Crabs are some of the best known arthropods—a terms that means jointed foot (Greek: arthron, joint; pous, foot). They are among the most successful of all living species (about 4,500 species have been described), with members adapted to living on land and in water; some species even succeed in living in both habitats. The majority, however, live in the marine environment. Unlike lobsters (to which they are closely related), which have a long and cylindrical body with an extended abdomen, crabs have a broad, flattened body and a short, symmetrical abdomen—adaptations that enable them to squeeze beneath rocks and into crevices for feeding purposes as well as concealment.
The bulk of the body is taken up by the abdomen. Attached to this is a small head which bears long eye stalks that fit into special sockets on the carapace. There are also several pairs of antennae of unequal length and feeding mouthparts known as maxillipeds. The first pair of walking legs are large in comparison with the remainder of the body and end in pinching claws. These are usually referred to as chelipeds. In most species, the tips of the remaining four pairs of legs terminate in pointed tips. When feeding, food is picked up by the chelipeds, torn apart, and passed to the maxillipeds in small portions, from where it is pushed towards the pharynx. While some species are active predators of small fish, others are detritus feeders and scoop large volumes of mud towards the mouth region using the chelipeds as spades. These species then filter out any food particles and reject the remainder of the materials. Some species of burrowing crabs, which remain concealed in the soft sea bed, create a water current down into their burrows and filter out food particles in a similar manner. Their chelipeds are also fringed with tiny hair-like structures known as setae, which help extract the largest unwanted materials from the water current before other parts are ingested.
When moving on land or on the sea bed, crabs usually move in a sideways manner: the leading legs pull the body forward and those on the opposite side assist by pushing. Some species may use just two or three pairs of legs when moving quickly, stopping occasionally to turn around and reverse the order in which the legs move. Contrary to popular opinion, few crabs actually swim. One group of specialized swimming crabs (the family Portunidae) have an oval shaped body and the last pair of walking legs are flattened and act as paddles that propel the animal. Examples of these swimming crabs include the common blue crab (Callinectes sapidus), the green crab (Carcinides maenas) and the lady, or calico crab (Ovalipes ocellatus).
The remaining species, the "true crabs" vary considerably in size and behavior. Some of the largest of these are the spider crabs (family Maiidae). These are all marine species that live in the littoral zone, frequently skulking around on the sea bed in harbors and estuaries. This group contains the largest known arthropod, the giant Japanese crab (Macrocheira kaempferi) which can measure up to 13 ft (4 m) in diameter when fully extended. Most members of this family are scavenging animals. Many elect to carry a range of small sponges and other marine organisms on their outer carapace for concealment.
The aptly named fiddler crabs (family Ocyopidae), are easily recognized by the massively enlarged front claw of the male. The claw is usually carried horizontally in front of the body and has been likened to a fiddle; the smaller opposing claw is known as the bow. When males are trying to attract females, they wave these large claws two and fro; crabs with larger claws seem to attract more suitors than those with tiny claws. These crabs are usually a light brown color with mottled purple and darker brown patches on the carapace—a pattern that helps to conceal them on the dark sands and mud flats on which they live.
Unlike all other crabs, the tiny hermit crab has a soft body which is inserted in the shell of a marine snail for protection. Hermit crabs never kill the original occupant of the shell and frequently change "homes" as they grow, slipping out of one shell and into another. House hunting is a demanding task, and hermit crabs spend considerable time inspecting new prospective pieces of real estate, checking for size and weight. The shell is held on through a combination of modified hind limbs, which grasp some of the internal rings of the shell, and the pressure of the body against the shell wall. When resting, the crab can withdraw entirely inside the shell, blocking the opening with its claws. Hermit crab shells are commonly adorned with sea anemones and hydroids, the reason seeming to be that these provide some protection against small predators due to the battery of specialized stinging cells that these organisms possess. In return for this service, the anemones and hydroids may benefit from the guarantee that they will always be in clean water and the possibility of obtaining food scraps from the crab when it is feeding. The importance of this relationship for the crab is seen when a hermit crab changes its shell, as they usually delicately remove the anemones and hydroids from their former home to their new abode.
Of the terrestrial species, one of the most distinctive groups are the robber, or coconut, crabs which live in deep burrows above the high water mark. These crabs rarely venture into the sea, apart from when they lay their eggs. They have overcome the problem of obtaining oxygen by converting their gill chambers to modified chambers lined with moisture, enabling them to breathe atmospheric oxygen. Closely related to the hermit crab, robber crabs have developed a toughened upper surface on their abdomen which means that have no need of a shell for protection. Coconut crabs—so called because of their habit of digging in the soft soils of coconut plantations—occasionally climb trees and sever the stems attaching young coconuts, on which they feed.
Crabs have a complicated life history. Mating is usually preceded by a short period of courtship. The eggs are laid shortly after copulation and are retained on the female's body until the larvae emerge. The tiny "zoea" larvae, as they are known, are free-living and grow through a series of body molts to reach a stage known as the "megalops" larvae, at which stage the first resemblance to the parent crabs is visible. Further development leads to the immature and mature adult form.