An oyster is a marine bivalve mollusk. Several of the 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. They are unable to move, but are often disloged from their resting place by waves. The shell consists of a left and right valve (bi-valve) joined together at the narrow 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 alves apart and the oyster opens.A short gullet connects the mouth to the stomach. The body also contains the digestive, reproductive, circulatory, excretory, and nervous system.

The oyster feeds on microorganisms that are brought into the shell with the current. They then filter out the food that they want and eliminate (not excrete) the rest.

Oysters have varying life processes. The oysters of the American Pacific coast are hermaphrodites-that is,their reproductive organs contain both eggs and sperm. The eggs are fertilzed within the body (internal fertilization) and are kept 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, wher fertiliztion occurs (external fertilization). 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.

Now the really important question. Why do we only eat oyster in the months that have R's in them? The answer to this also has something to do with an oysters sex life. During the month without R's, the adult oyster has littleinterest in sex and,in fact, even less wo work with. During cold weather the oysters body weight is made up of the glycogen and salts that make it fat and tasty. But in the late spring, the oyster's fancy lightly tunrns to reproduction and it assumes a sex for the season, or at least part of it. It may be either sex, primarily. Having made a choice of which sex it will be, the oyster converts 80 percent of its body weight to sex organs, which are thin and watery and taste like nothing at all. It isn't that oysters are inedible during the summer; just that they aren't as meaty and succulent. Then too, it;s just as well to leave them alone toget on with the business of making more oysters while thay are in the mood. In the fall they lose interst in reproduction and reconvert to glycogen and salts, leaving there gender up for grabs again.

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 30m (100 ft). Beds of American bluepont 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.

Oysters make up the familyl Osteidae of the order Ostreoda. Pearl oysters belong to the family Pteriidae of the order Arcidae. The American bluepont oyster is classified as Ostrea virginica.

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