Crabs are generally covered with a thick exoskeleton, composed primarily of calcium carbonate,[4] and armed with a single pair of chelae (claws). Crabs are found in all of the world's oceans, while many crabs live in fresh water and on land, particularly in tropical regions. Crabs vary in size from the pea crab, a few millimetres wide, to the Japanese spider crab, with a leg span of up to 4 metres (13 ft).[5]

About 850 species of crab are freshwater, terrestrial or semi-terrestrial species;[6] they are found throughout the world's tropical and semi-tropical regions. They were previously thought to be a monophyletic group, but are now believed to represent at least two distinct lineages, one in the Old World and one in the New World.[7]

The earliest unambiguous crab fossils date from the Jurassic,[8] although Carboniferous Imocaris, known only from its carapace, may be a primitive crab.[9] The radiation of crabs in the Cretaceous and afterward may be linked either to the break-up of Gondwana or to the concurrent radiation of bony fish, crabs' main predators.[10]