Crabs are generally covered with a thick exoskeleton, composed primarily of calcium carbonate, 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 upto 4metres (13 ft).
they are found throughout the sea tropical and semi-tropical regions. They were previously thought to be amonophyletic group, but are now believed to represent at least two distinct lineages, one in the Old World and one in the New World.