Crab, a land- or water-dwelling crustacean with a short, flat body that is nearly circular. The crab is related to the crayfish, lobster, and shrimp. The head and thorax are fused together to form a shell, or carapace. The carapace is composed of chitin, a horny substance that is waterproof. The two eyes, supported on stalks, flank two pairs of hairlike antennae (feelers). Crabs have 10 legs. In some species, the front two are modified to form large, pincerlike claws. In ocean-dwelling crabs that swim, the two hind legs function as paddles.

Crabs molt (shed their shells) as they grow. A new shell forms in several days. While the new shell is forming, the crab is known as a soft-shell crab. It is defenseless during this time.

Several kinds of crabs are used as food by humans. The most important is the common blue crab of the Atlantic coast. These crabs may be caught by using a piece of meat as bait on a line. The crab seizes the bait in its pincers and can be drawn to the surface and captured. Soft-shell crabs are considered delicacies.

The blue crabThe blue crab has a fan-shaped shell up to six inches across.
Life of the Crab

Most crabs are found in the ocean, but there are also freshwater and land species.

Even land crabs, however, lay their eggs in the water. Some species eat plants, others eat snails, minnows, and small shellfish. Some marine crabs are scavengers, and eat carrion (decayed flesh). At night they emerge from the sea and feed-on dead fish that have washed up on the shore.

Crabs have a peculiar way of walking. Although they can move in any direction, they almost always move sideways, pushing with the legs on one side of the body and pulling with those on the other.

Young crabs hatch from eggs. The larva, called a zoea, is tiny and does not resemble the adult crab. It sheds its skin several times and develops into a megalops, an intermediate form that eventually molts and develops into the adult crab.

Crabs belong to the order Decapoda of the class Crustacea. Crabs are distinguished by the shape of their shell and legs. The many types of crabs include the following.

The Beach Crab,

(also called the Sand, Sprite, Ghost, or White, Crab) is found from New Jersey to Florida. It burrows in the sand during the day and emerges at night to hunt for food. It belongs to the genus Ocypode.

Fiddler Crabs

are found on both coasts of North America. The male has one front claw that is much larger than the other, making him look like a fiddler holding his instrument, Fiddler crabs belong to the genus Uca.

Fiddler crabFiddler crab males have one front claw that is much larger than the other.
Green, or Shore, Crabs,

found from Nova Scotia to New Jersey, can remain out of water for long periods. The greenish shell is about two inches (5 cm) wide. A typical species is Carcinus maenas.

Land Crabs

come out of their holes at night to feed. A typical species is the mountain crab, Gecarcinus ruricola, which is found in southern Florida, and from the West Indies to Brazil. It has a black and purple carapace.

Rock Crabs

are found on rocky shores of both coasts of North America. Their shells range in width from five to eight inches (13 to 20 cm). Rock crabs make up the genus Cancer.

Spider Crabs

have rough, spiny bodies often covered with algae. The giant spider crab of Japan (Macrocheira kampferi) may have a shell one foot (30 cm) across.

Swimming Crabs

The edible blue crab has a fan-shaped shell up to six inches (15 cm) across. Unlike most other crabs, it is a good swimmer. A popular commercial species is Callinectes spaidus. The lady crab (Ovalipes ocellatus or Portunus puber) has a yellowish-gray shell with tiny purple spots. Its shell is about three inches (7.5 cm) in diameter.

Coconut, hermit, and horseshoe crabs are not true crabs.