Crab

Crab, common name for any of a group of crustaceans 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 divided into two groups: true crabs (about 4500 species) and hermit crabs and their allies (about 1400 species).

CHARACTERISTICS
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 tail of the hermit crabs and their allies. Instead, the abdomen of the true crabs serves, in the female, 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.

Crabs often reach a considerable size—the largest 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. They are able to resist changes in the external environment and 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 well developed, allow 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.

LIFE CYCLE
All crab species have separate sexes. In many species, mating occurs only when the female has just molted and her new shell is not yet hard. (So-called soft-shell crabs, a delicacy, are simply crabs in this transitional molting stage.) The eggs are held in a brood pouch and pass through two larval stages before 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 in its unprotected, soft state. Lost legs and chelae can be replaced when molting takes place. Crabs may live from 3 to 12 years.

DIVERSITY
True crab species are numerous. Among the 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 tiny pea crabs live in or on marine animals such as oysters. Male fiddler 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, such as the coconut crab, are tropical omnivores that migrate to the sea to release their larvae.

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. Squat lobsters are more primitive, lobsterlike forms, whereas porcelain crabs strongly resemble true crabs. Sand crabs have long bodies and burrow backward into sand. They filter suspended matter from the water.

In general, crabs 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, subphylum Crustacea, phylum Arthropoda. True crabs make up the infraorder Brachyura. Hermit crabs and their allies make up the infraorder Anomura. Fiddler crabs belong to the genus Uca, sand crabs to the genera Emerita and Blepharipoda, and hermit crabs to the genus Pagurus.

Contributed By:
Michael T. Ghiselin

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