Crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail", or where the reduced abdomen is entirely hidden under the thorax. Crabs have a soft body covered with a hard shell. They are generally covered with a thick exoskeleton, and armed with a single pair of chelae (claws). 6,793 species are known. Crabs are found in all of the world's oceans. Additionally, many crabs live in freshwater 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).