Oyster
From Wikipedia, the free encyclopedia
-
For other uses, see Oyster (disambiguation).
The name oyster is used for a number of different groups of molluscs which grow for the most part in marine or brackish water. The shell, usually highly calcified, surrounds a soft body. Gills filter plankton from the water, and strong adductor muscles are used to hold the shell closed. Some of these groups are highly prized as food, both raw and cooked.
Contents |
[edit] 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. virginica).
[edit] Physical characteristics
- Oysters are filter-feeders that draw water in over their gills through the beating of cilia. Suspended food plankton and particles are trapped in the mucus of the gills and transported to the mouth, where they are eaten, digested and expelled as feces or pseudofeces. 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. Scientists believe that the Chesapeake Bay's once-flourishing oyster populations historically filtered the estuary's entire water volume of excess nutrients every three or four days. Today that process would take almost a year, and sediment, nutrients, and algae can cause problems in local waters. 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. The mantle is lined with many small, thin-walled blood vessels which extract oxygen from the water and expel carbon dioxide. A small, three-chambered heart, lying under the abductor muscle, pumps colorless blood, with its supply of oxygen, to all parts of the body. At the same time two kidneys located on the underside of the muscle purify the blood of any waste products they have collected.
- 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. The gonads, organs responsible for producing both eggs and sperm, surround the digestive organs and are made up of sex cells, branching tubules and connective tissue.
[edit] Oyster habitat and lifestyle
As a keystone species, oysters provide habitat for an extensive array of marine life. The native oyster usually inhabits water depths of between 8 and 25 feet. The hard surfaces of oyster shells and the nooks between the shells provide places where a host of small animals can live. Hundreds of animals such as anemones, barnacles, and hooked mussels use oyster reefs as habitat. Many of these animals serve as food for larger animals, including striped bass, black drum and croakers. An oyster reef, with its many convolutions, can encompass 50 times the surface area of an equally extensive flat bottom. The oyster contributes to improved water quality through its filter feeding capacity. An oyster's mature shape often depends on the type of bottom to which it originally attached. It orients itself with its outer, flared shell tilted upward. One valve is cupped and the other is flat. The submerged shell opens periodically to permit the oyster to feed.
Oysters usually mature by one year of age. They are protandric, which means that during their first year they spawn as males (releasing sperm into the water). As they grow larger over the next two or three years and develop greater energy reserves, they release eggs, as females. Bay oysters are usually prepared to spawn by the end of June. An increase in water temperature prompts a few initial oysters to spawn. This triggers a spawning 'chain reaction', which clouds the water with millions of eggs and sperm. A single female oyster can produce up to 100 million eggs annually. 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. Attached oyster larvae are called 'spat'. 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."
[edit] 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. Preparation can be as simple as opening the shell, while cooking can be as spare as adding butter and/or salt, or can be very elaborate. Perhaps the definitive work on oysters as food is Consider the Oyster, by M. F. K. Fisher.
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; however, they should be consumed when fresh, as their taste reflects their age. Precautions should be taken when consuming them (see below). 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. The texture is soft and fleshy, but crisp to the tooth. This is often influenced by the water that they are grown in with variations in salinity, minerals, and nutrients.
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. To confirm if an open oyster is dead, tap the shell. 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). Inexperienced shuckers tend to apply excessive force, which may result in injuries if they slip. Always use a heavy glove; 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. There is also a second way in, referred to as the "sidedoor", which is about halfway along one side where the lips of the oyster widen so there is a slight indentation where a knife may successfully be inserted. This is generally a better way to open an oyster when it is a "crumbler" (i.e. one with a particularly soft shell either due to drills or the amount of calcium in the water). Either way, however, is tricky when an oyster's shell is in such a condition.
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; additionally their meats are much reduced in size. 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.[1]
Oysters are sometimes cited as an aphrodisiac.[2] It is disputed whether this is true. If there is such an effect, it may be due to the soft, moist texture and appearance of the oyster; it may also be due to their high zinc content. Another joking theory states, "If you can get a woman to eat a raw oyster, you can get her to do anything!" referring to their visual nature, unappealing for some people.
[edit] 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. The borough of Colchester (formerly the capital of Roman Britain) holds an annual Oyser Feast in October of each year, at which the "Colchester Natives" (the native oyster, Ostrea Edulis) are consumed. 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. Using his hydraulic knowledges, he built a complex breeding system including channels and preys to control sea tides. He was famous because of this, and Roman people used to say he could breed oysters on the roof of his house.[3]
In the early nineteenth century, oysters were very cheap and were mainly eaten by the working classes. However, increasing demands from the rapidly-growing cities led to many of the beds running short. To increase production, foreign varieties were introduced and this soon brought disease which, combined with pollution, and increasing sedimentation resulted in oysters becoming rare. This has been exacerbated worldwide by ever-increasing demands on wild oyster stocks.[4] This scarcity increased prices leading to their current status as a delicacy.
In the United Kingdom, the native variety is still held to be the finest, taking five years to mature and protected by an Act of Parliament during the May-August spawning season. The current market is dominated by the larger Pacific oyster and rock oyster varieties which are farmed all year round.
[edit] 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. The largest pearl-bearing oyster type is the Pinctada maxima, which is roughly the size of a dinner plate. 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). Over the years, the grit is covered with enough nacre to form what we know as a pearl. There are many different types and colours and shapes of pearl, but this depends on the pigment of the nacre and the shape of the piece of grit being covered over.
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. These pearls are not as valuable as natural pearls, but look exactly the same.
[edit] 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.
[edit] 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 Pilgrim oyster, a kind of scallop.
- the Saddle oyster (Anomia ephippium)
[edit] Other uses of the word oyster
- The Rocky Mountain oyster is not a mollusc but the testicles of a bull. It is also known as the Prairie oyster.
- There are several plants whose common name is "Oyster plant", for example the Purple Salsify.
- The Oyster Mushroom is a variety of mushroom available in most grocery stores in America. It is so named due to its shape and appearance.
- Oyster card - a form of electronic ticketing designed for use on Transport for London and National Rail services within the Greater London area of the United Kingdom.
- Chicken Oyster - is a piece of chicken which is oyster shaped and sits at the top of the muscle of the leg.
[edit] See also
[edit] External links
[edit] References
- ^ Nell J. A. (2002). "Farming triploid oysters". Aquaculture 210: 69-88. DOI:10.1016/S0044-8486(01)00861-4.
- ^ Stott, Rebecca (2004). Oyster. The University of Chicage Press. Retrieved on 2006-08-11.
- ^ Holland, Tom (2003). Rubicon.
- ^ Clover, Charles. 2004. The End of the Line: How overfishing is changing the world and what we eat. Ebury Press, London. ISBN 0-09-189780-7
