oyster

Edible shellfish with a rough, irregular hinged shell, found on the sea bottom in coastal areas. Oysters are bivalve molluscs; the upper valve (shell) is flat, the lower hollow, like a bowl, and the two are hinged by an elastic ligament. The mantle, a protective layer of skin, lies against the shell, shielding the inner body, which includes the organs for breathing, digesting food, and reproduction. Oysters commonly change their sex once a year, sometimes more often; females can release up to one million eggs during a spawning period. (Family Ostreidae.)

Oysters have been considered a delicacy since ancient times. Among the species commercially exploited for food today are the North American eastern oyster (Crassostrea virginica) of the Atlantic coast and the European oyster (Ostrea edulis). The former is oviparous (eggs are discharged straight into the water) and the latter is larviparous (eggs and larvae remain in the mantle cavity for a period before release). Oyster farming is increasingly practised, the beds being specially cleansed for the easy setting of the free-swimming larvae (known as ‘spats’), and the oysters later properly spaced for growth and fattened.

Valuable pearls are not obtained from members of the true oyster family; they occur in pearl oysters (family Pteriidae). There are also tree oysters (family Isognomonidae) and thorny oysters (family Spondylidae).