determine the
gender. It's not nice.)
Some say that on a large, older crab,
you can look at the legs to determine gender. The female will have
smoother legs, while the males will be hairy or spiny.
The female
land hermit crab has tiny openings (called "gonopores") that may or may
not be visible on the first section of the back pair of her walking
legs.
The good news is that even though you'll probably never
know for sure what the gender of your hermit crab is, they do not breed
in captivity, so you don't have to worry about waking up one day to find
a litter of newly-hatched baby hermit crabs.
It's very difficult to determine a hermit crab's gender. You'll probably have to guess.
It's very difficult to determine a hermit crab's gender. You'll probably have to guess.
warrenski via Flickr Commons
Taking Care of Your New Hermit Crab
What do hermit crabs need? How do you care for them?
Warmth: Hermit crabs are sensitive to the cold.
Fresh water: For drinking. This should be changed regularly.
Salt water: To clean them, bathe them in a very small amount of salt
water. They should be able to climb out, which they will do almost
immediately.