oyster,bivalve mollusk found in beds in shallow, warm waters of all oceans. The shell is made up of two valves, the upper one flat and the lower convex, with variable outlines and a rough outer surface. Since the wild oyster spends most of its life (except for the free-swimming larval stage) attached—having fused its valve with a sticky substance to a substratum of shells, rocks, or roots—the foot is rudimentary.
In some species the sexes are separate and the eggs are laid and fertilized in the water; in others the animal is hermaphroditic and the eggs are retained with the shell. Only a small proportion of the millions of eggs laid survive. Large numbers of the free-swimming larvae, called veligers, are consumed by fish and other animals. After the oyster becomes sessile, it may be victimized by oyster drills, starfish, and other enemies.
Edible oysters belong to the family Ostreidae, the true oysters. The eastern oyster, Crassostrea virginica, reaches a length of 4 to 6 in. (10–15 cm). These oysters are harvested in artificial beds on both coasts of the United States: on the Atlantic especially in the regions of the Delaware and Chesapeake bays and in the waters off Long Island, in the Gulf Coast off Louisiana, and in the Pacific off the state of Washington. Prepared beds are usually seeded with veligers or young sessile oysters called spats. In warm waters they mature in 11/2 years; in cooler waters the period of growth is about 4 to 5 years. They are usually transplanted several times before harvest to enhance their food supply and stimulate growth. Oysters are also farmed in mesh cages, trays, and bags (which may have floats incorporated), on ropes to which they have been attached, or in special tanks. The Pacific oyster, C. gigas, native to the coast of Asia and farmed there, also is farmed in the U.S. Northwest, Europe, and Australia and New Zealand. The edible oysters of the genus Ostrea, such as the Belon and Olympia oysters, are less significant commercially than they once were, due to overharvesting and disease.
The wing and the pearl oysters, of the family Pteriidae, are widespread in warmer seas; there is one eastern and one western species of each in American waters. The great pearl oyster, from which the pearl is obtained, is a large (12-in./30.5-cm) tropical species. The familiar jingle shells, delicate, shiny orange or yellow shells common on beaches, belong to the same order as the oyster.
Oysters are classified in the phylum Mollusca, class Pelecypoda or Bivalvia.