Crab

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 Latreille, 1802
Superfamilies
Dromiacea Homolodromioidea Dromioidea Homoloidea Eubrachyura Raninoidea Cyclodorippoidea Dorippoidea Calappoidea Leucosioidea Majoidea Hymenosomatoidea Parthenopoidea Retroplumoidea Cancroidea Portunoidea Bythograeoidea Xanthoidea Bellioidea Potamoidea Pseudothelphusoidea Gecarcinucoidea Cryptochiroidea Pinnotheroidea * Ocypodoidea * Grapsoidea * An asterisk (*) marks the crabs included in the clade Thoracotremata.

Crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short "tail" (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 chelae (claws). Crabs are found in all of the world's oceans; 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 .

True crabs

The term "crab" is often used for several different groups of decapod crustaceans, but only members of the Brachyura are true crabs. Other taxa, such as hermit crabs, porcelain crabs and king crabs are, despite superficial similarities, not crabs at all; rather, they belong to the Anomura and 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 ).

True crabs have five pairs of walking legs (the first of which is modified into a pair of claws or chelae) and typically a flattened shell. In all but a few crabs (for example, Raninoida), the abdomen is folded under the cephalothorax. The form of the abdomen usually reveals the sex of the crab; males have a narrow abdomen, while females have a much wider abdomen, under which they carry their eggs.

The mouthparts of crabs are covered by flattened maxillipeds, and the front of the carapace does not form a long "rostrum" . The gills of crabs are formed of flattened plates ("phyllobranchiate"), resembling those of shrimp, but of a different structure . Although famed for their tendency to walk sideways, many crabs are able to walk in any direction.

Diet

Crabs are omnivores, feeding primarily on algae , 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 .

Sexual dimorphism

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 . 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.

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 .

Evolution and classification

The infraclass Brachyura contains about 70 families, as many as the remainder of the Decapoda . 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 .

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 narower shape, the gonopores have moved towards the midline, away from the legs, and onto the sternum . 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 .

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 . 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 .

About 850 species  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 .

Gallery

• 
  Masked crab, Corystes cassivelaunus
• 
  Liocarcinus vernalis
• 
  Circular crab, Atelecyclus rotundatus
• The terrestrial halloween crab, Geocarcinus ruricola
  The terrestrial halloween crab, Geocarcinus ruricola
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  Arrow crab Stenorhynchus seticornis
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  "Sally lightfoot", Grapsus grapsus
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  Thumbnail crab, Thia scutellata
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  Japanese spider crab, Macrocheira kaempferi
• Ghost crab, Ocypode quadrata
  Ghost crab, Ocypode quadrata
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  Fiddler crab, Uca pugnax
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  Lyreidus tridentatus, a raninid
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  Calappa sulcata, a shame-faced crab