Crab: common name for any arthropod of two crustacean groups, or infraorders: hermit crabs and allies (about 1400 species) and true crabs (about 4500 species). Both of these distinct groups have a similar body shape characterized by a reduced abdomen and an enlarged and broadened anterior portion of the body. Although most common as bottom dwellers in the sea, crabs also occur in fresh water, and some venture onto land.
Crabs are related to lobsters and shrimps, but their evolutionary development has enabled them to walk or run sideways and to burrow as well as swim. The body is more or less covered by a chitinous shell, or carapace, with a waxy coating. The reduced abdomen, no longer used in locomotion in many species, is tucked under the body. This reduction is greatest among the true crabs, which lack the flaplike tail of the hermit crabs and their allies; instead, the abdomen serves as a brood pouch for the eggs. A crab's segmented body has several pairs of appendages, of which usually five pairs serve as walking legs and two as sensory antennae. The front legs bear pincers (chelae) used in feeding, defense, and display. By many criteria, crabs are “advanced” animals. They often reach a considerable size—the record being the spider crab, with long, thin legs spanning up to 3.6 m (up to 12 ft)—and they possess fairly complicated nervous systems. Crabs are able to resist changes in the external environment, which allows them to flourish in rather hostile habitats. Their food habits are varied. Mostly active animals with complex behavior patterns, crabs have compound eyes and can see well. The senses of smell and taste also are well developed, allowing them to identify both food and prospective mates. Reproductive and social behavior sometimes include complex mating rituals and communication techniques such as drumming or waving the pincers. Crabs tend to be aggressive toward one another, and the males often fight to gain access to the females.
All crab species have separate sexes. Mating often occurs only when the female has just molted and its new shell is not yet hard. (So-called soft-shell crabs are simply crabs in this transitional molting stage.) The eggs are held in a brood pouch and pass through two larval stages until they hatch into tiny larvae, which swim about in the water. In some species the larva first appears in a form called a zoea, which does not resemble the adult, and then in a more crablike stage called a megalops, in which the abdomen is still prominent. Each time the young crab molts, it increases its size considerably, but it is exposed to danger while it is still soft. Lost legs and chelae can be replaced when molting takes place. Crabs may live from 3 to 12 years.
Kinds of Crabs
The familiar hermit crabs have unprotected abdomens and must live in abandoned shells, usually those of snails. They have a reduced number of walking legs, and the enlarged right claw serves to block the shell entrance when a predator or rival threatens. (Hermits often have to fight for possession of a new shell, which they must seek each time they molt.) Some hermit crabs live inland except when breeding and have gill chambers that function as lungs. More primitive, lobsterlike forms are called squat lobsters, whereas porcelain crabs strongly resemble true crabs. Sand crabs have long bodies and burrow backward into sand. They filter suspended matter from the water. True crabs are too numerous to mention more than a few important families. The spider crabs have long limbs and a narrow body; some conceal themselves by decorating their bodies with plants and animals.
The so-called edible crabs and rock crabs have a broad, oval body. The swimming crabs, such as the blue crab, have legs modified as paddles. The pea crabs are small and live in or on marine animals such as oysters. Male fiddler crabs, one of the burrowing crabs, have a large claw used in mating and combat. The shore crabs are conspicuous at or above the water level at the seashore. The land crabs are tropical omnivores that migrate to the sea to release their larvae. In general, all crabs probably are edible, and the meat is rich in protein and low in fat. For a commercial fishery to succeed, the crabs must be abundant and cheaply caught, usually in “pots” or with nets. At the present time, because of overfishing, pollution, and the cost of energy, the prospect is small for increasing the yield of these types of fisheries.
Scientific classification: Crabs belong to the order Decapoda of the subphylum Crustacea. Hermit crabs and their allies make up the infraorder Anomura. True crabs make up the infraorder Brachyura.