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(oysters), a family of bivalve mollusks comprising approximately 25 genera. Oysters vary in size; individuals of some species may attain a height of 45 cm, for example, Crassostrea gigas. The shell may be rounded, triangular, or elongate and wedge-shaped. The left (lower) valve, which concresces to a solid substrate, is the more convex of the two. The surface of the shell usually consists of concentric, raised thickened plates and sometimes radial ribs or folds. The hinge has no teeth. The valves are joined by an internal ligament, and a single adductor muscle passes through the central part of the body. The intestine does not penetrate the pericardium. The foot and byssus gland are reduced, owing to the concrescence of the mollusk to the substrate.
Oysters are filter feeders; an individual of the species Ostrea edulis can filter 1–3 liters of water per hour, depending on the temperature. Oysters may be dioecious or hermaphroditic. Large individuals produce several million eggs. The larvae are plank-tonic for ten to 15 days. They then attach themselves to a substrate and are referred to as spat. Most species are distributed in tropical and subtropical seas. In temperate waters they are found only where the summer temperature of the water attains 16°C, a necessary condition for reproduction. Oysters live betweeen tidal levels and in littoral shallows; however, some species are found at depths to 60 m. A number of species form solid banks, for example, Ostrea edulis in the Black Sea and Crassostrea gigas in the southern part of the Sea of Japan. Many species can tolerate severe desalination. The principal enemies of oysters are some species of starfishes, gastropod mollusks, and boring sponges.
Some species, including O. edulis, C. gigas, and C. virginica, have commercial importance. They are eaten fresh (live) and canned. The average composition of the flesh is 11 percent proteins, 2 percent fats, 6 percent carbohydrates, 3 percent ash, and 78 percent water. Oyster flesh also contains vitamin C and B-group vitamins. In European countries, the USA, Japan, and other countries, oysters are raised on farms. In France the annual harvest exceeds 1 billion. In the USSR there are large reserves of oysters in the Black Sea and the Sea of Japan.
REFERENCESRukovodstvo po zoologii, vol. 2. Moscow-Leningrad, 1940.
Razin, A. I. “Morskie promyslovye molliuski Iuzhnogo Primor’ia.” Izv. Tikhookeanskogo n.-i. in-ta rybnogo khoziaistva i okeanografii, 1934, vol. 8.
Gonge, C. M. Oysters. London, 1960.
Treatise on Invertebrate Paleontology, part 6: “Mollusca.” (Bivalvia, vol. 3.) [Kansas City, Mo.] 1971.
O. A. SKARLATO