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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 •.
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 •. The gills of crabs are formed of flattened plates ("phyllobranchiate"), resembling those of shrimp, but of a different structure •. 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. 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 ••. Crab fishery 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 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 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 Similar animals Several other groups of animals 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 Anomura. The UK Food Standards Agency allows king crabs to be sold as "crab" •, but this practice is not followed outside the food industry. Others, such as horseshoe crabs 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 •). | |||||||||||||||||||||||||||||||||
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