Crabs, Brachyura, belong to the group of ten-limbed crustaceans, which means that it has ten limbs: 2 claw scissors and 4 pairs of limbs. The crab has a reduced hind body, which it carries folded up under the front body or chest
Crabs vary greatly in size; ranging from 5 mm up to 30 cm. The legs of the Japanese big-nosed crab span 2½ m.
Their primary food consists of carrion, but they can also actively find food by catching e.g. fish.
Crabs run sideways because they are wider than they are long and therefore provide less water resistance to run sideways than forward. When crabs have to be so wide and flat, it is so that they can quickly dig themselves down, into the sandy bottom. Crabs have 2 penises/vaginals.