There are about 4,500 species, or kinds, of the crustaceans known as crabs. Some types, including the blue crab, the Dungeness crab, and the king crab, are often eaten by humans. Crabs may be sold fresh to restaurants, or their meat may be canned.

Where crabs live

Crabs can be found in all oceans and in freshwater. Some crabs live on land, sometimes several miles from water. Species such as the fiddler crab live in burrows in the sand or mud, where they stay during winter and high tides. Others live within the shells of oysters or mussels.

Physical features

Crabs and other crustaceans have a hard covering known as the exoskeleton. Like its relative the lobster, a crab has five pairs of legs. The first pair has large pinching claws that help with feeding and defense. The last pair may be broad and flattened to serve as paddles for crabs that swim. The tail of a crab is curled under its midsection. Crabs breathe by using gills, but the gills of land crabs have developed in such a way that they act like lungs.

Crabs have broader bodies than other crustaceans. They come in a great range of sizes. Tiny pea crabs may measure less than an inch (2.5 centimeters) across. The giant crab of Japan, on the other hand, may be more than 1 foot (30 centimeters) across and measure 12 feet (4 meters) from tip to tip of its outstretched legs.

Behavior

Most crabs are scavengers, meaning they eat dead or decaying material. Some crabs may feed on vegetable matter, and others eat small living animals. The crab can see movement through its two large eyes, which extend from the head on movable stalks located above the two pairs of antennae. The mouth is on the underside of the head.

One group of crabs, the hermits, are known for their habit of taking over empty snail shells for shelter. A hermit crab drags the shell behind it as it walks about. As the crab grows larger, it seeks a larger shell.

Reproduction and growth

All female crabs, including land crabs, must return to the water to lay eggs. The eggs are carried on the female's body until they hatch. For the blue crab, this might mean carrying some 2 million eggs at a time. Although a few baby crabs leave the egg looking like small adults, most do not. Instead, a newly hatched crab is usually a tiny, see-through, legless creature that swims at the top of the water. At this stage the animal is called a zoea. As a result of a process called metamorphosis, during which it molts (sheds its outside covering) many times, the youngster becomes an adult crab.