common name for any of several different species of marine bivalve mollusks. Several of the more than 50 living species of oysters are edible. Species known as pearl oysters belong to a separate family.

Oysters attach themselves to rocks or lie on the sea bottom. The two valves of the oyster shell are joined together by a ligament that pulls the valves apart. Attached to both valves is a strong muscle that keeps the shell tightly closed. The gills (respiratory organs) are covered with hairlike structures called cilia that help direct food to the mouth, which is connected to the stomach by a short gullet. Oysters also have digestive, reproductive, circulatory, excretory, and nervous systems.

Oysters have varying methods of reproduction. Some species are hermaphrodites—that is, their reproductive organs contain both eggs and sperm. In other species, the sexes are separate.

Oysters are found throughout the world. They usually form large beds, which extend in warm waters from the tidal zone to a depth of up to 30 m (100 ft). Oyster culture is practiced in many countries. Young oysters, called seed oysters, are placed on suitable bottoms covered with tile or shells.

Scientific classification: Oysters make up the family Ostreidae of the order Ostreoida.