The Crab
crab crustacean with an increased cephalothorax covered by a broad, flat shell called the carapace. Leading from the cephalothorax are the various appendages: five pairs of legs, the first pair bearing pincers, are attached at the sides; two eyes on short, ambulant stalks, two short antennules, two lengthier antennae, and many mouthparts are attached at the face; at the behind the tiny abdomen is bent under the cephalothorax.
The belly of the female, broader and thinner than that of the male, forms an apronlike anatomical structure that endlessly circulates water over the eggs that are stocked on her undersurface. The free-swimming larva, which hatches in about two weeks, is easily recognized by the extended spine that sticks out from its carapace. After several sheds, the small crab settles to the bottom and begins to take on mature features.
Crabs are principally nautical, but some are onshore for long periods of time. They are omnivorous; some are scavengers and others predators. Although they are capable of locomotion in all directions, crabs incline to move sideways; swimming crabs have the last pair of legs flattened to form paddles.
The blue crab of the Atlantic coast of the United States is a swimming crab that is much used for food. It is commercialized as a soft-shelled crab after it has molted and before the new shell has toughened. Females of the oyster and mussel crabs exist inside the shells of bivalve mollusks. Often seen hurrying about near their tunnels in muddy banks are the fiddler crabs , the males of which have one much enlarged claw used in defense and in courting rituals. The sand, or ghost, crabs build tunnels high up on the sand into which they seem to disappear. The sluggish, long-legged spider crabs are often disguised by the algae, barnacles, and sea anemones that attach themselves to the shell. The huge spider crab of Japan, the largest living arthropod, has legs about 4 ft long and a shell over 1 ft wide. The nearly related kelp crabs are found in kelp beds in the Pacific. The name king crab is applied to the largest of the edible crabs, species native to the N Pacific and commercialized frozen, canned, or fresh; the red king crab has been presented into the Barents Sea.
Proper crabs are classified in the phylum Arthropoda , subphylum Crustacea, order Decapoda. Although the many species of true crabs are alike in appearance, DNA evidence indicates that that similarity is a result of convergent evolution among several groups of sometines only distantly associated decapods. The horseshoe crab , which also is called by the name king crab, is not a crustacean, and the hermit crab , although a crustacean, is not a true crab.
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