king crab: see crab; horseshoe crab.

horseshoe crab, large, primitive marine arthropod related to the spider, sometimes called a king crab (a name also used for the largest of the edible true crabs). The heavy dark brown exoskeleton, or carapace, is domed and shaped like a horseshoe. The body is divided into a broad, flattened, semicircular front part (the prosoma), a tapering middle part (the opisthosoma), and a pointed, spiky taillike part (the telson).

Horseshoe crabs have no jaws, and the mouth is flanked by a pair of pincerlike chelicera that are used to crush worms and other invertebrates taken as food. Five pairs of walking legs attached to the prosoma enable the animals to swim awkwardly or burrow through the sand or mud. The respiratory organs are called book gills and are unique to horseshoe crabs. Each book gill is made of about 100 thin leaves, or plates; these are fitted like pages of a book onto one pair of flaplike appendages on the opisthosoma. Rhythmic movement of the appendages circulates water over the gill surfaces and drives blood into and out of the gill leaves.

Horseshoe crabs first appeared in the Upper Silurian period, and a number of fossil species have been described. Five species still survive; four of these are found along the Pacific coast of Asia. The American species, Limulus polyphemus, is common along the Atlantic coast from Nova Scotia to Florida. It lives in shallow water, preferring soft or sandy bottoms, and reaches a maximum length of nearly 2 ft (61 cm).

Horseshoe crabs are considered living fossils; they resemble fossil trilobites and eurypterids of the Paleozoic era. They are classified in the phylum Chelicerata, class Merostomata.

hermit crab, a crustacean distinguished from true crabs by its long, soft, spirally coiled abdomen terminating in an asymmetrically hooked tail. Most hermit crabs protect this vulnerable portion of their bodies by occupying the empty shells of periwinkles, whelks, and other gastropod mollusks. A few find other homes; for example, a species that inhabits the Indian Ocean lives in sections of old bamboo cane. When the hermit crab grows out of one shell it seeks a larger one, fighting for it if challenged. Sea anemones often attach themselves to these shells, obtaining free transportation and scraps of food in return for protecting their hosts. Hermit crabs are common beach scavengers in most parts of the world. Most species are marine, but some tropical forms, such as the robber crab, Birgus latro, are largely terrestrial. This species, the largest hermit crab, generally reaches over 1 ft (30 cm) in length. It becomes increasingly terrestrial and develops heavy armor as it matures into an adult, at which stage it is able to completely discard its adopted shell. With its great pincers the robber crab has been known to crack coconuts, which it obtains by climbing palm trees. Hermit crabs are classified in the phylum Arthropoda, subphylum Crustacea, order Decapoda.

fiddler crab, common name for small, amphibious crabs belonging to the genus Uca. They are characterized by a rectangular carapace (shell) and a narrow abdomen, which is flexed under the body. They are called fiddler crabs because the males have one enormous claw, held in front of the body like a fiddle. This claw often contrasts in color with the rest of the body, and is used by the male at certain times of the year to attract females in a complicated courtship display procedure, characteristically different in each species of Uca. The claw also functions to warn off intruders and to establish territories. The female Uca has two small claws on the first appendages. Some species of fiddler crabs live on sandy beaches that are somewhat protected from extreme wave action. Others live in muddy marshes and estuaries. The Uca species living on sandy beaches, such as the common Atlantic fiddler, U. pugilator, make burrows about 1 ft (30 cm) deep, just below the high tide line. The sand is carried to the surface by specialized legs of the crab, and pushed away from the entrance. Fiddler crabs are poor swimmers and rarely enter the water during their adult lives. During the spring and summer, the fiddlers remain in their burrows only during high tide periods. The entrances of the burrows are covered with sand, and the burrows contain a bubble of air, which the crabs use for respiration. When the tide ebbs, the fiddlers emerge and scurry about, collecting food in the drift lines left by the ebbing water. Both claws of the female and the smaller claw of the male are used to scoop up sand and pass the grains to the mouthparts. Certain specialized appendages (the first and second maxillipeds) have spoon-shaped setae, used to scour organic matter from the sand grains and pass it to the mouth. The sand grains are then rejected in the form of small sand balls. After mating, the female fiddler crab carries the fertilized eggs under her flexed abdomen. Certain cyclic changes occur in Uca (as well as in some other crustaceans), such as changes in pigmentation. During the day the crabs are dark; at night they are pale. Fiddler crabs are classified in the phylum Arthropoda, subphylum Crustacea, order Decapoda.

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.

blue crab, common name for a crustacean, Callinectes sapidus, found on the S Atlantic and Gulf coasts of North America. The blue crab is a member of the family of swimming crabs known as the Portunidae and is characterized by a broad, semitriangular carapace (shell) covering the thorax, by a narrow abdomen tucked under its body, and by five pairs of appendages called pereiopods, of which the first two bear large claws (chelae) and the last two are flattened paddles modified for swimming. It is the most common edible crab of the Atlantic coast, and several million pounds are fished commercially by trapping or trawling each year. It is sold both as the hard-shell variety and as the familiar delicacy known as the soft-shelled crab. In the hard-shell form, the crab is in an intermolt phase (between molts) and the exoskeleton is fully hardened (sclerotized). In its soft-shell stage, the crab is in the phase just after the molt but before the exoskeleton has hardened. Since, in nature, the crab retires to secluded areas at the time of the molt and is thus difficult to collect, commercial fishermen collect the crabs at the so-called peeler stage, which occurs two to three days before the molt. The crabs are then held in pens, on floats in the water, until just after the molt, when they are marketable. The ovaries of the female begin to develop only after mating has taken place. The female carries the young under her abdomen until they hatch as tiny larvae, which are only 1/25 in. (0.1 cm) long. The crabs molt many times and grow to 7 in. (17.8 cm) in about 200 days. Blue crabs are classified in the phylum Arthropoda, subphylum Crustacea, class Malacostraca, order Decapoda, family Portunidae.

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

Crab Nebula, diffuse gaseous nebula in the constellation Taurus; cataloged as NGC 1952 and M1, the first object recorded in Charles Messier's catalog of nonstellar objects. It is the remnant of a supernova that was observed in 1054 by Chinese and Arab astronomers to be as bright as Venus; markings in northern New Mexico depict a star near a crescent moon that might be a record of this supernova. Only three other supernovas have been observed in our galaxy since then. The explosion of the Crab Nebula produced a large expanding shell of delicate filaments. The filaments contain ionized gas in which unusually energetic electrons twist through magnetic fields at speeds close to that of light, emitting synchrotron radiation. Although this radiation is what makes supernova remnants visible in radio wavelengths, in this nebula it is so strong that observers can see the filaments through moderate-sized optical telescopes under good conditions. The nebula is also a strong emitter of X rays. At its center is a pulsar, PSR 0531+21, that spins 30 times per second. The youngest pulsar observed, it gives off radiation at radio, optical, ultraviolet, X-ray, and gammay ray wavelengths, as well as electrons that power the synchrotron radiation in the surrounding nebula.

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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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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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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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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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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