Definitions

crab

crab

[krab]
crab, crustacean with an enlarged cephalothorax covered by a broad, flat shell called the carapace. Extending from the cephalothorax are the various appendages: five pairs of legs, the first pair bearing claws (or pincers), are attached at the sides; two eyes on short, movable stalks, two short antennules, two longer antennae, and numerous mouthparts are attached at the front; at the rear the tiny abdomen is bent under the cephalothorax.

The abdomen of the female, wider and flatter than that of the male, forms an apronlike structure that continuously circulates water over the eggs that are carried on her underside. The free-swimming larva, which hatches in about two weeks, is easily recognized by the large spine that projects from its carapace. After several molts, the young crab settles to the bottom and begins to take on adult features.

Crabs are chiefly marine, but some are terrestrial for long periods. They are omnivorous; some are scavengers and others predators. Although they are capable of locomotion in all directions, crabs tend 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 marketed as a soft-shelled crab after it has molted and before the new shell has hardened. Females of the oyster and mussel crabs live inside the shells of bivalve mollusks. Often seen scurrying about near their burrows in muddy banks are the fiddler crabs, the males of which have one much enlarged claw used in defense and in courtship rituals. The sand, or ghost, crabs build burrows high up on the sand into which they seem to vanish. The sluggish, long-legged spider crabs are often disguised by the algae, barnacles, and sea anemones that attach themselves to the carapace. The giant spider crab of Japan, the largest living arthropod, has legs about 4 ft (22 cm) long and a carapace over 1 ft (30 cm) wide. The closely related kelp crabs are found in kelp beds in the Pacific. The name king crab is applied to the largest (up to 20 lb/9 kg) of the edible crabs, species native to the N Pacific and marketed frozen, canned, or fresh; the red king crab has been introduced into the Barents Sea.

True crabs are classified in the phylum Arthropoda, subphylum Crustacea, order Decapoda. Although the many species of true crabs are similar in appearance, DNA evidence suggests that that similarity is a result of convergent evolution among several groups of sometines only distantly related 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.

Crab, The, English name for Cancer, a constellation.

Spider crab (Libinia)

Any species of sluggish marine crab in the widely distributed family Majidae (or Maiidae). Spider crabs have a beak-shaped head; thick, rounded body; and long, spindly legs. They use a mucuslike mouth secretion to fasten algae, sponges, and other organisms to the hairs, spines, and knobby projections covering the body. Most species are scavengers, especially of carrion. Their size varies greatly. The body of the European long-beaked spider crab (Macropodia rostrata) is less than 0.5 in. (1 cm) in diameter, whereas the Japanese giant crab (Macrocheira kaempferi), whose outstretched claws can measure 13 ft (4 m) from tip to tip, is perhaps the largest known arthropod.

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or Alaskan king crab or Japanese crab

Marine decapod (Paralithodes camtschatica), an edible crab. It is found in the shallow waters off Japan and along the Alaska coast; it also inhabits the Bering Sea. One of the largest crabs, it often weighs 10 lbs (4.5 kg) or more. Its size and tasty flesh make it a valued food, and large numbers are fished commercially each year.

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Horseshoe crab (Limulus polyphemus).

Any of four extant species of marine arthropods (order Xiphosura, subphylum Chelicerata), found on the eastern coasts of Asia (three species) and North America (one species). Despite the name, horseshoe crabs are not crabs; they are more closely related to scorpions. Fossil relatives date back 505 million years. They are most abundant in estuarine waters. The North American species, Limulus polyphemus, can grow to more than 2 ft (60 cm) long. The body consists of three parts hinged together: a broad, horseshoe-shaped cephalothorax; a much smaller, segmented abdomen; and a long, sharp tail-spine, or telson. They spawn on sandy beaches in spring and summer. Adults feed on marine worms; larvae feed on small organisms.

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Hermit crab (Pagurus samuelis).

Any crab (families Paguridae and Coenobitidae) that uses empty shells or other hollow objects as a shelter for partial containment and protection of the body. They are found worldwide in sandy- or muddy-bottomed waters and occasionally on land and in trees. They have two pairs of antennae and four pairs of legs; the first pair of legs is modified to form pincers, shaped to cover the shell entrance when the animal is inside. As the crab grows, it periodically leaves its shell and finds a larger one to occupy. The reddish brown large hermit crab (Pagurus pollicaris; 4–5 in., or 10–12 cm, long) and the small hermit crab (P. longicarpus) are found in North American Atlantic coastal waters.

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Any of about 65 species of decapods (genus Uca) whose males hold one claw, always much larger than the other, somewhat like a violin. Both claws of the female are relatively small. Fiddler crabs often live in large numbers on beaches in temperate to tropical regions of the world. They inhabit water-covered burrows up to about 1 ft (30 cm) deep and feed on algae and other organic matter. Common North American species (e.g., marsh fiddler, china-back fiddler) live all along the U.S. Atlantic coast. Brightly coloured, they range in body size from about 1 to 1.2 in. (2.5 to 3 cm).

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Common swimming crab (Portunus holsatus), showing its paddle-shaped feet

Any of 4,500 species of short-tailed decapod, found in all oceans, in freshwater, and on land. Its carapace (upper body shield) is usually broad, and its first pair of legs is modified into pincers. Most crabs live in the sea and breathe through gills, which in land crabs are modified to serve as lungs. They walk or crawl, generally with a sideways gait; some are good swimmers. Crabs are omnivorous scavengers, but many are predatory and some are herbivorous. Two of the largest known crustaceans are the giant crab of Japan (13 ft, or 4 m, from claw tip to claw tip), a spider crab; and the Tasmanian crab (up to 18 in., or 46 cm, long, and weighing more than 20 lbs, or 9 kg). Other species are less than an inch long. Well-known crabs include the hermit crab, edible crab (Britain and Europe), blue crab, Dungeness crab, fiddler crab, and king crab.

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Blue crab (Callinectes sapidus)

Any member of a genus (Callinectes) of decapods, particularly C. sapidus and C. hastatus, common edible crabs of the western Atlantic coast prized as delicacies. Their usual habitats are muddy shores, bays, and estuaries. The blue crab shell, greenish on top and dingy white below, is about 3 in. (15–18 cm) long. The legs are bluish. The chelae, or pincers, are large and somewhat unequal in size, and the fifth pair of legs is flattened for swimming. Blue crabs are scavengers.

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Bright nebula in the constellation Taurus, about 5,000 light-years from Earth. Roughly 12 light-years in diameter, it is the remnant of a supernova, first observed by Chinese and other astronomers in 1054, that was visible in daylight for 23 days and at night for almost two years. Identified as a nebula circa 1731, it was named (for its form) in the mid-19th century. In 1921 it was discovered to be still expanding; the present rate is about 700 mi/second (1,100 km/second). The Crab is one of the few astronomical objects from which electromagnetic radiation has been detected over the entire measurable spectrum. In the late 1960s a pulsar, thought to be the collapsed remnant star of the supernova, was found near its centre.

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Crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" (βραχύ/brachy = short, ουρά/οura = tail), or where the reduced abdomen is entirely hidden under the thorax. They are generally covered with a thick exoskeleton, and are armed with a single pair of chelae (claws). 6,793 species are known. Crabs are found in all of the world's oceans. Additionally, there are also many freshwater and terrestrial crabs, particularly in tropical regions. Crabs vary in size from the pea crab, only a few millimetres wide, to the Japanese spider crab, with a leg span of up to 4 m.

Diet

Crabs are decapods—ten-legged omnivores—but the front pair of legs are specialised with enlarged claws, so they are often described as eight-legged. Some species feed primarily on algae, while others take any type of food, including mollusks, worms, other crustaceans, fungi, bacteria, and detritus, depending on availability and on the species of crab. For many species, a mixed diet of plant and animal matter results in the fastest growth and greatest fitness.

Crab fishery

Crabs make up 20% of all marine crustaceans caught and farmed worldwide, with over 1½ million tonnes being consumed annually. Of that total, one species accounts for one fifth: Portunus trituberculatus. Other important taxa include Portunus pelagicus, several species in the genus Chionoecetes, the blue crab (Callinectes sapidus), Charybdis spp., Cancer pagurus, the Dungeness crab (Cancer magister) and Scylla serrata, each of which provides more than 20,000 tonnes annually .

Evolution and classification

The infraorder Brachyura contains about 93 families, as many as the remainder of the Decapoda. The evolution of crabs is characterised by an increasing robustness of the body, and a reduction in the abdomen. Although other groups have also undergone similar processes of carcinisation, it is most advanced in crabs. The telson is no longer functional in crabs, and the uropods are absent, having probably evolved into small devices for holding the reduced abdomen tight against the sternum.

In most decapodes, the gonopores (sexual openings) are found on the legs. However, since crabs use the first two pairs of pleopods (abdominal appendages) for sperm transfer, this arrangement has changed. As the male abdomen evolved into a narrower shape, the gonopores have moved towards the midline, away from the legs, and onto the sternum. A similar change occurred, independently, with the female gonopores. The movement of the female gonopore to the sternum defines the clade Eubrachyura, and the later change in the position of the male gonopore defines the Thoracotremata. It is still a subject of debate whether those crabs where the female, but not male, gonopores are situated on the sternum form a monophyletic group.

The earliest unambiguous crab fossils date from the Jurassic, although the Carboniferous Imocaris, known only from its carapace is thought to be a primitive crab. The radiation of crabs in the Cretaceous and afterwards may be linked either to the break-up of Gondwana or to the concurrent radiation of bony fish, the main predators of crabs.

About 850 species of crab are freshwater or (semi-)terrestrial species; they are found throughout the world's tropical and semi-tropical regions. They were previously thought to be a closely related group, but are now believed to represent at least two distinct lineages, one in the Old World and one in the New World.

Cultural influences of the crab

The Moche people of ancient Peru worshipped nature, especially the sea. They placed emphasis on animals and often depicted crabs in their art.

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