Bivalve mollusc with unequal valves, the left valve typically being cemented to a hard substrate; shell valves closed by a single muscle; often cultured for human consumption, regarded as a delicacy. Some species used for the production of pearls. (Class: Pelecypoda. Order: Ostreoida.)
The name oyster is used for a number of different groups of molluscs which grow for the most part in marine or brackish water.
True oysters
The "true oysters" are the members of the family Ostreidae, and this includes the edible oysters, which mainly belong to the genera Ostrea, Crassostrea, Ostreola or Saccostrea. Examples are the Edible Oyster, Ostrea edulis, Eastern Oyster Crassostrea virginica, Olympia Oyster Ostreola conchaphila, Pacific Oyster Crassostrea gigas, Sydney rock oyster Saccostrea glomerata, and the Wellfleet oyster (a variety of C. Feeding activity is greatest in oysters when water temperatures are above 50°F (~10°C). Healthy oysters consume algae and other water-borne nutrients, each one filtering up to five litres of water per hour. Oysters filter these pollutants, and either eat them or shape them into small packets that are deposited on the bottom where they are harmless. Oysters breathe much like fish, using both gills and mantle. There is no way of determining male oysters from females by examining their shells. While oysters have separate sexes, they may change sex one or more times during their life span.
Oyster habitat and lifestyle
As a keystone species, oysters provide habitat for an extensive array of marine life. The hard surfaces of oyster shells and the nooks between the shells provide places where a host of small animals can live.
Oysters usually mature by one year of age. Bay oysters are usually prepared to spawn by the end of June. An increase in water temperature prompts a few initial oysters to spawn. The eggs become fertilized in the water and develop into larvae, which eventually find suitable sites on which to settle, such as another oyster's shell. Spat are oysters 25 mm or less in length.
Some oysters in the tropics grow on mangrove roots and are exposed at low tide making them easy to collect. In Trinidad in the West Indies tourists are often astounded when they are told that "oysters grow on trees."
Culinary oysters
Oysters can be eaten raw, smoked, boiled, baked, fried, roasted, stewed, canned, pickled, steamed, broiled (grilled) or used in a variety of drinks. Perhaps the definitive work on oysters as food is Consider the Oyster, by M.
Oysters are low in food energy; one dozen raw oysters contain approximately 110 calories (0.460 kJ), and are rich in zinc, iron, calcium, and vitamin A.
Unlike most shellfish, oysters can have a fairly long shelf-life: up to around two weeks; Purists insist on eating oysters raw, with no dressing save perhaps lemon juice, vinegar, or cocktail sauce. Raw oysters are regarded like wines in that they have complex flavors that vary greatly among varieties and regions: some taste sweet, others salty or with a mineral flavor, or even like melon.
Oysters are generally an expensive food in places where they aren't harvested, and often they are eaten only on special occasions, such as Christmas. Whether oysters are predominantly eaten raw or cooked is a matter of personal preference. In the United States today, oysters are most often cooked before consumption, but there is also a high demand for raw oysters on the half-shell (shooters) typically served at oyster bars. Canned smoked oysters are also widely available as preserves with a long shelf life. Raw oysters were once a staple food along the East Coast of the US and are still easily found in states bordering the ocean. Oysters are nearly always eaten raw in France.
Fresh oysters must be alive just before consumption. There is a simple criterion: oysters must be tightly closed; oysters that are already open are dead and must be discarded. A live oyster will close and is safe to eat, a dead oyster can also be closed however it will make a distinct noise when tapped and are called "clackers." Opening oysters requires skill, for live oysters, outside of the water, shut themselves tightly with a powerful muscle sealing their fluids. The generally used method for opening oysters is to use a special knife (called an oyster knife, a variant of a shucking knife), with a short and thick blade about 2 inches long, inserting the blade (with some moderate force and vibration if necessary) at the hinge in the rear of the shell, and sliding it upward to cut the adductor muscle (which holds the shell closed). if you don't cut yourself with the knife you can just as easily cut yourself on the oyster shell itself which can be razor sharp.
An alternative to opening raw oysters before consumption is to cook them in the shell – the heat kills the oysters and they open by themselves. Cooked oysters are savory and slightly sweet-tasting, and the varieties are mostly equivalent.
A piece of folk wisdom concerning oysters is that they are best to eat in months containing the letter r. This is because oysters spawn in the warmer months, from roughly May to August in the Northern Hemisphere, and their flavor when eaten raw can be somewhat watery and bland during spawning season; Oysters from the Gulf of Mexico spawn throughout the year, but are delicious cooked or raw.
To avoid spawning, sterile oysters are now cultured by crossbreeding tetraploid and diploid oysters. Because the resulting triploid oyster cannot propagate, the oyster spawning season does not occur.
Oysters are sometimes cited as an aphrodisiac.
History
Within the United Kingdom, the town of Whitstable in the county of Kent is particularly noted for oyster farming from beds on the Kentish Flats that have been used since Roman times. Similarly the seaside resort of Cancale in France is noted for its oysters which also date from Roman times. In fact, Sergius Orata (from Roman Republic) is considered to be the first big merchant of oysters in History. He was famous because of this, and Roman people used to say he could breed oysters on the roof of his house.
In the early nineteenth century, oysters were very cheap and were mainly eaten by the working classes. To increase production, foreign varieties were introduced and this soon brought disease which, combined with pollution, and increasing sedimentation resulted in oysters becoming rare. The current market is dominated by the larger Pacific oyster and rock oyster varieties which are farmed all year round.
Pearl oysters
All oysters (and, indeed, many other shelled mollusks) can secrete pearls, but those from edible oysters have no market value. The Pearl Oysters come from a different family, the Pteriidae (Feathered Oysters). Both cultivated pearls and natural pearls are obtained from these oysters, though some other mollusks, for example freshwater mussels, also yield pearls of commercial value. Not all oysters produce pearls. In fact, in a haul of three tons of oysters, only around three or four oysters produce perfect pearls.
These oysters produce pearls by covering an invading piece of grit with nacre (or as most know it, mother-of-pearl).
Pearls can also be cultivated by pearl farmers placing a single piece of grit, usually a piece of polished mussel shell, inside the oyster. In three to six years, the oyster will produce a perfect pearl.
Dermo
"Dermo" (Perkinsus marinus) is marine disease of oysters, caused by a protozoan parasite. It is a prevalent pathogen of oysters, causing massive mortality in oyster populations and poses a significant economic threat to the oyster industry.
Other molluscs named "oyster"
A number of other molluscs not falling into either of these groups have common names that include the word "oyster", usually because they either taste or look like oysters, or because they yield noticeable pearls. Examples include:
the family Spondylidae, the Thorny Oysters; the Saddle oyster (Anomia ephippium)
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