Crab

Crab (Ger. Krabbe, Krebs), a name applied to the Crustacea of the order Brachyura, and to other forms, especially of the order Anomura, which resemble them more or less closely in appearance and habits.

The Brachyura, or true crabs, are distinguished from the long-tailed lobsters and shrimps which form the order Macrura, by the fact that the abdomen or tail is of small size and is carried folded up under the body. In most of them the body is transversely oval or triangular in outline and more or less flattened, and is covered by a hard shell, the carapace. There are five pairs of legs. The first pair end in nippers or chelae and are usually much more massive than the others which are used in walking or swimming. The eyes are set on movable stalks and can be withdrawn into sockets in the front part of the carapace. There are six pairs of jaws and foot-jaws (maxillipeds) enclosed within a “buccal cavern,” the opening of which is covered by the broad and flattened third pair of foot-jaws. The abdomen is usually narrow and triangular in the males, but in the females it is broad and rounded and bears appendages to which the eggs are attached after spawning (fig. 1).  
  
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Fig. 1.—Side view of Crab (Morse), the abdomen extended and carrying a mass of eggs beneath it; e, eggs.  
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Fig. 2.—Zoëa of Common Shore-Crab in its second stage. r, Rostral spine; s, Dorsal spine; m, Maxillipeds; t, Buds of thoracic feet; a, Abdomen. (Spence Bate.)  
  
As in most Crustacea, the young of nearly all crabs, when newly hatched, are very different from their parents. The first larval stage is known as a Zoëa, this name having been given to it when it was believed by naturalists to be a distinct and independent species of animal. The Zoëa is a minute transparent organism, swimming at the surface of the sea. It has a rounded body, armed with long spines, and a long segmented tail. The eyes are large but not set on stalks, the legs are not yet developed, and the foot-jaws form swimming paddles. After casting its skin several times as it grows in size, the young crab passes into a stage known as the Megalopa (fig. 2), also formerly regarded as an independent animal, in which the body and limbs are more crab-like, but the abdomen is large and not filled up. After a further moult the animal assumes a form very similar to that of the adult. There are a few crabs, living on land or in fresh water, which do not pass through a metamorphosis but leave the egg as miniature adults.

Most crabs live in the sea, and even the land-crabs, which are abundant in tropical countries, nearly all visit the sea occasionally and pass through their early stages in it. Many shore-crabs living between tide-marks are more or less amphibious, and the river-crab of southern Europe or Lenten crab (Potamon edule, better known as Thelphusa fluviatilis) is an example of the freshwater crabs which are abundant in most of the warmer regions of the world. As a rule, crabs breathe by gills, which are lodged in a pair of cavities at the sides of the carapace, but in the true land-crabs the cavities become enlarged and modified so as to act as lungs for breathing air.

Walking or crawling is the usual mode of locomotion, and the peculiar sidelong gait familiar to most people in the common shore-crab, is characteristic of most members of the group. The crabs of the family Portunidae, and some others, swim with great dexterity by means of their flattened paddle-shaped feet.

Like many other Crustacea, crabs are often omnivorous and act as the scavengers of the sea, but many are predatory in their habits and some are content with a vegetable diet.

Though no crab, perhaps, is truly parasitic, some live in relations of “commensalism” with other animals. The best known examples of this are the little “mussel-crabs” (Pinnotheridae) which live within the shells of mussels and other bivalve mollusca and probably share the food of their hosts. Some crabs live among corals, and one species at least gives rise to hollow swellings on the branches of a coral like the “galls” which are formed on plants by certain insects. Another crab (Melia tesselata) carries in each of its claws a living sea-anemone which it uses as an animated weapon of defence and an implement for the capture of prey. Many of the sluggish spider-crabs (Maiidae) have their shells covered by a forest of growing sea-weeds, zoophytes and sponges, which are “planted” there by the crab itself, and which afford it a very effective disguise.

Many of the larger crabs are sought for as food by man. The most important and valuable are the edible crab of British and European coasts (Cancer pagurus) and the blue crab of the Atlantic coast of the United States (Callinectes sapidus).

Among the Anomura, the best known are the hermit-crabs, which live in the empty shells of Gasteropod Mollusca, which they carry about with them as portable dwellings. In these, the abdomen is soft-skinned and spirally twisted so as to fit into the shells which they inhabit. The common hermit-crab of the British coasts (Pagurus or Eupagurus Bernhardus) is sometimes called the soldier-crab from its pugnacity. Small specimens are found between tide-marks inhabiting the shells of periwinkles and other small molluscs, but the full-grown specimens live in deeper water and are usually found in the shell of the whelk (Buccinum). As the crab grows it changes its dwelling from time to time, often having to fight with its fellows for the possession of an empty shell. Sometimes an annelid worm lives inside the shell along with the hermit and often the outside is covered with zoophytes. In some species, as in the British Eupagurus prideauxi, a sea-anemone is constantly found attached to the shell, profiting by the active locomotion of the crab and probably sharing the crumbs of its food, while it affords its host protection by its stinging powers.

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Fig. 3.—Gecarcinus ruricola (Violet Land Crab). | Fig. 4.—Portunus puber (Velvet Swimming Crab).  
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Fig. 6.—Eupagurus Bernhardus (Soldier Crab). | Fig. 5. Podophthalmus vigil (Sentinel Spinous Crab). | Fig. 7.—Pinnotheres pisum (Pea Crab).  
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Fig. 8.—Corystes Cassivelaunus (Masked Crab). | Fig. 9.—Eupagurus angulatus (a Hermit Crab).  
  
In tropical countries the hermit-crabs of the family Coenobitidae live on land, often at considerable distances from the sea, to which, however, they return for the purpose of hatching out their spawn. The large robber-crab or cocoa-nut crab of the Indo-Pacific islands (Birgus latro), which belongs to this family, has given up the habit of carrying a portable dwelling, and the upper surface of its abdomen has become covered by shelly plates. The stories of its climbing palm-trees to get the fruit were long doubted, but it has been seen, and even photographed in the act.

(W. T. Ca.)

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