Oysters range from 4 to 6 inches (10-15 cm) in length. The valves (shells) of all oysters are dark gray and rough in texture. They are hinged at the narrow end by a tough elastic ligament, and they are opened and closed by means of a large muscle called the adductor. Within the valves is a soft fleshy tissue, called the mantle, that secretes the shell. Along the mantle are tentacles that detect disturbances in the water.
After a brief swimming stage each little oyster attaches itself by a valve to a solid support and remains in place for life. Oysters inhabit shallow coastal waters in most parts of the world.
Certain species, especially the American oyster (Ostrea tiirgin-ica~), the Olympia oyster (O. lurida), and the Japanese oyster (O. gigas), are widely valued for their meat. Oyster meat is very nutritious. It contains protein, calcium, phosphorus, iron, and iodine, as well as vitamins A, B, C, D, and G. Some species, especially the pearl oysters (Pinctada), produce valuable pearls and are cultivated in large numbers.
During spring or summer, when the water reaches a temperature of about 80° F, male and female oysters shed their sex cells into the water, where fertilization occurs. It is believed that a single female may produce more than 100 million eggs in one season.
Young oysters are free-swimming, microscopic organisms called veliger larvae. Each larva has a tiny clam-shaped shell and a special swimming organ, called the velum, which has many beating cilia. The larva swims about for approximately two weeks and then settles to the bottom. It moves about by means of a fleshy foot, looking for a suitable place to attach itself. When it finds a rock, shell, or other object, the larva secretes a sticky substance, spreads the glue with its foot, and then presses its left valve against it.
After having attached itself, the young oyster is called a spat. A spat measures about a twentieth of an inch (1.27 mm) in diameter, but it grows rapidly and sometimes reaches the size of a quarter within a month. However, the rate of growth varies, depending mostly on the temperature of the water and the amount of food available. In warm regions, oysters mature in about a year and a half, while in cool climates they may take as long as four or five years to become adults.
Oysters have many enemies and rarely reach old age. During the veliger stage many oysters are devoured by fish and other animals. Many veligers also die if they cannot find a suitable place to attach themselves. Sometimes they are crowded out or buried by other oysters, as well as by barnacles and other animals.
One of their most serious enemies is the oyster drill, a sea snail that drills through the oyster's shell and eats the flesh inside. Among the other organisms that kill many oysters are the boring sponge, the pea crab, the mud-blister worm, and the starfish.
Artificial oyster beds were devised as early as the 1st century B.C. Today commercial farms are found in many parts of the world. In the United States the major oyster-producing areas are found along the Atlantic Coast from Rhode Island to Virginia, along the Gulf Coast of Louisiana, and along the coast of the state of Washington.
The ideal site for an oyster farm is a muddy sand bottom. After the site has been chosen, it is covered with shells and other materials on which the veliger larvae can settle. These materials are called cultch. Some farmers raise only those oysters that happen to settle on their cultch, but most farmers purchase large numbers of seed oysters, young oysters that are about an inch (2.5 cm) long. Seed oysters are usually spread evenly over the bottom, but sometimes they are raised in large wire trays suspended in the water. If the oysters are kept off the bottom, they are less likely to be infested with parasites. From 500 to 750 bushels (17,600-26,400 liters) of seed oysters are required to cover each acre.
During their growth, oysters are usually moved from one bed to another. This transplanting often stimulates growth, especially if the new bed has a rich supply of food. Sometimes, oysters are moved to areas where the food supply consists mostly of plant material. Oysters raised in such areas develop a green color, especially around the gills, and many people believe that these oysters have a superior flavor. It takes from two to four years to produce fully grown oysters.
Harvesting usually occurs in late spring. Oysters grown between high and low tidemarks are usually picked by hand. In deeper water, harvesters use long tongs or dredges to collect oysters from the bottom. The cultch, which is picked up along with the oysters, is cleaned and saved for the following year's planting.
After harvesting, the oysters are taken to large plants, where most of them are shucked, or shelled. The empty shells are cleaned and sold as cultch. The meat is washed and sold fresh, or it may be frozen, smoked, or packed in oil. Oysters not removed from the shell are washed, soaked in sterile water, and sold to be served on the half shell.