Oyster, an edible bivalve mollusk (one with a two-pieced shell). The two parts of an oyster shell are unlike in size and shape. The three groups, or genera, of true oysters are distantly related to scallops and mussels. Other mollusks called oysters, such as pearl oysters and Bermuda oysters, are members of related bivalve families.

Oyster shellsOyster shells are gray-white with a rough, uneven texture.

Oysters live attached to the ocean bottom in hard-surfaced areas called oyster beds. These beds are located in deep or shallow ocean water or in the brackish water of river estuaries. They are found in all temperate and tropical oceans. Humans have eaten oysters since prehistoric times and have cultivated oyster beds and raised oysters for at least 2,000 years.

Oysters are thin and less flavorful than usual during the period from May to August—their reproductive season. This fact might account for the false belief that it is unsafe to eat oysters during months having no “r's” in their names.

Oysters vary in length from 2 inches (5 cm) to more than 12 inches (30 cm). Oysters of one group produce roundish, flat shells; those of the other two groups produce elongated, deeply cupped shells. Oyster shells are gray or whitish with an uneven texture. Some species produce shells marked with purple or red. Oyster shells are composed of calcium carbonate, the main constituent of limestone.

An oyster's body occupies the deeper of the two shells. It is surrounded by a fleshy layer of tissue called the mantle. The mantle is folded into three distinct layers: the inner layer, containing muscles; the middle layer, containing two rows of sensory organs used to detect chemical and temperature changes; and the outer layer, containing the shell-secreting membrane.

An oyster holds the parts of its shell together by contracting the centrally located adductor muscle. When the oyster relaxes, its shell is opened by a hingelike elastic horny ligament in the narrow end of the shell.

A pair of gills is located under the mantle. It is composed of layers of folded filaments, giving it a pleated appearance. The gills are used for gathering food and for respiration. At the forward end of the gills is the mouth, which is surrounded by four fleshy lobes called palps. An oyster has neither head nor brain. Instead, it has two nerve centers, one that controls the mouth and mantle, and one that controls the internal organs.

Oysters are preyed upon by marine snails, such as oyster drills and sting winkles. These mollusks drill through the shell and eat the oyster. Other dangers to oyster beds are the pink slipper, a mollusk that occupies space in the beds and crowds out the oysters, and various disease-causing protozoan parasites. Man has depleted oyster beds by overfishing and by polluting the water with industrial wastes.