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{{Taxobox_begin | color=pink | name=Crabs}}
{{Taxobox_image | image = [[Image:Blue crab on market in Piraeus - Callinectes sapidus Rathbun 20020819-317.jpg|250px]] | caption = ''Callinectes sapidus'' (Blue crab)}}
{{Taxobox_begin_placement | color = pink}}
{{Taxobox_regnum_entry | taxon=[[Animal]]ia }}
{{Taxobox_phylum_entry | taxon=[[Arthropod]]a }}
{{Taxobox_subphylum_entry | taxon=[[Crustacea]] }}
{{Taxobox_classis_entry | taxon=[[Malacostraca]] }}
{{Taxobox_ordo_entry | taxon =[[Decapoda]] }}
{{Taxobox_subordo_entry | taxon=Pleocyemata }}
{{Taxobox_infraordo_entry | taxon='''Brachyura''' }}
{{Taxobox_authority | author = [[Pierre André Latreille|Latreille]] | date = 1802}}
{{Taxobox_end_placement}}

{{Taxobox_section_subdivision | color = pink | plural_taxon = Superfamilies}}
*[[Dromiacea]]
**[[Homolodromidae|Homolodromioidea]]
**[[Dromioidea]]
**[[Homoloidea]]
*[[Eubrachyura]]
**[[Raninoidea]]
**[[Cyclodorippoidea]]
**[[Dorippoidea]]
**[[Calappoidea]]
**[[Leucosioidea]]
**[[Majoidea]]
**[[Hymenosomatidae|Hymenosomatoidea]]
**[[Parthenopoidea]]
**[[Retroplumidae|Retroplumoidea]]
**[[Cancroidea]]
**[[Portunoidea]]
**[[Bythograeidae|Bythograeoidea]]
**[[Xanthoidea]]
**[[Belliidae|Bellioidea]]
**[[Potamoidea]]
**[[Psuedothelphusidae|Pseudothelphusoidea]]
**[[Gecarcinucoidea]]
**[[Cryptochiridae|Cryptochiroidea]]
**[[Pinnotheridae|Pinnotheroidea]] *
**[[Ocypodoidea]] *
**[[Grapsoidea]] *
<small>An asterisk (*) marks the crabs included in the clade [[Thoracotremata]].</small>
{{Taxobox_end}}


'''Crabs''' are decapod [[crustacean]]s of the '''Brachyura''' infraorder or division. 

As members of the order Decapoda, which also includes crayfish, [[lobster]]s, prawns, and [[shrimp]], crabs have ten legs: the last five pairs of the eight pairs of thoracic appendages characteristic of crustaceans. As members of the Brachyura taxon, crabs typically have a very short "tail" or the abdomen is entirely hidden under the thorax (([[Greek language|Greek]]: ''brachy'' = short, ''ura'' = tail). Crabs typically are characterized by the first pair of the five pairs of appendages being modified into chelae (claws), a thick [[exoskeleton]], and an upper body shield or carapace that is generally broad, hard, and flattened. 

Crabs offer important benefits to human beings. [[Economics|Economically]] and nutritionally, crabs are a popular food item. [[Ecology|Ecologically]], they are integral to [[food chain|food webs]], both as a consumer of [[animal]]s and [[plant]]s, and as a popular prey for many [[bird]]s, [[fish]], and [[mammal]]s. They also provide aesthetic benefit to humans, whether for the joy experienced from the wonder of observing their diverse forms and colors, or whether they appeal to other [[sense]]s via their texture and taste as a food item. 

There are about 4,500 known species of crabs. Crabs are found in all of the world's oceans; there are also many freshwater and terrestrial crabs, particularly in tropical regions. About 850 species of crab are freshwater or semi-terrestrial species (Sternberg and Cumberlidge 2001). Crabs vary in size from the pea crab, only a few millimeters wide, to the Japanese spider crab, with a leg span of up to 4 meters (Oceanlink 2006).

Some animals that bear the name "crab" are not true crabs. The horseshoe crab, which also is called by the name king crab, is not a crustacean, and the hermit crab, although a crustacean, is not a true crab. 

==Anatomy==
True crabs have five pairs of 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 maxillipeds, and the front of the carapace does not form a long rostrum (Dixon et al. 2004). The [[gill]]s of crabs are formed of flattened plates ("phyllobranchiate"), resembling those of [[shrimp]], but of a different structure (Taylor and Taylor 1992). 

Most crabs show clear sexual dimorphism and so can be easily sexed. The abdomen, which is held recurved under the thorax, is narrow and pencil-like in males. In females, however, the abdomen retains a greater number of pleopods (swimming legs, but also used for brooding the eggs) and is considerably wider. This relates to the carrying of the fertilized 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 (primarily walking legs and are also used for gathering food), 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, consuming both [[animal]] protein and [[plant]]s. They feed primarily on [[algae]] but take any other food, including [[mollusk]]s, [[worm]]s, other [[crustacean]]s, [[fungi]], [[bacteria]] and [[detritus]], depending on their availability and the crab species (Woods 1993). For many crabs, a mixed diet of plant and animal matter results in the fastest growth and greatest fitness (Kennish 1996; Buck et al. 2003). 

==Crab fishery==
Crabs make up 20 percent of all marine [[crustacean]]s caught and farmed worldwide, with over 1 ½ million tons 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 tons annually (FAO 2004).

==Evolution and classification==
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 (Schram and Mapes 1984). The radiation of crabs in the [[Cretaceous]] and afterwards may be linked either to the breakup of [[Gondwana]] or to the concurrent radiation of bony fish, the main predators of crabs (Wägele 1989). 

About 850 species of freshwater or semi-terrestrial species are found throughout the world's tropical and semi-tropical regions (Sternberg and Cumberlidge 2001). 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 (Sternberg, Cumberlidge, and Rodriguez 1999).

The infraclass Brachyura contains about 70 families, as many as the remainder of Decapoda (Martin and Davis 2001). The [[evolution]] of crabs is characterized by an increasing robustness of the body, and a reduction in the abdomen. Although other groups have also undergone similar processes of carcinization, it is most advanced in crabs. The telson is no longer functional in crabs, and the uropods are absent, possibly having evolved into small devices for holding the reduced abdomen tight against the sternum (Guinot and Bouchard 1998). 

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 is different in crabs. The male abdomen has a narrower shape, and the gonopores are more toward the midline, away from the legs, and onto the sternum (De Saint Laurent 1980). A similar change is seen 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 (Martin and Davis 2001).

==Gallery==
<gallery>
Image:Corystes cassivelaunus.jpg|Masked crab, ''Corystes cassivelaunus''
Image:Liocarcinus vernalis.jpg|''Liocarcinus vernalis''
Image:Atelecyclus rotundatus museum.jpg|Circular crab, ''Atelecyclus rotundatus''
Image:Gecarcinus ruricola.jpg|The terrestrial halloween crab, ''Geocarcinus ruricola''
Image:Stenorhynchus seticornis 2.jpg|Arrow crab ''Stenorhynchus seticornis'' 
Image:Grapsus grapsus.JPG|"Sally lightfoot," ''Grapsus grapsus''
Image:Thia scutellata.jpg|Thumbnail crab, ''Thia scutellata''
Image:Spider crabs at the Kaiyukan Aquarium in Osaka close.jpg|Japanese spider crab, ''Macrocheira kaempferi''
Image:Ocypode quadrata (2).jpg|Ghost crab, ''Ocypode quadrata''
Image:Fiddler crab.jpg|Fiddler crab, ''Uca pugnax''
Image:Lyreidus tridentatus.jpg|''Lyreidus tridentatus,'', a raninid
Image:Calappa sulcata.jpg|''Calappa sulcata'', a shame-faced crab
</gallery>

==Similar animals==
Several other groups of [[animal]]s are either called crabs or have the term "crab" in their names. These include hermit crabs, porcelain crabs, and king crabs, which, despite superficial similarities to true crabs, belong to the infraorder Anomura. The [[United Kingdom|UK]] Food Standards Agency allows king crabs to be sold as "crab," but this practice is not followed outside the food industry (Food Standards Agency 2003). Other animals, 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 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) (Guinot 2006).

==References==
* Buck, T. L., G. A. Breed, S. C. Pennings, M. E. Chase, M. Zimmer, and T. H. Carefoot. 2003. [http://www.springerlink.com/content/l7m3368427059312 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.
* 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'' 290: 1317–1320. 
* Dixon, C. J., F. R. Schram, and S. T. Ahyong. 2004. A new hypothesis of decapod phylogeny. ''Crustaceana'' 76(8): 935–975.
* Food and Agriculture Organization (FAO). 2005. [http://www.fao.org/figis/servlet/TabLandArea?tb_ds=Capture&tb_mode=TABLE&tb_act=SELECT&tb_grp=COUNTRY Global Capture Production 1950–2004.] Retrieved August 26, 2006.
* Food Standards Agency. 2003. [http://www.food.gov.uk/multimedia/webpage/namesoffish UK List of Legal Names for Fish Species.] Retrieved April 6, 2007.
* Guinot, D. 2006. Rediscovery of the holotype of ''Paeduma cylindraceum'' (Bell, 1859) and description of a new genus of Hexapodidae (Decapoda, Brachyura). ''Zoosystema'' 28(2): 553–571. 
* Guinot, D., and J. M. Bouchard. 1998. Evolution of the abdominal holding systems of brachyuran crabs (Crustacea, Decapoda, Brachyura). ''Zoosystema'' 20(4): 613–694.
* Kennish, R. 1996. Diet composition influences the fitness of the herbivorous crab ''Grapsus albolineatus.'' ''Oecologia'' 105(1): 22–29
* Martin, J. W., and G. E. Davis. 2001. [http://www.nhm.org/research/publications/CrustaceaClassification.pdf An Updated Classification of the Recent Crustacea.] Natural History Museum of Los Angeles County. Retrieved April 6, 2007.
* Oceanlink. 2006. [http://oceanlink.island.net/records.html Biggest, Smallest, Fastest, Deepest: Marine Animal Records.] Retrieved September 22, 2006.
* Schram, F. R., and R. Mapes. 1984. ''Imocaris tuberculata,'' n. gen., n. sp. (Crustacea: Decapoda) fror the upper Mississippian Imo Formation, Arkansas. ''Transactions of the San Diego Society of Natural History'' 20(11): 165–168. 
* Sternberg, R. von, and N. Cumberlidge. 2001. On the heterotreme-thoracotreme distinction in the Eubrachyura De Saint Laurent, 1980 (Decapoda: Brachyura). ''Crustaceana'' 74: 321–338. 
* Sternberg, R. von, N. Cumberlidge, and G. Rodriguez. 1999. On the marine sister groups of the freshwater crabs (Crustacea: Decapoda: Brachyura). ''J. Zool. Syst. Evol. Research'' 37: 19–38.
* Taylor, H. H., and E. W. Taylor. 1992. Gills and lungs: The exchange of gases and ions. ''Microscopic Anatomy of Invertebrates'' 10: 203–293.
* Wägele, J. W. 1989. On the influence of fishes on the evolution of benthic crustaceans. ''J. Zool. Syst. Evolut.-forsch.'' 27: 297–309. 
* 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'' 27: 309–315. 

{{credit2|Crab|91148907|Decapoda|91164150}}
[[Category:Life sciences]]
[[Category:Animals]]
[[Category:Invertebrates]]
[[Category:Arthropods]]
[[Category:Food]]