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The term crab is sometimes applied to several different groups of decapods, but only members of the Brachyura are true crabs; other taxa, such as hermit crabs, porcelain crabs, king crabs, and horseshoe crabs are, despite superficial similarities, not crabs at all. Hermit crabs, king crabs and porcelain crabs belong to the Anomala and can be distinguished from true crabs by counting the legs - in Anomala, the last pair of walking legs is hidden inside the carapace and so only four pairs are visible (counting the claws), whereas uninjured true crabs always have five visible pairs.
True crabs are crustaceans in the Order Decapoda, which also contains lobsters, shrimp, prawns, hermit crabs and a number of less well-known groups. They have five pairs of walking legs (the first of which is modified into a pair of claws or chelae) 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, in order to carry eggs. Crabs are a very diverse group, mostly found in salt water, but with some groups living in freshwater or even on land. Although famed for their tendency to walk sideways, crabs are in fact able to walk in any diretion.
Classification within the crabs is traditionally based on the position of the gonopores, whether they are found on the legs or on the thorax.
Groups of crabs include:
A number of animals with a similar shape are commonly called crabs, including the crab louse (an insect), the Horseshoe crab (in the class Merostomata). The crab louse is the origin of the slang term "crabs" for an infestation of pubic lice.
A crab is any of various machines used to hoist heavy objects.
A Crab landing is a method of landing an airplane in a crosswind, so named because many crabs walk sideways.
A crab is a type of misapplied stroke in rowing when the oar is pulled round past the oarsman.
A Crab is a type of scratch turntablists use, involving flickering the crossfader between your thumb and your fingers, starting with the pinky and ending with the index.
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