Crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" ( = short, ουρά/οura = 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). 6,793 species are known. 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.

Diet

Crabs are decapods—ten-legged omnivores—but the front pair of legs are specialised with enlarged claws, so they are often described as eight-legged. Some species feed primarily on algae, while others take any type of food, including mollusks, worms, other crustaceans, fungi, bacteria, and detritus, depending on availability and on the species of crab. For many species, a mixed diet of plant and animal matter results in the fastest growth and greatest fitness.

Crab fishery

Main: Crab fisheries

Evolution and classification

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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.

Cultural influences of the crab

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

Nutrition

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Gallery

Image:Corystes cassivelaunus.jpg|Masked crab, Corystes cassivelaunus Image:Liocarcinus vernalis.jpg|Liocarcinus vernalis Image:Atelecyclus rotundatus.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 Light Foot", 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:Hepatus epheliticus.jpg|Hepatus epheliticus, a calico crab Image:Crab larva.jpg|Crab larva (megalopa)