oyster

oyster, edible 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 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 is victimized by oyster drills, starfish, and other enemies. Most species are too small for food, but the American, or common, oyster reaches a length of 2 to 6 in. (5–15 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 1 1-2 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. The wing and the pearl oysters 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. Oysters are classified in the phylum Mollusca, class Pelecypoda or bivalvia, order Filibranchia, family Ostreidae.

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oyster

European flat oyster (Ostrea edulis)

(credit: G. Tomsich—Photo Researchers)

Any bivalve of two families, Ostreidae (true oysters) or Aviculidae (pearl oysters), found in temperate and warm coastal waters worldwide. Both valves (halves) have a rough, often dirty-gray outer surface and a smooth white inner lining (nacre). The lower valve, which affixes to a surface, is nearly flat. The smaller upper valve is convex and has rougher edges. The oyster filters its food, minute organic particles, from the water. Cultivated as food, oysters are regarded as a delicacy. Pearls are the accumulation of nacre around a piece of foreign matter.