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'''Crabs''' are [[Decapoda|decapod]] [[crustacean]]s of the infraorder '''Brachyura''', which typically have a very short projecting "tail" ([[Greek language|Greek]]: ''brachy'' = short, ''ura'' = tail), or where the reduced abdomen is entirely hidden under the thorax. They are generally covered with a thick [[exoskeleton]], and are armed with a single pair of [[chela]]e ([[claw]]s). Crabs are found in all of the world's oceans. Additionally, there are also many [[fresh water|freshwater]] and [[Land crab|terrestrial crabs]], particularly in tropical regions. Crabs vary in size from the [[pea crab]], only a few [[millimetre]]s wide, to the [[Japanese spider crab]], with a leg span of up to 4&nbsp;[[metre|m]].<ref>{{cite web |url=http://oceanlink.island.net/records.html |title=Biggest, Smallest, Fastest, Deepest: Marine Animal Records |publisher=OceanLink |accessmonthday=September 22 |accessyear=2006}}</ref>
'''Crabs''' are [[Decapoda|decapod]] [[crustacean]]s of the infraorder '''Brachyura''', which typically have a very short projecting "tail" ([[Greek language|Greek]]: ''brachy'' = short, ''ura'' = tail), or where the reduced abdomen is entirely hidden under the thorax. They are generally covered with a thick [[exoskeleton]], and are armed with a single pair of [[chela]]e ([[claw]]s). Crabs are found in all of the world's oceans. Additionally, there are also many [[fresh water|freshwater]] and [[Land crab|terrestrial crabs]], particularly in tropical regions. Crabs vary in size from the [[pea crab]], only a few [[millimetre]]s wide, to the [[Japanese spider crab]], with a leg span of up to 4&nbsp;[[metre|m]].<ref>{{cite web |url=http://oceanlink.island.net/records.html |title=Biggest, Smallest, Fastest, Deepest: Marine Animal Records |publisher=OceanLink |accessmonthday=September 22 |accessyear=2006}}</ref>


==Anatomy==
LAUREN BEESE HAS CRABS
[[Image:DSC02901 crab.JPG|Crab (''Atergatis floridus'') at Big Island of Hawaii|thumb|250px|left]]
True crabs have five pairs of [[arthropod leg|legs]], the first of which are modified into a pair of claws and are not used for locomotion. In all but a few crabs (for example, [[Raninoida]]), the [[abdomen]] is folded under the [[cephalothorax]] in the adult stage. The [[mouthparts]] of crabs are covered by flattened [[maxilliped]]s, and the front of the [[carapace]] does not form a long [[Rostrum (anatomy)|rostrum]].<ref>{{cite journal | quotes=no|author = Dixon, C. J., [[Frederick Schram|F. R. Schram]] & S. T. Ahyong|year=2004|title=A new hypothesis of decapod phylogeny|journal=[[Crustaceana]]|volume=76|issue=8|pages=935–975}}</ref> The [[gill]]s of crabs are formed of flattened plates ("phyllobranchiate"), resembling those of [[shrimp]], but of a different structure.<ref>{{cite journal | quotes=no |author=Taylor, H. H. & E. W. Taylor |year=1992 |title=Gills and Lungs: The Exchange of Gases and Ions |journal=Microscopic Anatomy of Invertebrates |volume=10 |pages=203–293}}</ref> They can also be the size of a pea, or even smaller.


Lauren Beese has crabs


Most crabs show clear [[sexual dimorphism]] and so can be easily sexed. The abdomen, which is held recurved under the thorax, is narrow in males. In females, however, the abdomen retains a greater number of [[pleopod]]s and is considerably wider.<ref>{{cite web |url=http://www.dnr.sc.gov/marine/sertc/Glossary.pdf |publisher=Southeastern Regional Taxonomic Center |title=Glossary of terms for decapods |format=[[Portable Document Format|PDF]]}}</ref> This relates to the carrying of the [[fertilisation|fertilised]] eggs by the female crabs (as seen in all [[Pleocyemata|pleocyemates]]). In those species in which no such dimorphism is found, the position of the gonopores must be used instead. In females, these are on the third [[pereiopod]], or nearby on the sternum in higher crabs; in males, the gonopores are at the base of the fifth pereiopods or, in higher crabs, on the sternum nearby.
Ronnete has a fishy fanny

==Diet==
Crabs are [[omnivore]]s, some feeding primarily on [[alga]]e,<ref>{{cite journal |quotes=no |title=Natural diet of the crab ''Notomithrax ursus'' (Brachyura, Majidae) at Oaro, South Island, New Zealand |author=Woods, C. M. C. |journal=[[New Zealand Journal of Marine and Freshwater Research]] |year=1993 |Volume=27 |pages=309=315 |url=http://www.rsnz.org/publish/nzjmfr/1993/29.php}}</ref> others taking any type of food, including [[mollusk]]s, [[worm]]s, other [[crustacean]]s, [[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 (biology)|fitness]].<ref>{{cite journal |quotes=no |title=Diet composition influeces the fitness of the herbivorous crab ''Grapsus albolineatus'' |journal=[[Oecologia]] |author=Kennish, R. |volum=105 |issue=1 |year=1996 |pages=22-29 |url=http://www.springerlink.com/content/l7m3368427059312/}}</ref><ref>{{cite journal |quotes=no |title=Diet choice in an omnivorous salt-marsh crab: different food types, body size, and habitat complexity |author=Buck, T. L., G. A. Breed, S. C. Pennings, M. E. Chase, M. Zimmer & T. H. Carefoot |journal=[[Journal of Experimental Marine Biology and Ecology]] |year=2003 |volume=292 |issue=1 |pages=103-116 |url=http://cat.inist.fr/?aModele=afficheN&cpsidt=14847986}}</ref>

==Crab fishery==
[[Image:Sorting Crabs Ffionphort.jpg|350px|thumb|left|[[Edible crab]]s being sorted by fishermen at [[Fionnphort]], [[Scotland]]]]
Crabs make up 20% of all marine [[crustacean]]s caught and farmed worldwide, with over 1½ million [[tonne]]s being consumed annually. Of that total, one species accounts for one fifth: ''[[Portunus trituberculatus]]''. Other important [[taxon|taxa]] include ''[[Portunus pelagicus]]'', several species in the genus ''[[Chionoecetes]]'', the [[blue crab]] (''Callinectes sapidus''), ''[[Charybdis (genus)|Charybdis spp.]]'', ''[[Edible crab|Cancer pagurus]]'', the [[Dungeness crab]] (''Cancer magister)'' and ''[[Scylla serrata]]'', each of which provides more than 20,000&nbsp;tonnes annually&nbsp;<ref>{{cite web |url=http://www.fao.org/figis/servlet/TabLandArea?tb_ds=Capture&tb_mode=TABLE&tb_act=SELECT&tb_grp=COUNTRY |title=Global Capture Production 1950-2004 |publisher=[[Food and Agriculture Organization|FAO]] |accessmonthday=August 26 |accessyear=2006}}</ref>.
<br clear="left">
[[Image:Crab larva.jpg|thumb|right|Crab larva (megalopa)]]


==Evolution and classification==
==Evolution and classification==

Revision as of 15:20, 11 March 2008

For other uses, see Crab (disambiguation).

Crabs
Callinectes sapidus
Scientific classification
Kingdom:
Animalia
Phylum:
Arthropoda
Subphylum:
Crustacea
Class:
Malacostraca
Order:
Decapoda
Suborder:
Pleocyemata
Infraorder:
Brachyura

Linnaeus, 1758
Superfamilies

Crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" (Greek: brachy = short, ura = tail), or where the reduced abdomen is entirely hidden under the thorax. They are generally covered with a thick exoskeleton, and are armed with a single pair of chelae (claws). Crabs are found in all of the world's oceans. Additionally, there are also many freshwater and terrestrial crabs, particularly in tropical regions. Crabs vary in size from the pea crab, only a few millimetres wide, to the Japanese spider crab, with a leg span of up to 4 m.[1]

Anatomy

File:DSC02901 crab.JPG
Crab (Atergatis floridus) at Big Island of Hawaii

True crabs have five pairs of legs, the first of which are modified into a pair of claws and are not used for locomotion. In all but a few crabs (for example, Raninoida), the abdomen is folded under the cephalothorax in the adult stage. The mouthparts of crabs are covered by flattened maxillipeds, and the front of the carapace does not form a long rostrum.[2] The gills of crabs are formed of flattened plates ("phyllobranchiate"), resembling those of shrimp, but of a different structure.[3] They can also be the size of a pea, or even smaller.


Most crabs show clear sexual dimorphism and so can be easily sexed. The abdomen, which is held recurved under the thorax, is narrow in males. In females, however, the abdomen retains a greater number of pleopods and is considerably wider.[4] This relates to the carrying of the fertilised eggs by the female crabs (as seen in all pleocyemates). In those species in which no such dimorphism is found, the position of the gonopores must be used instead. In females, these are on the third pereiopod, or nearby on the sternum in higher crabs; in males, the gonopores are at the base of the fifth pereiopods or, in higher crabs, on the sternum nearby.

Diet

Crabs are omnivores, some feeding primarily on algae,[5] others taking any type of food, including mollusks, 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.[6][7]

Crab fishery

Edible crabs being sorted by fishermen at Fionnphort, Scotland

Crabs make up 20% of all marine crustaceans caught and farmed worldwide, with over 1½ million tonnes being consumed annually. Of that total, one species accounts for one fifth: Portunus trituberculatus. Other important taxa include Portunus pelagicus, several species in the genus Chionoecetes, the blue crab (Callinectes sapidus), Charybdis spp., Cancer pagurus, the Dungeness crab (Cancer magister) and Scylla serrata, each of which provides more than 20,000 tonnes annually [8].

File:Crab larva.jpg
Crab larva (megalopa)

Evolution and classification

The infraorder Brachyura contains about 93 families[9], as many as the remainder of the Decapoda.[10] The evolution of crabs is characterised by an increasing robustness of the body, and a reduction in the abdomen. Although other groups have also undergone similar processes of carcinisation, it 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.[11]

In most decapods, the gonopores (sexual openings) are found on the legs. However, since crabs use the first two pairs of pleopods (abdominal appendages) for sperm transfer, this arrangement has changed. As the male abdomen evolved into a narrower shape, the gonopores have moved towards the midline, away from the legs, and onto the sternum.[12] 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 those crabs where the female, but not male, gonopores are situated on the sternum form a monophyletic group.[10]

The earliest unambiguous crab fossils date from the Jurassic, although the Carboniferous Imocaris, known only from its carapace is thought to be a primitive crab.[13] The radiation of crabs in the Cretaceous and afterwards may be linked either to the break-up of Gondwana or to the concurrent radiation of bony fish, the main predators of crabs.[14]

About 850 species[15] of crab are freshwater or (semi-)terrestrial species; they are found throughout the world's tropical and semi-tropical regions. They were previously thought to be a closely related group, but are now believed to represent at least two distinct lineages, one in the Old World and one in the New World.[16]

Similar animals

Several other groups of animals are either called crabs or have the term "crab" as part of their common names. These include hermit crabs, porcelain crabs and king crabs, which, despite superficial similarities to true crabs, belong to the Anomura. The UK Food Standards Agency allows king crabs to be sold as "crab",[17] but this practice is not followed outside the food industry.

Anomuran "crabs" can be distinguished from true crabs by counting the legs. In Anomura, the last pair of pereiopods (walking legs) is hidden inside the carapace, so only four pairs are visible (counting the claws), whereas uninjured true crabs generally have five visible pairs (in the family Hexapodidae, the last pair of pereiopods is vestigial [18]).

Others, such as horseshoe crabs, are much more distantly related.

Cultural influences of the crab

The Moche people of ancient Peru worshipped nature, especially the sea.[19] They placed emphasis on animals and often depicted crabs in their art.[20]

References

Wikimedia Commons has media related to Brachyura.
Wikispecies has information related to Brachyura.
  1. ^ "Biggest, Smallest, Fastest, Deepest: Marine Animal Records". OceanLink. {{cite web}}: Unknown parameter |accessmonthday= ignored (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)
  2. ^ Dixon, C. J., F. R. Schram & S. T. Ahyong (2004). "A new hypothesis of decapod phylogeny". Crustaceana. 76 (8): 935–975. {{cite journal}}: Unknown parameter |quotes= ignored (help)CS1 maint: multiple names: authors list (link)
  3. ^ Taylor, H. H. & E. W. Taylor (1992). "Gills and Lungs: The Exchange of Gases and Ions". Microscopic Anatomy of Invertebrates. 10: 203–293. {{cite journal}}: Unknown parameter |quotes= ignored (help)
  4. ^ "Glossary of terms for decapods" (PDF). Southeastern Regional Taxonomic Center.
  5. ^ Woods, C. M. C. (1993). "Natural diet of the crab Notomithrax ursus (Brachyura, Majidae) at Oaro, South Island, New Zealand". New Zealand Journal of Marine and Freshwater Research: 309=315. {{cite journal}}: Unknown parameter |Volume= ignored (|volume= suggested) (help); Unknown parameter |quotes= ignored (help)
  6. ^ Kennish, R. (1996). "Diet composition influeces the fitness of the herbivorous crab Grapsus albolineatus". Oecologia (1): 22–29. {{cite journal}}: Unknown parameter |quotes= ignored (help); Unknown parameter |volum= ignored (help)
  7. ^ Buck, T. L., G. A. Breed, S. C. Pennings, M. E. Chase, M. Zimmer & T. H. Carefoot (2003). "Diet choice in an omnivorous salt-marsh crab: different food types, body size, and habitat complexity". Journal of Experimental Marine Biology and Ecology. 292 (1): 103–116. {{cite journal}}: Unknown parameter |quotes= ignored (help)CS1 maint: multiple names: authors list (link)
  8. ^ "Global Capture Production 1950-2004". FAO. {{cite web}}: Unknown parameter |accessmonthday= ignored (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)
  9. ^ Systema Brachyurorum: Part 1. An Annotated checklist of extant Brachyuran crabs of the world Raffles Bulletin of Zoology, 2008, 286pp.
  10. ^ a b Martin, J. W. & G. E. Davis (2001). An Updated Classification of the Recent Crustacea (PDF). Natural History Museum of Los Angeles County. pp. 132 pp.
  11. ^ Guinot, D & J.–M. Bouchard (1998). "Evolution of the abdominal holding systems of brachyuran crabs (Crustacea, Decapoda, Brachyura)" (PDF). Zoosystema. 20 (4): 613–694. {{cite journal}}: Unknown parameter |quotes= ignored (help)
  12. ^ De Saint Laurent, M. (1980). "Sur la classification et la phylogénie des Crustacés Décapodes Brachyoures. II. Heterotremata et Thoracotremata Guinto, 1977". C. R. Acad. Sc. Paris. t. 290: 1317–1320. {{cite journal}}: Unknown parameter |quotes= ignored (help)
  13. ^ Schram, F. R. & R. Mapes (1984). "Imocaris tuberculata, n. gen., n. sp. (Crustacea: Decapoda) fro the upper Mississippian Imo Formation, Arkansas". Transactions of the San Diego Society of Natural History. 20 (11): 165–168. {{cite journal}}: Unknown parameter |quotes= ignored (help)
  14. ^ Wägele, J. W. (1989). "On the influence of fishes on the evolution of benthic crustaceans". J. zool. Syst. Evolut.-forsch. 27: 297–309. {{cite journal}}: Unknown parameter |quotes= ignored (help)
  15. ^ Sternberg, R. von & N. Cumberlidge (2001). "On the heterotreme-thoracotreme distinction in the Eubrachyura De Saint Laurent, 1980 (Decapoda: Brachyura)". Crustaceana. 74: 321–338. {{cite journal}}: Unknown parameter |quotes= ignored (help)
  16. ^ Sternberg, R. von, N. Cumberlidge & G. Rodriguez (1999). "On the marine sister groups of the freshwater crabs (Crustacea: Decapoda: Brachyura)". J. Zool. Syst. Evol. Research. 37: 19–38. {{cite journal}}: Unknown parameter |quotes= ignored (help)CS1 maint: multiple names: authors list (link)
  17. ^ "UK list of legal names for fish species". Food Standards Agency. 2003-09-26. {{cite web}}: Check date values in: |date= (help)
  18. ^ Guinot, D. (2006). "Rediscovery of the holotype of Paeduma cylindraceum (Bell, 1859) and description of a new genus of Hexapodidae (Decapoda, Brachyura)" (PDF). Zoosystema. 28 (2): 553–571. {{cite journal}}: Unknown parameter |quotes= ignored (help)
  19. ^ Benson, Elizabeth, The Mochica: A Culture of Peru. New York, NY: Praeger Press. 1972
  20. ^ Berrin, Katherine & Larco Museum. The Spirit of Ancient Peru:Treasures from the Museo Arqueológico Rafael Larco Herrera. New York: Thames and Hudson, 1997.

See also

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