Crab

Crabs are decapod crustaceans of the infraorder Brachyura (meaning "short tailed" in Greek), which typically have a very short projecting tail-like abdomen, usually hidden entirely under the thorax. Their exoskeleton is often thickened and hard. They generally have five pairs of legs, and they have "pincers" or "claws" on the ends of the frontmost pair, scientifically termed the chelae. They are present in all the world's oceans, in freshwater, and on land, often hiding themselves in small crevices or burrowing into sediment. Crabs are omnivores, feeding on a variety of food, including a significant proportion of algae, as well as detritus and other invertebrates. Crabs are widely consumed by humans as food, with over 1.5 million tonnes caught annually.

True crabs first appeared in the fossil record during the Jurassic period, around 200 million years ago, achieving great diversity by the Cretaceous period; around 7,000 extant species in 96 families are known. A number of other crustacean groups converged on the body plan of crabs, sometimes closely resembling them; this phenomenon is called carcinisation.