oyster
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What is an oyster?
Oysters are bivalve mollusks found in temperate and warm coastal waters. As filter feeders, they draw water through their gills to extract minute organic particles for food.
True oysters belong to the family Ostreidae and pearl oysters to Aviculidae. Though almost all bivalves are edible, the most important edible oysters come from the genus Crassostrea. Oysters can be eaten raw, cooked, or smoked and have been harvested as food for over 2,000 years.
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What type of animal is an oyster?
An oyster is a bivalve mollusk found in temperate and warm coastal waters of all oceans. As bivalves, they are characterized by a shell divided into left and right valves connected by a hinge. Oysters filter minute organic particles from the water for food using tiny hairlike structures (cilia) on their gills. They are preyed upon by birds, sea stars, snails, and fishes. True oysters belong to the family Ostreidae, while pearl oysters belong to Aviculidae.
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Where do oysters live?
Oysters of the families Ostreidae (true oysters) and Aviculidae (pearl oysters) inhabit temperate and warm coastal waters across the world's oceans. True oysters like the European flat oyster (Ostrea edulis) populate waters from Norway to Morocco, including the Mediterranean and Black Seas. The Olympia oyster (O. lurida) is native to the Pacific coast of North America. The North American oyster (Crassostrea virginica) thrives from the Gulf of Saint Lawrence to the West Indies and has been introduced to the Pacific coast of North America.
Oysters are cultivated in Asia, Europe, and North America. In Japan, oyster farms cultivate seed oysters and raise bivalves for food and export.
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How do oysters get their food?
Oysters are bivalve mollusks that filter feed to obtain their food. They draw water in through tiny, hair-like structures called cilia on their gills using wave-like motions. As much as two to three gallons of water may pass through an oyster in one hour. Oysters filter minute organic particles from the water, which serve as their food source. Filter feeding is a form of food procurement where food particles or small organisms are randomly strained from water. Oysters, in turn, are a food source for birds, sea stars, snails, and fishes.
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What are the different parts of an oyster's body?
An oyster's body consists of several key components:
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Mantle: A soft covering that lines the shell and secretes it. The mantle cavity, formed between the mantle and the body, facilitates water flow and gas exchange.
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Shell: Oysters have two valves (halves) that make up their shell. The upper valve is convex, while the lower valve is flatter and attaches to surfaces. The valves are connected by an elastic ligament and are closed by a large adductor muscle.
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Gills (ctenidia): These structures draw water in through cilia, filtering out minute organic particles for food. They also extract oxygen from the water.
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Visceral mass: Located within the shell, it contains the digestive, reproductive, excretory, and respiratory systems.
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Foot: The foot is reduced in size and positioned anteriorly.
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Adductor muscle: A large central muscle that serves to close the valve against the pull of the ligament.
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How are oysters important to the environment and people?
Oysters play a vital role in their ecosystems and are important to people. They are filter feeders that filter water, which helps to keep their habitats clean. They also provide habitats for other marine life and are a food source for various animals, including birds and fish.
Oysters have been cultivated as food for over 2,000 years and are commercially important in many parts of the world. They are raised on farms and harvested for consumption. Additionally, some oyster species produce pearls, which are highly valued and used in jewelry.
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oyster, any member of the families Ostreidae (true oysters) or Aviculidae (pearl oysters), bivalve mollusks found in temperate and warm coastal waters of all oceans. Bivalves known as thorny oysters (Spondylus) and saddle oysters (Anomia) are sometimes included in the group.
True oysters have been cultivated as food for more than 2,000 years. Pearl oysters also have long been valued for the precious pearls that develop in them. (See also pearl.)
The two valves of the oyster shell, which differ in shape, have rough surfaces that are often a dirty gray. The upper valve is convex, or higher at the middle than at the edges. The lower valve, fixed to the bottom or to another surface, is larger, has smoother edges, and is rather flat. The inner surfaces of both valves are smooth and white.
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The valves are held together at their narrow ends by an elastic ligament. A large central muscle (adductor muscle) serves to close the valve against the pull of the ligament. As the valves are held slightly open, tiny hairlike structures (cilia) on the gills draw water inward by means of wavelike motions. Two to three gallons may pass through the oyster in an hour. Minute organic particles, filtered from the water, serve as food.
Oysters, in turn, are eaten by birds, sea stars, and snails, as well as by fishes. The oyster drill (Urosalpinx cinenea), a widely occurring snail, drills a tiny hole through the oyster shell, then sucks out the living tissue.
Like other bivalves, most oysters are either male or female, although hermaphroditism also occurs. Ostrea edulis exhibits a phenomenon called sequential hermaphroditism, in which an individual alternates sexes seasonally or with changes in water temperature. Oysters breed in the summer. The eggs of some species are released into the water before fertilization by the sperm; the eggs of others are fertilized within the female. The young are released as ciliated larvae known collectively as veligers, which swim for several days before permanently attaching themselves to a site and metamorphosing. Edible oysters are ready for harvesting in three to five years.
True oysters (family Ostreidae) include species of Ostrea, Crassostrea, and Pycnodonte. Common Ostrea species include the European flat, or edible, oyster, O. edulis; the Olympia oyster, O. lurida; and O. frons. Crassostrea species include the Portuguese oyster, C. angulata; the North American, or Virginia, oyster, C. virginica; and the Japanese oyster, C. gigas. Pearl oysters (family Aviculidae) are mostly of the genus Meleagrina, sometimes called Pinctada or Margaritifera.
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O. edulis occurs from the coast of Norway to waters near Morocco, through the Mediterranean Sea, and into the Black Sea. It is hermaphroditic and attains lengths of about 8 cm (about 3 inches). O. lurida, of the Pacific coastal waters of North America, grows to about 7.5 cm (3 inches). C. virginica, native to the Gulf of Saint Lawrence to the West Indies and about 15 cm (6 inches) long, has been introduced into Pacific coastal waters of North America. Up to 50,000,000 eggs may be released by the female at one time. Commercially, C. virginica is the most important North American mollusk. C. angulata occurs in coastal waters of western Europe. C. gigas, of Japanese coastal waters, is among the largest oysters, attaining lengths of about 30 cm (1 foot). Like C. virginica, the Sydney rock oyster (Crassostrea commercialis) changes sex; born male, it changes to female later in life. It is the most economically important Australian edible oyster.
Oysters are shucked and eaten raw, cooked, or smoked. Popular varieties include the blue point and lynnhaven—forms of C. virginica (harvested, respectively, from the Blue Point, Long Island, and Lynnhaven Bay, Va., regions); as well as the colchester of Britain and the marennes of France. The colchester and marennes are forms of O. edulis.
Pearls are formed in oysters by the accumulation of nacre, the material lining the oyster shell, around a solid piece of foreign matter that has become lodged inside the shell. Pearls formed in edible oysters are lustreless and of no value. The best natural pearls occur in a few Oriental species, particularly Meleagrina vulgaris, native to the Persian Gulf. This species is found mainly at depths of 8 to 20 fathoms (48 to 120 feet). Pearls are taken mostly from oysters more than five years old. Cultured pearls are grown around bits of mother-of-pearl inserted manually into the oyster. Most cultured pearls are grown in Japanese or Australian coastal waters.