Difference between revisions of "Crab"

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{{otheruses}}
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The crab comes in many varieties. Many crabs have shells, which can be ground into powder to make fertilizer. They have claws and legs, just like scorpions. Crabs have small eyes on stalks, which can be used for seeing, along with many other exciting functions. Some crabs can weigh as much as 9 pounds. The largest crabs live in the waters around alaska.
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{{Taxobox
 
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| color = pink
 
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| name = Crabs
 
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| image = Blue crab on market in Piraeus - Callinectes sapidus Rathbun 20020819-317.jpg
 
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| image_width = 250px
 
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| image_caption = ''[[Blue crab|Callinectes sapidus]]''
 
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| regnum = [[Animal]]ia
 
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| phylum = [[Arthropod]]a
 
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| subphylum = [[Crustacean|Crustacea]]
 
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| classis = [[Malacostraca]]
 
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| ordo = [[Decapoda]]
 
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| subordo = [[Pleocyemata]]
 
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| infraordo = '''Brachyura'''
 
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| infraordo_authority = [[Pierre André Latreille|Latreille]], 1802
 
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| subdivision_ranks = Superfamilies
 
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| subdivision =
 
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*[[Dromiacea]]
 
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**[[Homolodromidae|Homolodromioidea]]
 
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**[[Dromioidea]]
 
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**[[Homoloidea]]
 
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*[[Eubrachyura]]
 
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**[[Raninoidea]]
 
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**[[Cyclodorippoidea]]
 
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**[[Dorippoidea]]
 
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**[[Calappoidea]]
 
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**[[Leucosioidea]]
 
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**[[Majoidea]]
 
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**[[Hymenosomatidae|Hymenosomatoidea]]
 
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**[[Parthenopoidea]]
 
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**[[Retroplumidae|Retroplumoidea]]
 
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**[[Cancroidea]]
 
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**[[Portunoidea]]
 
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**[[Bythograeidae|Bythograeoidea]]
 
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**[[Xanthoidea]]
 
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**[[Belliidae|Bellioidea]]
 
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**[[Potamoidea]]
 
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**[[Psuedothelphusidae|Pseudothelphusoidea]]
 
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**[[Gecarcinucoidea]]
 
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**[[Cryptochiridae|Cryptochiroidea]]
 
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**[[Pinnotheridae|Pinnotheroidea]] *
 
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**[[Ocypodoidea]] *
 
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**[[Grapsoidea]] *
 
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<small>An asterisk (*) marks the crabs included in the clade [[Thoracotremata]].</small>
 
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}}
 
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'''Crabs''' are [[Decapoda|decapod]] [[crustacean]]s of the infraorder '''Brachyura''', which typically have a very short "tail" ([[Greek language|Greek]]: ''brachy'' = short, ''ura'' = tail), or where the 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; there are also many [[fresh water|freshwater]] and [[terrestrial animal|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]]&nbsp;<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>.
 
  
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==Anatomy==
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{{stub2}}
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True crabs have five pairs of [[arthropod leg|legs]], the first of which is modified into a pair of claws and is not used for locomotion. In all but a few crabs (for example, [[Raninoida]]), the [[abdomen]] is folded under the [[cephalothorax]]. The [[mouthparts]] of crabs are covered by flattened [[maxilliped]]s, and the front of the [[carapace]] does not form a long [[Rostrum (anatomy)|rostrum]]&nbsp;<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&nbsp;<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>.
 
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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&nbsp;<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.
 
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==Diet==
 
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Crabs are [[omnivore]]s, feeding primarily on [[alga]]e&nbsp;<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>, and taking any other food, including [[mollusc]]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]]&nbsp;<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>.
 
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==Crab fishery==
 
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[[Image:Sorting Crabs Ffionphort.jpg|350px|thumb|left|[[Edible crab]]s being sorted by fishermen at [[Fionnphort]], [[Scotland]]]]
 
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Crabs make up 20% of all marine crustaceans 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>.
 
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<br clear="left">
 
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==Evolution and classification==
 
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The infraorder Brachyura contains about 70 [[family (biology)|families]], as many as the remainder of the [[Decapoda]]&nbsp;<ref name="Martin & Davis">{{cite book |url=http://www.nhm.org/research/publications/CrustaceaClassification.pdf |title=An Updated Classification of the Recent Crustacea |author=Martin, J. W. & G. E. Davis |year=2001 |pages=132 pp. |publisher=Natural History Museum of Los Angeles County |format=[[Portable Document Format|PDF]]}}</ref>.  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 [[uropod]]s are absent, having probably evolved into small devices for holding the reduced abdomen tight against the sternum&nbsp;<ref>{{cite journal | quotes=no |author=Guinot, D & J.–M. Bouchard |year=1998 |title=Evolution of the abdominal holding systems of brachyuran crabs (Crustacea, Decapoda, Brachyura) |journal=[[Zoosystema]] |volume=20 |issue=4 |pages=613–694 |url=http://www.mnhn.fr/publication/zoosyst/z98n4a4.html |format=[[Portable Document Format|PDF]]}}</ref>.
 
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In most decapods, the [[gonopore]]s (sexual openings) are found on the legs. However, since crabs use the first two pairs of [[pleopod]]s (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 (arthropod)|sternum]]&nbsp;<ref>{{cite journal | quotes=no |author=De Saint Laurent, M. |year=1980 |title=Sur la classification et la phylogénie des Crustacés Décapodes Brachyoures. II. Heterotremata et Thoracotremata Guinto, 1977 |journal=[[C. R. Acad. Sc. Paris]] |volume=t. 290 |pages=1317–1320}}</ref>.  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 [[Monophyly|monophyletic group]]&nbsp;<ref name="Martin & Davis"/>.
 
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The earliest unambiguous crab [[fossil]]s date from the [[Jurassic]], although the [[Carboniferous]] ''Imocaris'', known only from its [[carapace]] is thought to be a primitive crab&nbsp;<ref>{{cite journal | quotes=no |author=[[Frederick Schram|Schram, F. R.]] & R. Mapes |year=1984 |title=''Imocaris tuberculata'', n. gen., n. sp. (Crustacea: Decapoda) fro the upper Mississippian Imo Formation, Arkansas |journal=[[Transactions of the San Diego Society of Natural History]] |volume=20 |issue=11 |pages=165–168}}</ref>. The [[Adaptive radiation|radiation]] of crabs in the [[Cretaceous]] and afterwards may be linked either to the break-up of [[Gondwana]] or to the concurrent radiation of [[Osteichthyes|bony fish]], the main [[predator]]s of crabs&nbsp;<ref>{{cite journal | quotes=no |author=Wägele, J. W. |year=1989 |title=On the influence of fishes on the evolution of benthic crustaceans |journal=J. zool. Syst. Evolut.-forsch. |volume=27 |pages=297–309}}</ref>.
 
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About 850 species&nbsp;<ref>{{cite journal | quotes=no |author=Sternberg, R. von & N. Cumberlidge |year=2001 |title=On the heterotreme-thoracotreme distinction in the Eubrachyura De Saint Laurent, 1980 (Decapoda: Brachyura) |journal=[[Crustaceana]] |volume=74 |pages=321–338}}</ref> of crab are freshwater or (semi-)terrestrial species; they are found throughout the world's [[tropic]]al and [[Subtropics|semi-tropical]] regions. They were previously thought to be a closely related group, but are now believed to represent at least two distinct [[Lineage (evolution)|lineages]], one in the [[Old World]] and one in the [[New World]]&nbsp;<ref>{{cite journal | quotes=no |author=Sternberg, R. von, N. Cumberlidge & G. Rodriguez |year=1999 |title=On the marine sister groups of the freshwater crabs (Crustacea: Decapoda: Brachyura) |journal=[[J. Zool. Syst. Evol. Research]] |volume=37 |pages=19–38}}</ref>.
 
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==Gallery==
 
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<gallery>
 
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Image:Corystes cassivelaunus.jpg|[[Masked crab]], ''Corystes cassivelaunus''
 
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Image:Liocarcinus vernalis.jpg|''[[Liocarcinus vernalis]]''
 
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Image:Atelecyclus rotundatus museum.jpg|[[Circular crab]], ''Atelecyclus rotundatus''
 
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Image:Gecarcinus ruricola.jpg|The terrestrial [[halloween crab]], ''Geocarcinus ruricola''
 
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Image:Stenorhynchus seticornis 2.jpg|[[Arrow crab]] ''Stenorhynchus seticornis''
 
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Image:Grapsus grapsus.JPG|"Sally lightfoot", ''[[Grapsus grapsus]]''
 
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Image:Thia scutellata.jpg|[[Thumbnail crab]], ''Thia scutellata''
 
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Image:Spider crabs at the Kaiyukan Aquarium in Osaka close.jpg|[[Japanese spider crab]], ''Macrocheira kaempferi''
 
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Image:Ocypode quadrata (2).jpg|[[Ghost crab]], ''Ocypode quadrata''
 
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Image:Fiddler crab.jpg|[[Fiddler crab]], ''Uca pugnax''
 
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Image:Lyreidus tridentatus.jpg|''Lyreidus tridentatus'', a [[Raninidae|raninid]]
 
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Image:Hepatus epheliticus.jpg|''Hepatus epheliticus'', a calico crab
 
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</gallery>
 
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==Similar animals==
 
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Several other groups of animals are either called crabs or have the term "crab" in their names. These include [[hermit crab]]s, [[porcelain crab]]s and [[king crab]]s, which, despite superficial similarities to true crabs, belong to the [[Anomura]]. The [[United Kingdom|UK]] [[Food Standards Agency]] allows [[king crab]]s to be sold as "crab"&nbsp;<ref>{{cite web |url=http://www.food.gov.uk/multimedia/webpage/namesoffish |title=UK list of legal names for fish species |publisher=[[Food Standards Agency]] |date=[[2003-09-26]]}}</ref>, but this practice is not followed outside the [[food industry]]. Others, such as [[horseshoe crab]]s are much more distantly related. Anomuran "crabs" can be distinguished from true crabs by counting the legs. In Anomura, the last pair of [[pereiopod]]s (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 structure|vestigial]]&nbsp;<ref>{{cite journal |quotes=no |title=Rediscovery of the holotype of ''Paeduma cylindraceum'' (Bell, 1859) and description of a new genus of Hexapodidae (Decapoda, Brachyura) |author=Guinot, D. |journal=[[Zoosystema]] |volume=28 |issue=2 |pages=553&ndash;571 |year=2006 |url=http://www.mnhn.fr/museum/front/medias/publication/8150_z06n2a26.pdf |format=[[Portable Document Format|PDF]]}}</ref>).
 
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==References==
 
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{{commonscat|Brachyura}}
 
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{{Wikispecies|Brachyura}}
 
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<div class="references-small"><references/></div>
 
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[[Category:Crabs| ]]
 
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[[Category:Edible crustaceans]]
 

Revision as of 18:23, March 19, 2007

The crab comes in many varieties. Many crabs have shells, which can be ground into powder to make fertilizer. They have claws and legs, just like scorpions. Crabs have small eyes on stalks, which can be used for seeing, along with many other exciting functions. Some crabs can weigh as much as 9 pounds. The largest crabs live in the waters around alaska.

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