Crab, common name for 2 groups of decapod ("ten-footed") Crustaceans. The Anomura, including hermit, mole and lithoid crabs, have relatively normal abdomens. In true crabs, infraorder Brachyura, including genera Cancer and Carcinus, the abdomen is greatly reduced in size and is flexed beneath the thorax. Worldwide there are 4500 species inhabiting marine, freshwater and terrestrial environments. Crabs comprise one of the largest groups of the subphylum Crustacea; as individuals, they are among the largest of arthropods. Giant Japanese and Australian spider crabs, larger in size than the largest lobsters but weighing somewhat less, may have a claw span of over 3 m and weigh up to 14 kg. In Canada crabs are found in marine deep-water and in intertidal and estuarine environments on both coasts. Some, eg, East Coast snow or queen crab (Chionoecetes opilio) and West Coast Dungeness crab (Cancer magister), are of considerable economic value.

See also Crustacean Resources.