crab

The tropical land crab, seen here in Costa Rica. As their name suggests, these crabs live on land. They visit the sea occasionally, principally in order to breed.

Any decapod (ten-legged) crustacean of the division Brachyura, with a broad, rather round, upper body shell (carapace) and a small abdomen tucked beneath the body. Crabs are related to lobsters and crayfish. Mainly marine, some crabs live in fresh water or on land. They are alert carnivores and scavengers. They have a typical sideways walk, and strong pincers on the first pair of legs, the other four pairs being used for walking. Periodically, the outer shell is cast to allow for growth. The name ‘crab’ is sometimes used for similar arthropods, such as the horseshoe crab, which is neither a true crab nor a crustacean.

There are many species of true crabs worldwide. The North American blue crab Callinectes sapidus, called the soft-shelled crab after moulting, is about 15 cm/6 in wide. It is extensively fished along the Atlantic and Gulf coasts. Other true crabs include fiddler crabs (Uca), the males of which have one enlarged claw to wave at and attract females, and spider crabs, with small bodies and very long legs, including the Japanese spider crab Macrocheira kaemperi with a leg span of 3.4 m/11 ft. Hermit crabs (division Anomura) have a soft, spirally twisted abdomen and make their homes in empty shells of sea snails for protection.

Some tropical hermit crabs are found a considerable distance from the sea. The robber crab Birgus latro grows large enough to climb palm trees and feed on coconuts.

Crab

In World War II, modified Sherman tank used for clearing minefields.

A power-driven spindle from which chains hung was mounted on the front of the tank. When the spindle revolved, the chains beat the ground in front of the tank, detonating any mines which were buried in the ground. The Crab could clear a lane 3 m/10 ft wide at a speed of about 2 kph/1.25 mph. When not ‘flailing’ for mines in this way, it could use its gun to fight as a tank.