Oyster

I

Introduction

Oyster, 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 (see Pearl).

II

Structure

Oysters attach themselves to rocks or lie on the sea bottom. They are unable to move but are often dislodged from their resting place by waves. The shell of the oyster is irregularly oval in shape. It consists of a left and right valve joined together at the narrow anterior end by an elastic ligament that acts as a hinge. Attached to both valves is a strong muscle called the adductor, which keeps the shell tightly closed. When the adductor relaxes, the elastic ligament pulls the valves apart. The left valve, upon which the oyster rests, is deeper and thicker than the right one. Except for the dark, pigmented areas where the shell is connected to the adductor, the inner surfaces of the valves are white. Two folds of fleshy membrane, called the mantle, cover the oyster's soft body and line the inside of the shell. The mantle secretes the organic and inorganic substances that make up the shell. At the anterior end of the body, between two pairs of thin lips, or palps, is an opening that constitutes the mouth of the oyster. Two pairs of sickle-shaped respiratory organs, the gills, are covered with hairlike structures called cilia. A short gullet connects the mouth to the stomach. The body also contains the digestive, reproductive, circulatory, excretory, and nervous systems.

The oyster feeds on microorganisms that are brought into the shell with the current produced by the movement of the cilia and sorted out by the labial palps before they reach the mouth.

III

Reproduction

Oysters have varying methods of reproduction. The European oyster and the Olympia oyster of the American Pacific Coast are hermaphrodites—that is, their reproductive organs contain both eggs and sperm. The eggs are fertilized within the body and are retained in the gills until shell-bearing larvae are formed. In the American bluepoint oyster of the Atlantic Coast, the sexes are separate. Females discharge millions of eggs into the water, where fertilization occurs. The larvae develop within six hours, swim actively for about two or three weeks, and then settle on stones or shells, where they mature by the end of the first year. The reproductive season varies according to the latitude of the habitat.

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IV

Distribution

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). Beds of American bluepoint oysters are found along the eastern coast of the continent. Chesapeake Bay is the largest oyster-producing body of water in the world, although many of its oyster beds have been depleted through overfishing or pollution. Large beds of edible oysters also exist in Japan and Australia. The native American Olympia oyster, which is much smaller and has a thinner shell than other edible species, is found on the west coast of North America.

Oyster culture is practiced in many countries. Young oysters, called seed oysters, are placed on suitable bottoms provided with artificial collectors, such as tile or shells. The most elaborate system of oyster cultivation is practiced in Japan, France, and the Netherlands.

Scientific classification: Oysters make up the family Ostreidae of the order Ostreoida. The European oyster is classified as Ostrea edulis, the Olympia oyster as Ostrea lurida, and the American bluepoint oyster as Crassostrea virginica. Pearl oysters belong to the family Pteriidae of the order Pterioida.

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