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Related to Decapoda: order Decapoda, Isopoda, Cirripedia, Eucarida, Dendrobranchiata
an order of invertebrate animals of a subclass of higher Crustacea.
The characteristic feature of Decapoda is the fusion of the maxillary segments of the head with the thoracic segments and the transformation of the three front pairs of thoracic legs into maxillipeds, which help to pass food to the mouth opening. Only the last five pairs of thoracic legs serve for locomotion (hence the name). Decapoda reach large dimensions; the span between the ends of the extended legs of the giant crab (Macrocheira kdmpheri) exceeds 3 m, and the body length of the Madagascar crayfish (Astacoides) reaches 80 cm. The head and thorax are covered above by a single carapace that is concrescent with all of the thoracic segments. The abdomen is often well developed (for example, in the crayfish and shrimp) and may be asymmetric (in hermit crabs), housed within the empty spiral shell of a gastropod; sometimes it is reduced and tucked under the cephalothorax (in crabs). One, two, or three pairs of the gressorial legs may be clawed. The abdominal legs are used for swimming, but in most species the two front pairs of abdominal legs have changed in the males into a copulative apparatus; in the females they hold the eggs.
Decapoda are dioecious. The males are usually larger and stronger than the females. The paired genital openings are located in the male at the base of the last pair of gressorial legs; in the female, at the base of the third pair of gressorial legs. The eggs produce larvae (usually at the zoea stage, but in some species at the nauplius stage). Most freshwater and many marine species have direct development—that is, the eggs produce a completely formed animal.
The order Decapoda contains about 8,500 species, most of them marine; a smaller number live in fresh waters or on dry land. In the seas of the USSR the greatest variety is found in the Sea of Okhotsk (94 species) and the Sea of Japan (85 species); 36 species are known in the Black Sea, 25 in the Barents, 13 in the White Sea, and four each in the Caspian and the Azov. Fresh waters of the Soviet Union contain nine species of crayfish, five species of shrimp, and four species of crab. The common freshwater crab can be found on dry land as well, in damp areas such as the Crimea, the Caucasus, and Middle Asia. Decapoda feed on diverse animal (chiefly invertebrate) and plant life. They are in turn food for many fish, seals, and walruses.
Decapoda are of interest commercially. The annual world catch of various shrimps, crabs, and lobsters is about 1 million tons. In the USSR the Kamchatka crab (related not to the crabs but to the hermit crabs) and certain shrimps and crayfish have the greatest commercial value. A number of freshwater Decapoda are intermediate hosts of flukes (trematodes). The Chinese crab, introduced to Europe accidentally, has multiplied greatly in rivers and has caused serious damage to the fishing industry by eating the fish that are caught in nets, ruining the nets, and destroying dams and other hydrotechnical structures with its burrows.
REFERENCESVinogradov, L. G. “Opredelitel’ krevetok, rakov i krabov Dal’nego Vostoka.” Izv. Tikhookeanskogo nauchno-issledovatel’skogo instituta rybnogo khoziaistva i okeanografii, 1950, vol. 33.
Zhiz’ zhivotnykh, vol. 2. Moscow, 1968.
Balss, H. “Decapoda.” In H. G. Bronn, Klassen und Ordnungen des Tierreichs, vol. 5, book 7. Munich, 1954-57.
IA. A. BIRSHTEIN