Debbie Burcham, age 15, of Keen, N.H., for her question:
HOW DO OYSTERS REPRODUCE?
An oyster is a marine bivalve mollusk. There are more than 50 species and most are used for food. Certain inedible species known as pearl oysters belong to a separate family. Oysters have varying life processes of reproduction.
The European oyster and the Olympia oyster of the American Pacific coast are hermaphrodites, which means 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 and the Portuguese oyster, the sexes are separate. Females produce many millions of eggs that are discharged 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 of the oyster varies according to the latitude of the habitat.
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 100 feet.
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 the world, although many of its oyster beds have been depleted through overfishing or pollution.
Large beds of edible oysters also exist in Japan. The Japanese oyster was successfully introduced to the Pacific Coast of North America and to Australia. The native American Olympia oyster is much smaller and has a thinner shell than the Japanese oyster.
Oyster culture is practiced in many countries. Young oysters, called seed oysters, are placed on suitable bottoms and provided with artificial collectors, such as tile or shells. The most elaborate systems of oyster cultivation is practiced in Japan, France and the Netherlands.
Oysters attach themselves to rocks or lie on the sea bottom. They are unable to move, but are often dislodged from their resting places 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 valve. 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 soft body of the oyster and adhere closely to the inner surfaces 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.