The term crab is often applied to several different groups of short (nose to tail) crustaceans with thick exoskeletons, but only members of the Brachyura are true crabs. Others, such as hermit crabs, porcelain crabs and king crabs are, despite superficial similarities, not crabs at all; they can be distinguished from true crabs by counting the legs (Uninjured true crabs always have five visible pairs of legs.)

True crabs have five pairs of walking legs (the first of which is modified into a pair of claws) and typically a flattened shell. In all but a few crabs, the abdomen is folded under the cephalothorax. The form of the abdomen usually reveals the sex of the crab; males have a narrow abdomen, while females have a much wider abdomen, under which they carry their eggs. Crabs are a very diverse group, mostly found in saltwater, but with some groups living in freshwater or on land. Although famed for their tendency to walk sideways, crabs are in fact able to walk in any direction.

Twenty percent: Crabs make up 20% of all marine crustaceans caught and farmed worldwide, with over 1½ million tonnes being consumed annually. Of that total, one species accounts for one fifth: Portunus trituberculatus. Other important taxa include Portunus pelagicus, several species in the genus Chionoecetes, Callinectes sapidus, Charybdis spp., Cancer pagurus, Cancer magister and Scylla serrata, each of which provides more than 20,000 tonnes annually.

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