Crabs are decapod crustaceans of the infraorder Brachyura. They are generally covered with a thick exoskeleton, and are armed with a single pair of chelae (claws). About 6000 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.
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