Difference between revisions of "Oyster" - New World Encyclopedia

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[[Image:Huitre p1040848.jpg|thumb|250px|''[[Crassostrea gigas]],'' Marennes-Oléron]]
[[Image:Huitre p1040848.jpg|thumb|''[[Crassostrea gigas]]'', Marennes-Oléron]]
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[[Image:Huitre p1040847.jpg|thumb|250px|''Crassostrea gigas,'' Marennes-Oléron]]
[[Image:Huitre p1040847.jpg|thumb|''Crassostrea gigas'', Marennes-Oléron]]
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'''Oyster''' is the common name applied to various [[bivalve]] [[mollusk]]s, and in particular the '''"true oysters"''' comprising family '''Ostreidae''' of the Ostreoida order and the '''pearl oysters''' of family '''Pteriidae''' (syn. Aviculidae) in the Pterioida order. The shell of oysters consists of two usually highly calcified valves (shells) that surround a soft body. Gills [[Filter feeder|filter]] [[plankton]] from the water, and strong [[adductor]] muscles are used to hold the shell closed. Most oysters live in marine habitats or [[brackish water]] and are widely distributed in shallow, warm, coastal waters of the world's oceans.
[[Image:Ostrea edulis Marennes p1050142.jpg|thumb|thumb|''Crassostrea gigas'', Marennes-Oléron, opened]]
 
[[Image:OysterBall.JPG|thumb|thumb|An ostreolith from the Middle [[Jurassic]] Carmel Formation of southern Utah.  This ball of oysters is made up almost entirely of the species ''Liostrea strigilecula''.]]
 
The common name '''oyster''' is used for a number of different groups of [[bivalve]] [[mollusk]]s, most of which live in marine or [[brackish water]]. The shell consists of two usually highly calcified valves which surround a soft body. Gills [[Filter feeder|filter]] [[plankton]] from the water, and strong adductor muscles are used to hold the shell closed.
 
  
Some of the groups known as oysters (true oysters) are highly prized as [[food]], both raw and cooked, but some other groups that are called oysters (such as the pearl oysters) are not widely eaten, at least not in recent times. True (edible) oysters are in fact incapable of making gem-quality pearls, although the opposite idea is a commonly-encountered misapprehension, often seen in illustrations or photographs where an edible oyster shell is mistakenly paired with a gem-quality pearl.
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The true oysters of family Ostreidae include many that are highly prized as [[food]], consumed both raw and cooked. The pearl oysters of the Pteriidae genus ''Pinctada'' are well-known for producing large, commercially valuable [[pearl]]s. On the other hand, the true oysters are incapable of making gem-quality pearls, although the opposite idea is a commonly-encountered misapprehension, often seen in illustrations or photographs where an edible oyster shell is mistakenly paired with a gem-quality pearl.  
  
The oyster is used as a metaphor in an [[idiom]]atic saying, "The world is your oyster," which means that the whole world is laid out before you like a wonderful living buffet.
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In addition to their commercial importance for culinary purposes and for the production of pearls, oysters provide important [[ecology|ecological]] values. Oyster reefs provide [[Habitat (ecology)|habitat]] for many organisms, and the oysters themselves provide food for various [[fish]], [[marine mammal]]s, and [[invertebrate]]s. As filter feeders, they have a remarkable ability to filter water, removing pollutants and excess nutrients.
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Despite these important functions for the [[ecosystem]] and humans, many oyster beds have been plundered without thought to future generations, with the overfishing resulting in serious decline in population size.
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[[Image:Ostrea edulis Marennes p1050142.jpg|thumb|thumb|''Crassostrea gigas,'' Marennes-Oléron, opened]]
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[[Image:OysterBall.JPG|thumb|thumb|An ostreolith from the Middle [[Jurassic]] Carmel Formation of southern Utah. This ball of oysters is made up almost entirely of the species ''Liostrea strigilecula.'']]
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[[Image:OysterBallInterior.jpg|thumb|Ostreolith interior showing an aragonitic shell cast as the nucleus and ''Gastrochaenolites'' borings.]]
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A number of bivalve mollusks other than edible oysters and pearl oysters also have common names that include the word "oyster." Among these are the "gryphaeate oysters" comprising family [[Gryphaeidae]] and the "thorny oysters" comprising family [[Spondylidae]], both of which are placed in the Ostreoida order. The "tree oysters" are part of the Isognomonidae family in the Pterioida order. This article will provide an overview of the Ostreidae (true oysters) and Pteriidae (pearl oysters), with emphasis on the true oysters.  
  
 
==True oysters==
 
==True oysters==
The "true oysters" are members of the family [[Ostreidae]]. This family 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'').  
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The members of the [[family (biology)|family]] Ostreidae of order Ostreoida and class [[Bivalvia]] are the "true oysters," and include all the [[species]] that are commonly eaten under the title "oyster." There are about eight or nine genera in Ostreidae. The [[edible oysters]] 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'').
  
===Physical characteristics===
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===Description===
*Oysters are [[Filter feeder|filter-feeders]]: they 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 from there are transported to the mouth, where they are eaten, digested and expelled as feces or pseudofeces. Feeding activity is greatest in oysters when the water temperatures are above 50°F (10°C). Healthy oysters consume algae and other water-borne nutrients, each one filtering up to five liters 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.
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Like [[scallop]]s (family [[Pectinidae]]), true oysters have a central [[adductor muscle]], which means that the shell has a characteristic central scar, marking its point of attachment. The shell shape tends to be irregular as a result of attaching to other objects. Adult oysters become permanently attached to a hard surface early in development and lack a foot as an adult. An oyster's mature shape often depends on the type of bottom to which it is originally attached, but it always 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.  
  
*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.
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The two shells (valves) of oysters tend to be highly calcified. Members of genus ''Ostrea'' differ from most bivalves by having shells completely comprised of [[calcite]] but with internal muscle scars of [[aragonite|aragonitic]] composition.
  
===Oyster habitat and lifestyle===
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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 adductor muscle, pumps colorless blood, with its supply of oxygen, to all parts of the body. At the same time two [[kidney]]s, located on the underside of the muscle, purify the blood of any waste products they have collected.
As a [[keystone species]], oysters provide [[Habitat (ecology)|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  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 is 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.
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Oysters are [[Filter feeder|filter-feeders]], drawing water in over their gills through the beating of [[cilia]]. Suspended food [[plankton]] and particles are trapped in the mucus of a gill, and from there are transported to the mouth, where they are eaten, digested, and expelled as feces or [[pseudofeces]]. Feeding activity is greatest in oysters when the water temperatures are above 50°F (10°C). Healthy oysters consume algae and other water-borne nutrients, with each filtering up to five liters of water per hour.  
  
Some tropical oysters in a different family, the family Isognomonidae, grow best 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 in the Caribbean, "oysters grow on trees."
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===Habitat and ecology===
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Members of genus ''Ostrea'' generally are subtidal and live continually immersed in sea water. They brood their fertilized eggs for various proportions of the period from fertilization to hatching. They do best in water with a not too thick concentration of [[phytoplankton]].  
  
The oyster's greatest predators include crabs, sea birds, sea stars, and humans.
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Members of genera ''Saccostrea'' and ''Crassostrea'' live mainly in the [[intertidal zone]], broadcast sperm and eggs into the sea, and can thrive in water that is very rich in phytoplankton. One of the most commonly cultivated oysters is ''Crassostrea gigas'', the Pacific or [[Japanese oyster]], which is ideally suited for [[oyster cultivation in seawater ponds]].
  
[[Image:Oysters p1040741.jpg|thumb|left|Raw oysters presented on a plate]]
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Note that some tropical oysters in a different family, the family [[Isognomonidae]], grow best 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 in the Caribbean, "oysters grow on trees."
  
===Oysters as food===
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A group of oysters is commonly called a ''bed.''
Although [[Jonathan Swift]] is often quoted as having said, "He was a bold man that first ate an oyster",<ref>[http://www.starnewsonline.com/apps/pbcs.dll/article?AID=/20041124/MAGAZINE53/41129007/-1/magazine50&template=printart Oyster Heaven]. Wilmington Star-News. Retrieved November 6, 2007.</ref> evidence of oyster consumption goes back into prehistory, as evidenced by [[Midden#Shell middens|oyster middens]] found worldwide. Oysters were an important foodstuff in all coastal areas where they could be found, and oyster fisheries were an important industry where they were plentiful. Overfishing and pressure from diseases and pollution have sharply reduced supplies, but they remain a popular treat, celebrated in [[oyster festival]]s in many cities and towns.
 
  
Oysters are a favorite among exotic foods and research now shows this shellfish to be a rich source of zinc, one of the minerals required for the production of testosterone.
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===Reproduction and life history===
  
====Oyster fishing and oyster cultivation====
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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 [[gonad]]s, organs responsible for producing both eggs and sperm, surround the digestive organs and are made up of sex cells, branching tubules, and connective tissue.
Oysters are fished by simply gathering them from their beds. A variety of means are used. In very shallow waters they can be gathered by hand or with small [[rake (tool)|rakes]]. In somewhat deeper water, long-handled rakes or oyster [[tongs]] are used to reach the beds. Patent tongs can be lowered on a line to reach beds which are too deep to reach directly. In all cases the manner of operation is the same: the waterman scrapes together a small pile of oysters, and then collects these by scooping them up with the rake or tongs.
 
  
In some areas a [[dredge]] is used. This is a toothed bar attached to a chain bag. The dredge is towed through an oyster bed by a boat, picking up those oysters in its path. While dredges collect oysters more quickly, they can be very damaging to the oyster beds, and their use is in general strictly limited. In the state of [[Maryland]], dredging was until 1965 limited to [[sailboat]]s, and even since that date motor power can only be used on certain days of the week. These regulations prompted the development of specialized sailboats (the [[bugeye (boat)|bugeye]] and later the [[skipjack (boat)|skipjack]]) for dredging.
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Both [[oviparous]] (egg bearing) and [[larviparous]] (larvae bearing) species are known within the Ostreidae. Both types are [[hermaphrodite]]s. However, the larviparous species (which include the [[edible oyster]], ''Ostrea edulis'') show a pattern of alternating sex within each individual (sequential hermaphrodites), whereas the oviparous species (such as the [[eastern oyster]], ''Crassostrea virginica'') are simultaneous hermaphrodites, in which an individual may possess fully functional male and female gonads, producing either female or male [[gamete]]s according to circumstances.
 
 
Oysters can also be collected by [[underwater diving|divers]].
 
  
In any case, when the oysters are collected, they are sorted to eliminate dead shells, unwanted catch, and other debris. Then they are taken to market where they are either canned or sold live.
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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.  
  
Oysters have been cultured for well over a century. Two methods are commonly used. In both cases oysters are cultivated to the size of "spat," the point at which they attach themselves to a substrate. They may be allowed to mature further to form "seed" oysters. In either case they are then set out to mature. They may be distributed over existing oyster beds and left to mature naturally, to be collected using the methods for fishing wild oysters. Or they may be put in racks or bags and held above the bottom. The oysters are harvested by lifting the bags or rack to the surface and removing mature oysters. The latter method avoids losses to some predators, but is more expensive.<ref>[http://www.lamer.lsu.edu/classroom/edonahalfshell/pdf/cycle_info.pdf Oyster Farming in Louisiana]. Louisiana State University. Retrieved November 6, 2007.</ref>
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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 millimeters or less in length. Many species of bivalve, oysters included, seem to be stimulated to settle by the proximity of adults of their species.
  
In many areas non-native oysters have been introduced in attempts to prop up failing harvests of native varieties. For example, the eastern oyster was introduced to California waters in 1875, while the Pacific oyster was introduced there in 1929.<ref>Conte, Fred S. [http://aqua.ucdavis.edu/dbweb/outreach/aqua/ASAQ-A07.PDF California Oyster Culture]. University of California, Davis. Retrieved November 6, 2007.</ref> Proposals for further such introductions remain controversial.
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The oyster's greatest predators include [[crab]]s, sea birds, [[sea star]]s, and humans. Some oysters contain live crabs, known as an [[oyster crab]].
  
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.<ref>Nell, J.A. 2002. Farming triploid oysters. ''Aquaculture''. 210:69-88.</ref>
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===Importance===
  
====Preparation and storage====
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====Ecological====
Oysters can be eaten half shelled, 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 [[edible salt|salt]], or can be very elaborate.
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As a [[keystone species]], oysters provide [[Habitat (ecology)|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. Hundreds of animals, such as [[anemones]], [[barnacles]], and hooked [[mussels]] use oyster reefs as habitat. Many of these animals, and the oysters themselves, serve as food for larger animals, including such commercially important fish such as [[striped bass]], [[black drum]], [[croaker]], herring, and anchovies.  
 
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]].
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====Filtering of water====
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As filter feeders, oysters are known for their role of filtering and removing pollutants and nutrients, such as [[nitrogen]], from water. Scientists believe that the [[Chesapeake Bay]]'s once-flourishing oyster populations historically filtered the estuary's entire water volume of excess nutrients in approximately three to four days. Today, with the decreased oyster populations, that process would take nearly a year (NOAA 2008), 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.  
  
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.
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====Oysters as food====
 
 
[[Image:oyster(L).jpg|thumb|A greenish oyster.]]
 
Oysters are generally an expensive food in places where they are not 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.
 
  
[[Image:Oyster knife DSC09237.jpg|thumb|left|Special knives for opening live oysters, such as this one, have short and stout blades and the best have a downward curve at the tip.]]
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[[Image:Whaleback Shell Midden gully - 20070722 07986.JPG|thumb|right|The [[Whaleback Shell Midden]] in [[Maine]] contains the shells from oyster harvesting for food consumption dating to 2,200 - 1,000 ago]]
Fresh oysters must be alive just before consumption. There is a simple criterion: oysters must be capable of closing the shell tightly. Any open oysters should be tapped on the shell: a live oyster will close up and is safe to eat. Oysters which are open and unresponsive are dead, and must be discarded.  Some dead oysters, or oyster shells which are full of sand may also be closed, but they will make a distinctive noise when tapped: they are known as "clackers" for this reason.
 
  
Opening oysters requires skill, for live oysters, outside of the water, tend to shut themselves tightly with a powerful muscle thus sealing in 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.  
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Although eighteenth century author [[Jonathan Swift]] is often quoted as having said, "He was a bold man that first ate an oyster" (Herbst 2001), evidence of oyster consumption goes back into prehistory, as evidenced by [[Midden#Shell middens|oyster middens]] (debris of human activity)  found worldwide. Archaeologists have found mounds of oyster shells dating back to 6950 B.C.E. and already by the first century B.C.E.. they were being cultivated by a Roman engineer, Sergius Orata (''Ecologist'' 2008). Oysters were an important food source in all coastal areas where they could be found, and oyster fisheries were an important industry where they were plentiful. Overfishing and pressure from diseases and pollution have sharply reduced supplies, but they remain a popular culinary dish and are celebrated in [[oyster festival]]s in many cities and towns.  
  
The blade needs to be inserted (with some moderate force and vibration if necessary) at the hinge in the rear of the shell. Then the blade should be slid upward to cut the adductor muscle (which holds the shell closed). Inexperienced shuckers tend to apply excessive force, which may result in injuries if the blade slips. A heavy glove should always be worn: 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.  
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[[Image:Oysters p1040741.jpg|thumb|left|Raw oysters presented on a plate.]]
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Oysters can be eaten half shelled, 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 and eating the contents including juice or adding [[butter]] and/or [[edible salt|salt]], or can be very elaborate. They are sometimes served on edible seaweed, such as [[brown algae]]. They are an exceptionally rich source of [[vitamin B12|vitamin <sub>12</sub>]], a rich source of iron, selenium, vitamin D, calcium, and iodine, and a good source of protein and niacin, as well as a source of [[vitamin]]s A, B<sub>1</sub>, and B<sub>2</sub> (Bender and Bender 2005; Herbst 2001). Oysters are low in [[food energy]]; one dozen raw oysters contain approximately 110 [[kilocalories]] (460 [[kilojoules|kJ]]).
  
A good demonstration of the opening technique is available [http://fishersislandoysters.com/how_to_open.htm here]. 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, can be tricky when an oyster's shell is in such a poor condition.
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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. For maximum shelf life, oysters should be stored out of water in refrigeration but not frozen and in 100 percent humidity. Oysters stored in water under refrigeration will open, utilize the small reserves of oxygen and die. Precautions should be taken when consuming them. Purists insist on eating oysters raw, with no dressing save perhaps [[lemon]] juice, vinegar (most commonly [[shallot]] 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.
  
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 slightly sweet-tasting and considered savory, and all the different varieties are mostly equivalent.  
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Oysters are generally an expensive food in places where they are not 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 for the poor in many countries with coastal access such as the United Kingdom and along the East Coast of the US and are thus still easily found in any areas bordering a sea or ocean. Oysters are commonly eaten raw in [[France]] in bars and as a "bar fast food" but the home use tends to be mixed with a large usage in cooking&mdash;steamed or in [[paella]] or soups.
  
A piece of folk wisdom concerning oysters is that they are best to eat in months containing the letter ''r'', as illustrated by the famous phrase: "oysters 'r' in season." 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 the oyster connoisseur.
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An alternative to opening raw oysters before consumption is to cook them in the shell&ndash;the heat kills the oysters and they open by themselves. Cooked oysters are slightly sweet-tasting and considered savory, and all the different varieties are mostly equivalent. Oysters can be successfully cooked in a microwave oven.
  
Oysters are sometimes cited as an [[aphrodisiac]]. It is disputed whether this is true. According to the [http://www.telegraph.co.uk/ Telegraph of London] a team of "American and Italian researchers analyzed bivalve molluscs - a group of shellfish that includes oysters - and found they were rich in rare amino acids that trigger increased levels of sex hormones."[http://www.smh.com.au/articles/2005/03/23/1111525227607.html] 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.
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A common food myth is that oysters are not suitable for eating during the summer months, and should only be eaten during months containing the letter "R" (in English). There are a few reasons why this myth actually has factual basis. First, months without "R" (May, June, July, and August) are the warmer months, and in warm months the oysters breed. This makes them less full bodied, less nutritious, and less enjoyable altogether. A health reason to not eat oysters in the warmer months is that the moment a raw oyster dies, it becomes tainted and poisonous. Consuming a dead raw oyster in the warm months make it harder to keep oysters alive on the plate. Another health issue is depending on the region of the oysters, they can contain harmful bacteria. Gulf Coast oysters do follow the "R rule" due to the fact that they contain high bacterial loads of human pathogens in these warm months, most notably ''[[Vibrio vulnificus]]'' and ''[[Vibrio parahaemolyticus]].'' The main danger is with immuno-compromised individuals who will be unable to fight off these infections and will almost surely succumb to septicemia leading to death. Oysters are filter feeders and will naturally concentrate any anything present in the surrounding sea water. ''Vibrio vulnificus'' is the most deadly seafood-borne [[pathogen]], with a higher case/death ratio than even ''Salmonella enterica'' and ''Escherichia coli.''
  
====History====
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In the early nineteenth century, oysters were very cheap and were mainly eaten by the working classes in the areas they were found. However, increasing demands from the rapidly-growing cities led to many of the [[Oyster Injustice|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 (Clover 2004). This scarcity increased prices leading to their current status as a delicacy.
[[Midden]]s testify to the prehistoric importance of oysters as a foodstuff. 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 Britain|Roman]] times. The borough of [[Colchester]] (which was briefly the capital of Roman Britain - during the Roman invasion) holds an annual [[Oyster Feast]] in October of each year, at which the "Colchester Natives" (the native oyster, Ostrea Edulis) are consumed. There are several oyster festivals held annually in the UK, e.g. Woburn Oyster Festival which is held in September.  
 
  
Similarly the seaside resort of Cancale in [[France]] is noted for its oysters, which also date from Roman times. In fact, [[Sergius Orata]] of the [[Roman Republic]] is considered to have been the first major merchant and cultivator of oysters. Using his very considerable hydraulic knowledge, he built a complex cultivation system including channels and locks to control the [[Tides|sea tides]]. He was famous because for this, and Roman people used to say he was so good that he could breed oysters on the roof of his house.<ref>Holland, Tom. 2003. ''Rubicon''. London, UK: Little, Brown. ISBN 0316861308</ref>
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====Oyster fishing====
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[[Image:Ostra gigante em Angola.jpg|thumb|right|200px|Giant oyster in south Angola.]]
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[[Image:2005oyster.PNG|thumb|240px|right|Oyster output in 2005]]
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Oysters are fished by simply gathering them from their beds. A variety of means are used. In very shallow waters, they can be gathered by hand or with small [[rake (tool)|rakes]]. In somewhat deeper water, long-handled rakes or oyster [[tongs]] are used to reach the beds. Patent tongs can be lowered on a line to reach beds which are too deep to reach directly. In all cases the manner of operation is the same: the waterman scrapes together a small pile of oysters, and then collects these by scooping them up with the rake or tongs.
  
The world-famous Clarenbridge and Galway Oyster Festivals are held in Galway, Ireland. each September. Ireland enjoys a long-standing tradition with regard to oysters where, typically, the shellfish is eaten live in conjunction with the national beverage, Guinness.
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In some areas, a [[Scallop dredge|dredge]] is used. This is a toothed bar attached to a chain bag. The dredge is towed through an oyster bed by a boat, picking up those oysters in its path. While dredges collect oysters more quickly, they can be very damaging to the oyster beds, and their use is in general strictly limited. In the state of [[Maryland]], dredging was until 1965 limited to [[sailboat]]s, and even since that date motor power can only be used on certain days of the week. These regulations prompted the development of specialized sailboats (the [[bugeye (boat)|bugeye]] and later the [[skipjack (boat)|skipjack]]) for dredging.
  
In the early nineteenth century, oysters were very cheap and were mainly eaten by the working classes. (Oysters were quite popular in [[New York City]] during the middle and late 19th century. [http://www.amazon.com/Big-Oyster-History-Half-Shell/dp/0345476387]{{Fact|date=February 2007}}) However, increasing demands from the rapidly-growing cities led to many of the [[Oyster Injustice|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.<ref>Clover, Charles. 2004. ''The End of the Line: How overfishing is changing the world and what we eat''. London, UK: Ebury Press. ISBN 0-09-189780-7.</ref> This scarcity increased prices leading to their current status as a delicacy.
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[[Image:Oyster culture in Belon, France 03.jpg|thumb|right|200px|Oyster culture in Belon, France.]]
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Oysters can also be collected by [[underwater diving|divers]].
  
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.
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In any case, when the oysters are collected, they are sorted to eliminate dead shells, unwanted catch, and other debris. Then they are taken to market where they are either canned or sold live.
  
==Pearl oysters==
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====Oyster farming====
[[Image:Pearl Oysters.jpg|thumb|Pearls being removed from pearl oysters]]
 
All types of oysters (and, indeed, many other shelled molluscs) can secrete [[pearl]]s, but those that sometimes form in edible oysters are unattractive and have no market value at all. [[Pearl Oyster]]s are in a totally different family, the [[Pteriidae]] (Feathered Oysters). Both [[cultured pearl]]s and natural pearls can be obtained from these oysters, though other molluscs, such as the freshwater [[mussel]]s, also yield pearls of commercial value. The largest pearl-bearing oyster type is the saltwater [[Pinctada maxima]], which is roughly the size of a dinner plate. Not all individual oysters produce pearls. In fact, in a haul of three [[ton]]s of oysters, only around three or four oysters produce perfect pearls.{{Fact|date=June 2007}}
 
  
These oysters produce [[pearls]] by covering an invading piece of grit with [[nacre]]. Over the years, the grit is covered with enough nacre to form what we know as a pearl. There are many different types and colors and shapes of pearl, but this depends on the pigment of the nacre and the shape of the piece of grit being covered over.
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Oysters have been cultured for a long time. [[Sergius Orata]] (fl. c. 95 B.C.E.) of the [[Roman Republic]] is considered to have been the first major merchant and cultivator of oysters. Using his very considerable hydraulic knowledge, in the first century B.C.E. he built a complex cultivation system including channels and locks to control the [[Tides|sea tides]]. He was famous for this, and Roman people used to say he was so good that he could breed oysters on the roof of his house (Holland 2003). A technique that he used to cultivate them in southern Italian lakes involved getting them to spawn on rock piles, with the larval oysters settling on twigs that were placed surrounding the rocks; when they grew to proper size, they were harvested ''(Economist 2008)''.
  
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. In fact since the beginning of the 20th century when several researchers discovered how to produce artificial pearls, the cultured pearl market has far outgrown the natural pearl market. Natural pearls have become scarcer and scarcer and a necklace with only natural pearls can easily cost several hundred thousand (US) dollars.<ref>[http://www.pearl-guide.com/ Home Page]. Pearl Guide. Retrieved November 6, 2007.</ref>
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Two methods are commonly used today. In both cases, oysters are cultivated to the size of "spat," the point at which they attach themselves to a substrate. They may be allowed to mature further to form "seed" oysters. In either case they are then set out to mature. They may be distributed over existing oyster beds and left to mature naturally, to be collected using the methods for fishing wild oysters. Or they may be put in racks or bags and held above the bottom. The oysters are harvested by lifting the bags or rack to the surface and removing mature oysters. The latter method avoids losses to some predators, but is more expensive (LaMER 2008).
  
==Oyster diseases==
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The Pacific (Japanese) oyster, ''Crassostrea gigas'' has also been grown in the outflow of [[mariculture]] ponds.  
Oysters are subject to various diseases which can reduce oyster harvests and often severely deplete local populations.  Control focuses on containing infections and breeding resistant strains and is the subject of much ongoing research.
 
  
===Dermo===
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In many areas, non-native oysters have been introduced in attempts to prop up failing harvests of native varieties. For example, the [[eastern oyster]] was introduced to California waters in 1875, while the Pacific oyster was introduced there in 1929 (Conte). The Pacific oyster prospered in Pendrell Sound, BC, Canada, where the surface water is typically warm enough for [[spawning]] in the summer. Over the following years, spat spread out sporadically and populated adjacent areas. Eventually, possibly following adaptation to the local conditions, the Pacific oyster spread up and down the coast and now is the basis of the West Coast oyster industry. Proposals for further such introductions remain controversial.
"''Dermo''" ''([[Perkinsus marinus]])'' is caused by a [[protozoa]]n [[parasite]]. It is a prevalent [[pathogen]] of oysters, causing massive mortality in oyster populations and poses a significant economic threat to the oyster industry.  The disease is of no direct threat to any humans consuming infected oysters.<ref name=CTAG>[http://www.ct.gov/doag/cwp/view.asp?a=1369&q=259180 Oyster Diseases]. Connecticut Department of Agriculture. Retrieved November 6, 2007.</ref>
 
  
Dermo first appeared in the [[Gulf of Mexico]] in the 1950s, and until 1978 it was believed to be caused by a [[fungus]].  While it is most serious in warmer southern waters, it has gradually spread up the East coast of the United States.<ref name=CBP>[http://www.chesapeakebay.net/newsoystersstress090905.htm Damaging Diseases - Some of Native Oysters' Worst Enemies]. Chesapeake Bay Program. Retrieved November 6, 2007.</ref>
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==Pearl oysters==
 
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[[Image:Pearl Oysters.jpg|thumb|right|Pearls being removed from pearl oysters.]]
===MSX===
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"Pearl oysters" are members of the [[family (biology)|family]] Pteriidae (feathered oysters) of the order Pterioida and class [[Bivalvia]]. They are renowned for producing [[pearl]]s of economic importance, valued as gemstones and also crushed and used in cosmetics or paint formulations. All types of oysters (and, indeed, almost all other shelled [[mollusk]]s) can secrete concretions that are known by biologists as pearls. However, those that sometimes form in edible oysters are unattractive and lack any market value. However, both [[cultured pearl]]s and natural pearls of value can be obtained from pearl oysters, as well as some other mollusks, such as freshwater [[mussel]]s.  
''MSX'' ('''M'''ultinucleated '''S'''phere '''X''') is caused by the protozoan ''[[Haplosporidium nelsoni]]'', generally seen as a multi-nucleated [[plasmodium]]. It is infectious and causes heavy mortality in the Eastern Oyster; survivors, however, are seen to develop resistance and can be used to help propagate resistant populations. It is associated with high salinity and water temperatures.<ref name=CTAG/>
 
 
 
MSX was first noted in [[Delaware Bay]] in 1957 and is now found all up and down the Eastern coast of the United States. Evidence suggests that it was brought to the United States when  ''Crassostrea gigas'', a Japanese oyster variety, was introduced to Delaware Bay.<ref name=CBP/>
 
 
 
==Other mollusks named "oyster"==
 
A number of other mollusks 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)''
 
 
 
==See also==
 
* [[Oyster sauce]]
 
* [[Pearl]]
 
* [[Oyster festival]]
 
* [[Oyster card]]
 
  
==Notes==
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In nature, pearl oysters produce natural pearls by covering a minute invading parasite with [[nacre]]. Over the years, the irritating object is covered with enough layers of nacre to form what we know as a pearl. There are many different types and colors and shapes of pearl; these qualities depend on the natural pigment tone of the nacre, and the shape of the original irritant which was being covered over. The largest pearl-bearing oyster type is the saltwater ''Pinctada maxima,'' which is roughly the size of a dinner plate.
<references/>
 
  
==External links==
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Pearls can also be cultivated by pearl farmers placing a nucleus, usually a piece of polished mussel shell, inside the oyster. In three to six years, the oyster will produce the desired pearl. These pearls are not as valuable as natural pearls, but externally appear exactly the same. (Cultured pearls and imitation pearls can be distinguished from natural pearls by [[X-ray]] examination.) Since the beginning of the twentieth century, when several researchers discovered how to produce artificial pearls, the cultured pearl market has far outgrown the natural pearl market. Natural pearls have become scarcer and scarcer and a necklace with only natural pearls can easily cost several hundred thousand (US) dollars.
  
* [http://www.west-mersea.co.uk/Mersea_Island_Company_Shed.htm Colchester Native Oysters]. Retrieved November 6, 2007.
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==References==
* [http://www.geographyinaction.co.uk/Assets/Photo_albums/Eleven/pages/Oysters1.html Oysters grown on trestles in Ireland]. Retrieved November 6, 2007.
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* Bender, D. A., and A. E. Bender. 2005. ''A Dictionary of Food and Nutrition.'' New York: Oxford University Press. ISBN 0198609612.
* [http://www.umces.edu/oysterlarvae.html Why Do Oysters Choose to Live Where They Could be Eaten?] University of Maryland Center for Environmental Science. Retrieved November 6, 2007.
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* Clover, Charles. 2004. ''The End of the Line: How Overfishing is Changing the World and What We Eat.'' London: Ebury Press. ISBN 0091897807.
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* Conte, F. S. n.d. [http://aqua.ucdavis.edu/dbweb/outreach/aqua/ASAQ-A07.PDF California oyster culture]. ''University of California, Davis: Department of Animal Science''. Retrieved January 6, 2009.
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* ''Economist''. 2008. [http://www.economist.com/world/unitedstates/displaystory.cfm?story_id=12795573 Gem of the ocean: A dozen ocean-cleaners and a pint of Guinness, please]. ''The Economist'' December 18, 2008. Retrieved January 6, 2009.
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* Herbst, S. T. 2001. ''The New Food Lover's Companion: Comprehensive Definitions of Nearly 6,000 Food, Drink, and Culinary Terms. Barron's Cooking Guide.'' Hauppauge, NY: Barron's Educational Series. ISBN 0764112589.
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* Holland, T. 2003. ''Rubicon: The Last Years of the Roman Republic.'' New York: Doubleday. ISBN 038550313X.
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* Integrated Taxonomic Information System (ITIS). 1999a. [http://www.itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=79588 Pterioida Newell, 1965] ''ITIS Taxonomic Serial No.: 79588''. Retrieved January 6, 2009.
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* Integrated Taxonomic Information System (ITIS). 1999b. [http://www.itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=79856 Ostreoida] ''ITIS Taxonomic Serial No.: 79856''. Retrieved January 6, 2009.
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* Integrated Taxonomic Information System (ITIS). 1999c. [http://www.itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=79866 Ostreidae Rafinesque, 1815] ''ITIS Taxonomic Serial No.: 79866''. Retrieved January 6, 2009.
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* Louisiana Sea Grant College Program (LaMER). 2008. [http://www.lamer.lsu.edu/classroom/edonahalfshell/pdf/cycle_info.pdf Oyster farming in Louisiana]. ''Louisiana State University''. Retrieved January 6, 2009.  
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* Malchus, N., and M. Aberhan. 1998. [http://jpaleontol.geoscienceworld.org/cgi/content/abstract/72/4/619 Transitional gryphaeate/exogyrate oysters (Bivalvia, Gryphaeidae) from the Lower Jurassic of northern Chile]. ''Journal of Paleontology'' 72(4): 619-631.
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* Maryland Department of Natural Resources. 2005. [http://www.dnr.state.md.us/dnrnews/pressrelease2005/081505.html Oyster restoration projected to provide significant boost to bay grasses while removing nitrogen pollution from the bay]. ''Maryland Department of Natural Resources''. Retrieved January 6, 2009.
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* National Oceanic and Atmospheric Administration (NOAA). 2008. [http://habitat.noaa.gov/restorationtechniques/public/habitat.cfm?HabitatID=2&HabitatTopicID=11 Oyster reefs: Ecological importance]. ''US National Oceanic and Atmospheric Administration''. Retrieved January 6, 2009.
  
{{commons|Oyster}}
 
{{cookbook}}
 
  
 
[[Category:Life sciences]]
 
[[Category:Life sciences]]
 
[[Category:Animals]]
 
[[Category:Animals]]
  
{{Credit|169272731}}
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{{Credit|Oyster|261481662|Ostreidae|261244241}}

Latest revision as of 16:01, 5 October 2016

Crassostrea gigas, Marennes-Oléron
Crassostrea gigas, Marennes-Oléron

Oyster is the common name applied to various bivalve mollusks, and in particular the "true oysters" comprising family Ostreidae of the Ostreoida order and the pearl oysters of family Pteriidae (syn. Aviculidae) in the Pterioida order. The shell of oysters consists of two usually highly calcified valves (shells) that surround a soft body. Gills filter plankton from the water, and strong adductor muscles are used to hold the shell closed. Most oysters live in marine habitats or brackish water and are widely distributed in shallow, warm, coastal waters of the world's oceans.

The true oysters of family Ostreidae include many that are highly prized as food, consumed both raw and cooked. The pearl oysters of the Pteriidae genus Pinctada are well-known for producing large, commercially valuable pearls. On the other hand, the true oysters are incapable of making gem-quality pearls, although the opposite idea is a commonly-encountered misapprehension, often seen in illustrations or photographs where an edible oyster shell is mistakenly paired with a gem-quality pearl.

In addition to their commercial importance for culinary purposes and for the production of pearls, oysters provide important ecological values. Oyster reefs provide habitat for many organisms, and the oysters themselves provide food for various fish, marine mammals, and invertebrates. As filter feeders, they have a remarkable ability to filter water, removing pollutants and excess nutrients.

Despite these important functions for the ecosystem and humans, many oyster beds have been plundered without thought to future generations, with the overfishing resulting in serious decline in population size.

Crassostrea gigas, Marennes-Oléron, opened
An ostreolith from the Middle Jurassic Carmel Formation of southern Utah. This ball of oysters is made up almost entirely of the species Liostrea strigilecula.
Ostreolith interior showing an aragonitic shell cast as the nucleus and Gastrochaenolites borings.

A number of bivalve mollusks other than edible oysters and pearl oysters also have common names that include the word "oyster." Among these are the "gryphaeate oysters" comprising family Gryphaeidae and the "thorny oysters" comprising family Spondylidae, both of which are placed in the Ostreoida order. The "tree oysters" are part of the Isognomonidae family in the Pterioida order. This article will provide an overview of the Ostreidae (true oysters) and Pteriidae (pearl oysters), with emphasis on the true oysters.

True oysters

The members of the family Ostreidae of order Ostreoida and class Bivalvia are the "true oysters," and include all the species that are commonly eaten under the title "oyster." There are about eight or nine genera in Ostreidae. The edible oysters 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).

Description

Like scallops (family Pectinidae), true oysters have a central adductor muscle, which means that the shell has a characteristic central scar, marking its point of attachment. The shell shape tends to be irregular as a result of attaching to other objects. Adult oysters become permanently attached to a hard surface early in development and lack a foot as an adult. An oyster's mature shape often depends on the type of bottom to which it is originally attached, but it always 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.

The two shells (valves) of oysters tend to be highly calcified. Members of genus Ostrea differ from most bivalves by having shells completely comprised of calcite but with internal muscle scars of aragonitic composition.

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 adductor 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.

Oysters are filter-feeders, drawing water in over their gills through the beating of cilia. Suspended food plankton and particles are trapped in the mucus of a gill, and from there are transported to the mouth, where they are eaten, digested, and expelled as feces or pseudofeces. Feeding activity is greatest in oysters when the water temperatures are above 50°F (10°C). Healthy oysters consume algae and other water-borne nutrients, with each filtering up to five liters of water per hour.

Habitat and ecology

Members of genus Ostrea generally are subtidal and live continually immersed in sea water. They brood their fertilized eggs for various proportions of the period from fertilization to hatching. They do best in water with a not too thick concentration of phytoplankton.

Members of genera Saccostrea and Crassostrea live mainly in the intertidal zone, broadcast sperm and eggs into the sea, and can thrive in water that is very rich in phytoplankton. One of the most commonly cultivated oysters is Crassostrea gigas, the Pacific or Japanese oyster, which is ideally suited for oyster cultivation in seawater ponds.

Note that some tropical oysters in a different family, the family Isognomonidae, grow best 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 in the Caribbean, "oysters grow on trees."

A group of oysters is commonly called a bed.

Reproduction and life history

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.

Both oviparous (egg bearing) and larviparous (larvae bearing) species are known within the Ostreidae. Both types are hermaphrodites. However, the larviparous species (which include the edible oyster, Ostrea edulis) show a pattern of alternating sex within each individual (sequential hermaphrodites), whereas the oviparous species (such as the eastern oyster, Crassostrea virginica) are simultaneous hermaphrodites, in which an individual may possess fully functional male and female gonads, producing either female or male gametes according to circumstances.

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 millimeters or less in length. Many species of bivalve, oysters included, seem to be stimulated to settle by the proximity of adults of their species.

The oyster's greatest predators include crabs, sea birds, sea stars, and humans. Some oysters contain live crabs, known as an oyster crab.

Importance

Ecological

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. Hundreds of animals, such as anemones, barnacles, and hooked mussels use oyster reefs as habitat. Many of these animals, and the oysters themselves, serve as food for larger animals, including such commercially important fish such as striped bass, black drum, croaker, herring, and anchovies.

Filtering of water

As filter feeders, oysters are known for their role of filtering and removing pollutants and nutrients, such as nitrogen, from water. Scientists believe that the Chesapeake Bay's once-flourishing oyster populations historically filtered the estuary's entire water volume of excess nutrients in approximately three to four days. Today, with the decreased oyster populations, that process would take nearly a year (NOAA 2008), 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 as food

The Whaleback Shell Midden in Maine contains the shells from oyster harvesting for food consumption dating to 2,200 - 1,000 ago

Although eighteenth century author Jonathan Swift is often quoted as having said, "He was a bold man that first ate an oyster" (Herbst 2001), evidence of oyster consumption goes back into prehistory, as evidenced by oyster middens (debris of human activity) found worldwide. Archaeologists have found mounds of oyster shells dating back to 6950 B.C.E. and already by the first century B.C.E. they were being cultivated by a Roman engineer, Sergius Orata (Ecologist 2008). Oysters were an important food source in all coastal areas where they could be found, and oyster fisheries were an important industry where they were plentiful. Overfishing and pressure from diseases and pollution have sharply reduced supplies, but they remain a popular culinary dish and are celebrated in oyster festivals in many cities and towns.

Raw oysters presented on a plate.

Oysters can be eaten half shelled, 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 and eating the contents including juice or adding butter and/or salt, or can be very elaborate. They are sometimes served on edible seaweed, such as brown algae. They are an exceptionally rich source of vitamin 12, a rich source of iron, selenium, vitamin D, calcium, and iodine, and a good source of protein and niacin, as well as a source of vitamins A, B1, and B2 (Bender and Bender 2005; Herbst 2001). Oysters are low in food energy; one dozen raw oysters contain approximately 110 kilocalories (460 kJ).

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. For maximum shelf life, oysters should be stored out of water in refrigeration but not frozen and in 100 percent humidity. Oysters stored in water under refrigeration will open, utilize the small reserves of oxygen and die. Precautions should be taken when consuming them. Purists insist on eating oysters raw, with no dressing save perhaps lemon juice, vinegar (most commonly shallot 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 are not 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 for the poor in many countries with coastal access such as the United Kingdom and along the East Coast of the US and are thus still easily found in any areas bordering a sea or ocean. Oysters are commonly eaten raw in France in bars and as a "bar fast food" but the home use tends to be mixed with a large usage in cooking—steamed or in paella or soups.

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 slightly sweet-tasting and considered savory, and all the different varieties are mostly equivalent. Oysters can be successfully cooked in a microwave oven.

A common food myth is that oysters are not suitable for eating during the summer months, and should only be eaten during months containing the letter "R" (in English). There are a few reasons why this myth actually has factual basis. First, months without "R" (May, June, July, and August) are the warmer months, and in warm months the oysters breed. This makes them less full bodied, less nutritious, and less enjoyable altogether. A health reason to not eat oysters in the warmer months is that the moment a raw oyster dies, it becomes tainted and poisonous. Consuming a dead raw oyster in the warm months make it harder to keep oysters alive on the plate. Another health issue is depending on the region of the oysters, they can contain harmful bacteria. Gulf Coast oysters do follow the "R rule" due to the fact that they contain high bacterial loads of human pathogens in these warm months, most notably Vibrio vulnificus and Vibrio parahaemolyticus. The main danger is with immuno-compromised individuals who will be unable to fight off these infections and will almost surely succumb to septicemia leading to death. Oysters are filter feeders and will naturally concentrate any anything present in the surrounding sea water. Vibrio vulnificus is the most deadly seafood-borne pathogen, with a higher case/death ratio than even Salmonella enterica and Escherichia coli.

In the early nineteenth century, oysters were very cheap and were mainly eaten by the working classes in the areas they were found. 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 (Clover 2004). This scarcity increased prices leading to their current status as a delicacy.

Oyster fishing

Giant oyster in south Angola.
Oyster output in 2005

Oysters are fished by simply gathering them from their beds. A variety of means are used. In very shallow waters, they can be gathered by hand or with small rakes. In somewhat deeper water, long-handled rakes or oyster tongs are used to reach the beds. Patent tongs can be lowered on a line to reach beds which are too deep to reach directly. In all cases the manner of operation is the same: the waterman scrapes together a small pile of oysters, and then collects these by scooping them up with the rake or tongs.

In some areas, a dredge is used. This is a toothed bar attached to a chain bag. The dredge is towed through an oyster bed by a boat, picking up those oysters in its path. While dredges collect oysters more quickly, they can be very damaging to the oyster beds, and their use is in general strictly limited. In the state of Maryland, dredging was until 1965 limited to sailboats, and even since that date motor power can only be used on certain days of the week. These regulations prompted the development of specialized sailboats (the bugeye and later the skipjack) for dredging.

Oyster culture in Belon, France.

Oysters can also be collected by divers.

In any case, when the oysters are collected, they are sorted to eliminate dead shells, unwanted catch, and other debris. Then they are taken to market where they are either canned or sold live.

Oyster farming

Oysters have been cultured for a long time. Sergius Orata (fl. c. 95 B.C.E.) of the Roman Republic is considered to have been the first major merchant and cultivator of oysters. Using his very considerable hydraulic knowledge, in the first century B.C.E. he built a complex cultivation system including channels and locks to control the sea tides. He was famous for this, and Roman people used to say he was so good that he could breed oysters on the roof of his house (Holland 2003). A technique that he used to cultivate them in southern Italian lakes involved getting them to spawn on rock piles, with the larval oysters settling on twigs that were placed surrounding the rocks; when they grew to proper size, they were harvested (Economist 2008).

Two methods are commonly used today. In both cases, oysters are cultivated to the size of "spat," the point at which they attach themselves to a substrate. They may be allowed to mature further to form "seed" oysters. In either case they are then set out to mature. They may be distributed over existing oyster beds and left to mature naturally, to be collected using the methods for fishing wild oysters. Or they may be put in racks or bags and held above the bottom. The oysters are harvested by lifting the bags or rack to the surface and removing mature oysters. The latter method avoids losses to some predators, but is more expensive (LaMER 2008).

The Pacific (Japanese) oyster, Crassostrea gigas has also been grown in the outflow of mariculture ponds.

In many areas, non-native oysters have been introduced in attempts to prop up failing harvests of native varieties. For example, the eastern oyster was introduced to California waters in 1875, while the Pacific oyster was introduced there in 1929 (Conte). The Pacific oyster prospered in Pendrell Sound, BC, Canada, where the surface water is typically warm enough for spawning in the summer. Over the following years, spat spread out sporadically and populated adjacent areas. Eventually, possibly following adaptation to the local conditions, the Pacific oyster spread up and down the coast and now is the basis of the West Coast oyster industry. Proposals for further such introductions remain controversial.

Pearl oysters

Pearls being removed from pearl oysters.

"Pearl oysters" are members of the family Pteriidae (feathered oysters) of the order Pterioida and class Bivalvia. They are renowned for producing pearls of economic importance, valued as gemstones and also crushed and used in cosmetics or paint formulations. All types of oysters (and, indeed, almost all other shelled mollusks) can secrete concretions that are known by biologists as pearls. However, those that sometimes form in edible oysters are unattractive and lack any market value. However, both cultured pearls and natural pearls of value can be obtained from pearl oysters, as well as some other mollusks, such as freshwater mussels.

In nature, pearl oysters produce natural pearls by covering a minute invading parasite with nacre. Over the years, the irritating object is covered with enough layers of nacre to form what we know as a pearl. There are many different types and colors and shapes of pearl; these qualities depend on the natural pigment tone of the nacre, and the shape of the original irritant which was being covered over. The largest pearl-bearing oyster type is the saltwater Pinctada maxima, which is roughly the size of a dinner plate.

Pearls can also be cultivated by pearl farmers placing a nucleus, usually a piece of polished mussel shell, inside the oyster. In three to six years, the oyster will produce the desired pearl. These pearls are not as valuable as natural pearls, but externally appear exactly the same. (Cultured pearls and imitation pearls can be distinguished from natural pearls by X-ray examination.) Since the beginning of the twentieth century, when several researchers discovered how to produce artificial pearls, the cultured pearl market has far outgrown the natural pearl market. Natural pearls have become scarcer and scarcer and a necklace with only natural pearls can easily cost several hundred thousand (US) dollars.

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