Oyster

Oysteris thecommon namefor a number of different families ofsalt-waterbivalvemolluscsthat live inmarineorbrackishhabitats. In some species, the valves are highlycalcified, and many are somewhat irregular in shape. Many, but not all oysters, are in the superfamilyOstreoidea.
Some species of oyster are commonly consumed and are regarded as adelicacyin some localities. Some types ofpearl oystersare harvested for thepearlproduced within themantle. Others, such as the translucentWindowpane oysters, are harvested for their shells.
The wordoystercomes fromOld Frenchoistre, and first appeared in English during the 14th century.The French derived from theLatinostrea, the feminine form ofostreum,which is thelatinisationof theAncient Greekὄστρεον(ostreon) 'oyster'.Compareὀστέον(osteon) 'bone'.
True oystersare members of the familyOstreidae. This family includes the edible oysters, which mainly belong to the generaOstrea,Crassostrea,Magallana, andSaccostrea. Examples include theEuropean flat oyster,eastern oyster,Olympia oyster,Pacific oyster, and theSydney rock oyster. Ostreidae evolved in theEarly Triassicepoch: ThegenusLiostreagrew on the shells of livingammonoids.
Almost all shell-bearing mollusks can secrete pearls, yet most are not very valuable. Pearls can form in both saltwater and freshwater environments.
Pearl oystersare not closely related to true oysters, being members of a distinct family, the feathered oysters (Pteriidae). Bothcultured pearlsandnatural pearlscan be extracted from pearl oysters, though other molluscs, such as the freshwatermussels, also yield pearls of commercial value.
The largest pearl-bearing oyster is the marinePinctada maxima, which is roughly the size of a dinner plate. Not all individual oysters produce pearls.
In nature, pearl oysters produce pearls by covering a minute invasive object withnacre. Over the years, the irritating object is covered with enough layers of nacre to become a pearl. The many different types, colours and shapes of pearls depend on the naturalpigmentof the nacre, and the shape of the original irritant.
Pearl farmers can culture a pearl by placing a nucleus, usually a piece of polished mussel shell, inside the oyster. In three to seven years, the oyster can produce a perfect pearl. 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.
A number of bivalve molluscs (other than true oysters and pearl oysters) also have common names that include the word "oyster", usually because they either taste like or look somewhat like true oysters, or because they yield noticeable pearls. Examples include:
In thePhilippines, a local thorny oyster species known asTikod amois a favorite seafood source in the southern part of the country.Because of its good flavor, it commands high prices.
Oysters breathe primarily viagills. In addition to their gills, oysters can exchange gases across theirmantles, which are lined with many small, thin-walledblood vessels. A small, three-chamberedheart, lying under theadductor muscle, pumps colorlessbloodto all parts of the body. At the same time, twokidneys, located on the underside of the muscle, remove waste products from the blood. Their nervous system includes two pairs of nerve cords and three pairs ofganglia. There is no evidence that oysters have a brain.
While some oysters have two sexes (European oysterandOlympia oyster), their reproductive organs contain both eggs and sperm. Because of this, it is technically possible for an oyster to fertilize its own eggs. Thegonadssurround the digestive organs, and are made up of sex cells, branching tubules, and connective tissue.
Once her millions of eggs are fertilized, the female discharges them into the water. Thelarvaedevelop in about six hours and exist suspended in the water column asveliger larvaefor two to three weeks before settling on a bed and reaching sexual maturity within a year.
Oysters arefilter feeders, drawing water in over theirgillsthrough the beating ofcilia. Suspendedplanktonand non-food particles are trapped in themucusof a gill, and from there are transported to the mouth, where they are eaten, digested, and expelled asfecesorpseudofecesthat fall to the bottom and remain out of the water column. Oysters feed most actively at temperatures ranging from the high 60s to the high 70s (20–26 °C).Under ideal laboratory conditions, an oyster can filter up to 190 L (50 US gal) of water per day. Under average conditions, mature oysters filter 11–45 L (3–12 U.S. gal).Chesapeake Bay's once-flourishing oyster population historically filtered excess nutrients from the estuary's entire water volume every three to four days. As of 2008 it was estimated that a complete cycle would take nearly a year.
A group of oysters is commonly called a bed oroyster reef.
As akeystone species, oysters providehabitatfor many marine species.CrassostreaandSaccostrealive mainly in theintertidal zone, whileOstreaissubtidal. 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 assea anemones,barnacles, andhooked mussels, inhabitoyster reefs. Many of these animals are prey to larger animals, including fish, such asstriped bass,black drumandcroakers.
An oyster reef can increase the surface area of a flat bottom 50-fold. 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.
Oysters usually reach maturity in one year. They areprotandric; during their first year, they spawn as males by releasingsperminto the water. As they grow over the next two or three years and develop greater energy reserves, they spawn as females by releasingeggs.Bay oystersusually spawn from the end of June until mid-August. An increase in water temperature prompts a few oysters to spawn. This triggers spawning in the rest, clouding 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, such as another oyster's shell, on which to settle. Attached oyster larvae are called spat. Spat are oysters less than 25 mm (1 in) long. Many species of bivalves, oysters included, seem to be stimulated to settle near adultconspecifics.
Oysters filter large amounts of water to feed and breathe (exchangeO2and CO2with water) but they are not permanently open. They regularly shut their valves to enter a resting state, even when they are permanently submersed. Their behaviour follows very strict circatidal and circadian rhythms according to the relative moon and sun positions. During neap tides, they exhibit much longer closing periods than during the spring tide.
Some tropical oysters, such as themangrove oysterin the familyOstreidae, grow best onmangroveroots. Low tide can expose them, making them easy to collect.
The largest oyster-producing body of water in theUnited Statesis theChesapeake Bay, although these beds have decreased in number due to overfishing and pollution. Other largeoyster farmingareas in the US include the bays and estuaries along the coast of theGulf of MexicofromApalachicola, Florida, in the east toGalveston, Texas, in the west. Large beds of edible oysters are also found in Japan and Australia. In 2005, China accounted for 80% of the global oyster harvest.In Europe, France remained the industry leader.
Common oyster predators includecrabs,seabirds,starfish, andhumans. Some oysters contain crabs, known asoyster crabs.
Bivalves, including oysters, are effective filter feeders and can have large effects on the water columns in which they occur.As filter feeders, oysters remove plankton and organic particles from the water column.Multiple studies have shown individual oysters are capable of filtering up to 190 litres (42 imp gal; 50 US gal) of water per day, and thus oyster reefs can significantly improve water quality and clarity.Oysters consume nitrogen-containing  compounds (nitratesandammonia),phosphates, plankton, detritus, bacteria, and dissolved organic matter, removing them from the water.What is not used for animal growth is then expelled as solid waste pellets, which eventually decompose into the atmosphere as nitrogen.InMaryland, theChesapeake Bay Programhad implemented a plan to use oysters to reduce the amount of nitrogen compounds entering theChesapeake Bayby 8,600 t (9,500 short tons) per year by 2010.Several studies have shown that oysters andmusselshave the capacity to dramatically alter nitrogen levels in estuaries.In the U.S.,Delawareis the only East Coast state without aquaculture, but making aquaculture a state-controlled industry of leasing water by the acre for commercial harvesting of shellfish is being considered.Supporters of Delaware's legislation to allow oyster aquaculture cite revenue, job creation, and nutrient cycling benefits. It is estimated that one acre (0.40 ha) can produce nearly 750,000 oysters, which could filter between 57,000 to 150,000 m(2.0–5.3 million cu ft) of water daily.Also seenutrient pollutionfor an extended explanation ofnutrient remediation.
As anecosystem engineer, oysters provide supportingecosystem services, along with provisioning, regulating and cultural services. Oysters influencenutrient cycling,water filtration, habitat structure,biodiversity, andfood webdynamics.Oyster reef habitats have been recognized asgreen infrastructurefor shoreline protection.Assimilation of nitrogen and phosphorus into shellfish tissues provides an opportunity to remove these nutrients from the water column.In California'sTomales Bay, native oyster presence is associated with higher species diversity of benthic invertebrates.As the ecological and economic importance of oyster reefs has become more acknowledged, restoration efforts have increased.
Middenstestify to the prehistoric importance of oysters as food, with some middens inNew South Wales, Australia, dated at ten thousand years.They have been cultivated in Japan from at least 2000 BC.In the United Kingdom, the town ofWhitstableis noted for oyster farming from beds on theKentish Flatsthat have been used sinceRomantimes. The borough ofColchesterholds an annualOyster Feasteach October, at which "Colchester Natives" (the native oyster,Ostrea edulis) are consumed. The United Kingdom hosts several other annual oyster festivals; for example,Woburn Oyster Festivalis held in September. In fact, in Victorian England, it was quite common for people to go to the pub and enjoy their favorite beer with some oysters. They quickly realized that the "rich, sweet, malty stouts" were great with the "briny, creamy oyster". Then brewers found that oyster shells naturally clarify a beer and they started putting crushed oyster shells into their brews. The first known brewery to start this was in 1938 at the Hammerton Brewery in London. That is where theoyster stoutwas first started.
TheFrenchseaside resort ofCancaleinBrittanyis noted for its oysters, which also date from Roman times.Sergius Orataof theRoman Republicis considered the first major merchant and cultivator of oysters. Using his considerable knowledge ofhydraulics, he built a sophisticated cultivation system, including channels and locks, to control thetides. He was so famous for this, the Romans used to say he could breed oysters on the roof of his house.
In the early 19th century, oysters were cheap and mainly eaten by theworking class. Throughout the 19th century,oyster bedsinNew York Harborbecame the largest source of oysters worldwide. On any day in the late 19th century, six million oysters could be found on barges tied up along the city's waterfront. They were naturally quite popular inNew York City, and helped initiate the city's restaurant trade.New York's oystermen became skilled cultivators of their beds, which provided employment for hundreds of workers and nutritious food for thousands. Eventually, rising demand exhausted many of the beds. To increase production, they introduced foreign species, which brought disease;effluentand increasing sedimentation from erosion destroyed most of the beds by the early 20th century. Oysters' popularity has put ever-increasing demands on wild oyster stocks.This scarcity increased prices, converting them from their original role as working-class food to their current status as an expensive delicacy.
In Britain, the native species (European flat oyster) has five years to mature and is protected by the people during their May-to-August spawning season. The current market is dominated by the largerPacific oysterandRock oysterspecies which are farmed year-round.
Oysters are harvested by simply gathering them from their beds. In very shallow waters, they can be gathered by hand or with smallrakes. In somewhat deeper water, long-handled rakes or oystertongsare used to reach the beds.Patent tongscan be lowered on a line to reach beds that are too deep to reach directly. In all cases, the task is the same: theoystermanscrapes oysters into a pile, and then scoops them up with the rake or tongs.
In some areas, ascallop dredgeis used. This is a toothed bar attached to a chain bag. The dredge is towed through an oyster bed by a boat, picking up the oysters in its path. While dredges collect oysters more quickly, they heavily damage the beds, and their use is highly restricted. Until 1965,Marylandlimited dredging tosailboats, and even since then motor boats can be used only on certain days of the week. These regulations prompted the development of specializedsailboats(thebugeyeand later theskipjack) for dredging.
Similar laws were enacted in Connecticut before World War I and lasted until 1969. The laws restricted the harvesting of oysters in state-owned beds to vessels under sail. These laws prompted the construction of the oyster sloop-style vessel to last well into the 20th century.Hopeis believed to be the last-built Connecticut oyster sloop, completed in 1948.
Oysters can also be collected bydivers.
In any case, when the oysters are collected, they are sorted to eliminate dead animals, bycatch (unwanted catch), and debris. Then they are taken to market, where they are either canned or sold live.
Oysters have been cultured since at least the days of theRoman Empire. ThePacific oyster(Magallana gigas) is presently the most widely grown bivalve around the world.Two methods are commonly used, release and bagging. In both cases, oysters are cultivated onshore to the size of spat, when they can attach themselves to a substrate. They may be allowed to mature further to form "seed oysters". In either case, they are then placed in the water to mature. The release technique involves distributing the spat throughout existing oyster beds, allowing them to mature naturally to be collected like wild oysters. Bagging has the cultivator putting spat in racks or bags and keeping them above the bottom. Harvesting involves simply lifting the bags or rack to the surface and removing the mature oysters. The latter method prevents losses to some predators, but is more expensive.
The Pacific oyster has been grown in the outflow ofmaricultureponds. When fish orprawnsare grown in ponds, it takes typically 10 kg (22 lb) of feed to produce1 kg (2+1⁄4lb) of product (dry-drybasis). The other 9 kg (20 lb) goes into the pond and after mineralization, provides food for phytoplankton, which in turn feeds the oyster.
To prevent spawning, sterile oysters are now cultured by crossbreedingtetraploidanddiploidoysters. The resultingtriploidoyster cannot propagate, which prevents introduced oysters from spreading into unwanted habitats.
In many areas, non-native oysters have been introduced in attempts to prop up failing harvests of native varieties. For example, theeastern oyster(Crassostrea virginica) was introduced to California waters in 1875, while the Pacific oyster was introduced there in 1929.Proposals for further such introductions remain controversial.
The Pacific oyster prospered inPendrell Sound, 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 North American west coast oyster industry. Pendrell Sound is now a reserve that supplies spat for cultivation.Near the mouth of theGreat Wicomico Riverin theChesapeake Bay, five-year-oldartificial reefsnow harbor more than 180 million nativeCrassostrea virginica. That is far lower than in the late 1880s, when the bay's population was in the billions, and watermen harvested about 910,000 m(25 million imp bsh) annually. The 2009 harvest was less than 7,300 m(200,000 imp bsh). Researchers claim the keys to the project were:
The "oyster-tecture" movement promotes the use of oyster reefs for water purification and wave attenuation.   An oyster-tecture project has been implemented at Withers Estuary, Withers Swash, South Carolina, by Neil Chambers-led volunteers, at a site where pollution was affecting beach tourism.Currently, for the installation cost of $3000, roughly 4.8 million liters of water are being filtered daily. In New Jersey, however, the Department of Environmental Protection refused to allow oysters as a filtering system in Sandy Hook Bay and the Raritan Bay, citing worries that commercial shellfish growers would be at risk and that members of the public might disregard warnings and consume tainted oysters. New Jersey Baykeepers responded by changing their strategy for utilizing oysters to clean up the waterway, by collaborating with Naval Weapons Station Earle. The Navy station is under 24/7 security and therefore eliminates anypoachingand associated human health risk.Oyster-tecture projects have been proposed to protect coastal cities, such as New York, from the threat of rising sea levels due toclimate change.AdditionallyOyster reef restorationhas shown to increase the population of oyster beds within the oceans while also conserving the biolife within the oyster reefs.
The accidental or intentionalintroduction of speciesby humans has the potential to negatively impact native oyster populations. For example, non-native species inTomales Bayhave resulted in the loss of half of California'sOlympia oysters.Oyster reefs occupy a small fraction of their distribution prior to mass harvesting during the last three centuries.
In October 2017, it was reported thatunderwater noise pollutioncan affect oysters as they close their shells when exposed to lowfrequenciesof sounds inexperimentalconditions. Oysters rely on hearing waves and currents to regulate theircircadian rhythms, and perception of weather events—such asrain—may inducespawning.Cargo ships,pile drivers, and explosions conducted underwater produce low frequencies that may be detected by oysters.
Environmental stressors as a result ofglobal changeare also negatively impacting oysters around the world, with many impacts affecting molecular, physiological, and behavioral processes in species includingMagallana gigas.
Recycled oyster shells can help restore oyster reefs to provide marine life habitat that reduces flooding, and protects shorelines from storms.Shell-recycling non-profits retrieve shells from restaurants, wash and dry them, and set them in the sun for up to a year to kill bacteria. Some states encourage shell recycling by offering tax incentives.
Jonathan Swiftis quoted as having said, "He was a bold man that first ate an oyster".Evidence of oyster consumption goes back into prehistory, evidenced byoyster middensfound worldwide. Oysters were an important food source in all coastal areas where they could be found, and oysterfisherieswere 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 inoyster festivalsin many cities and towns.
It was once assumed that oysters were only safe to eat in months with the letter 'r' in their English and French names. This myth is based in truth, in that in the Northern Hemisphere, oysters are much more likely to spoil in the warmer months of May, June, July, and August.In recent years, pathogens such asVibrio parahaemolyticushave caused outbreaks in several harvesting areas of the eastern United States during the summer months, lending further credence to this belief.
Oysters can be eaten on the half shell, raw,smoked,boiled,baked,fried,roasted,stewed,canned,pickled,steamed, orbroiled, or used in a variety of drinks. Eating can be as simple as opening the shell and eating the contents, including juice.Butterandsaltare often added. Poached oysters can be served on toast with a creamroux.In the case ofOysters Rockefeller, preparation can be very elaborate. They are sometimes served on edible seaweed, such asbrown algae.
Care should be taken when consuming oysters. They may be eaten raw, with no dressing or withlemonjuice,vinegar(most commonlyshallotvinegar), orcocktail sauce. Upscale restaurants pair raw oysters withmignonette sauce, which consists primarily of fresh choppedshallot, mixedpeppercorn, drywhite wineand lemon juice orsherry vinegar. Raw oysters have complex flavors that vary among varieties and regions: salty, briny, buttery, metallic or fruity. The texture is soft and fleshy. North American varieties includeKumamotoandYaquina Bayfrom Oregon,DuxburyandWellfleetfrom Massachusetts,MalpequefromPrince Edward Island, Canada,Blue Pointfrom Long Island, New York,PemaquidfromMaine,Rappahannock RiverandJames Riverfrom Virginia,Chesapeakefrom Maryland andCape Mayfrom New Jersey. Variations in water salinity, alkalinity, and mineral and nutritional content influence their flavor.
Oysters are an excellent source ofzinc,iron,calcium, andselenium, as well asvitamin Aandvitamin B12. Oysters are low infood energy; one dozen raw oysters provides only 460 kilojoules (110 kilocalories).They are rich in protein (approximately 9 g in 100 g of Pacific oysters).Two oysters (28 grams or 1 ounce) provide theReference Daily Intakeof zinc and vitamin B12.
Traditionally, oysters are considered to be anaphrodisiac, partially because they resemble female sex organs.A team of American and Italian researchers analyzed bivalves and found they were rich in amino acids that trigger increased levels ofsex hormones.Their highzinccontent aids the production of testosterone.
Opening oysters, referred to as "oyster-shucking", requires skill. The preferred method is to use a special knife (called anoyster knife, a variant of ashucking knife), with a short and thick blade about 5 cm (2 in) long.
While different methods are used to open an oyster (which sometimes depend on the type), the following is one commonly accepted oyster-shucking method.
Inexperienced shuckers can apply too much force, which can result in injury if the blade slips. Heavy gloves, sometimes sold asoyster gloves, are recommended; apart from the knife, the shell itself can be razor-sharp. Professional shuckers require fewer than three seconds to open the shell.
If the oyster has a particularly soft shell, the knife can be inserted instead in the "sidedoor", about halfway along one side where the oyster lips widen with a slight indentation.
Oyster-shucking has become a competitive sport; competitions are staged around the world.The Guinness World Oyster Opening Championship was held annually in September at theGalway International Oyster Festival, inGalway, Ireland until 2010.Since 2011, "Guinness" has been dropped from the title.
Unlike most shellfish, oysters can have a fairly long shelf life of up to four weeks. However, their taste becomes less pleasant as they age. Fresh oysters must be alive just before consumption or cooking.
Cooked oysters that do not open are generally assumed to be previously dead and therefore unsafe.There is only one criterion: the oyster must be capable of tightly closing its shell.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 be closed. These make a distinctive noise when tapped, and are known as "clackers".
Oysters can contain harmfulbacteria. Oysters are filter feeders, so will naturally concentrate anything present in the surrounding water. Oysters from theGulf Coastof the United States, for example, contain high bacterial loads of human pathogens in the warm months, most notablyVibrio vulnificusandVibrio parahaemolyticus. In these cases, the main danger is forimmunocompromisedindividuals, who are unable to fight off infection and can succumb tosepsis, leading to death.Vibrio vulnificusis the most deadly seafood-bornepathogen.
Depurationof oysters is a common industry practice and widely researched in the scientific community but is not commonly known by end consumers. The main objective of seafood depuration is to remove fecal contamination in seafood before being sold to end consumers. Oyster depuration is useful since they are generally eaten raw and in many countries, the requirement to process is government-regulated or mandatory. TheUnited Nations Food and Agriculture Organization(FAO) formally recognizes depuration and has published detailed documents on the process,whereas theCodex Alimentarius, encourages the application of seafood depuration.
Oyster depuration begins after the harvest of oysters from farmed locations. The oysters are transported and placed into tanks pumped with clean water for periods of 48 to 72 hours. The holding temperatures and salinity vary according to species. The seawater that the oysters were originally farmed in does not remain in the oyster, since the water used for depuration must be fully sterilized, and the depuration facility would not necessarily be located near the farming location.Depuration of oysters can remove moderate levels of contamination of most bacterial indicators and pathogens. Well-known contaminants includeVibrio parahaemolyticus, a temperature-sensitive bacterium found in seawater animals, andEscherichia coli, a bacterium found in coastal waters near highly populated cities having sewage systems discharging waste nearby, or in the presence of agricultural discharges.Depuration expands beyond oysters into many shellfish and other related products, especially in seafood that is known to come from potentially polluted areas; depurated seafood is effectively a product cleansed from inside-out to make it safe for human consumption.
As shellfish, consumption of oyster is forbidden byJewish dietary law. Similarly, inIslam,JaʽafariShiaandHanafiSunnidietary jurisprudenceprohibit consuming bivalves, including oysters, as it ismakruh(highly disliked).
Oysters are subject to various diseases which can reduce harvests and severely deplete local populations. Disease control focuses on containing infections and breeding resistant strains, and is the subject of much ongoing research.
Some oysters also harbor bacterial species which can cause human disease; of importance isVibrio vulnificus, which causesgastroenteritis, which is usually self-limiting, andcellulitis. Cellulitis can be severe and rapidly spreading, requiring antibiotics, medical care, and in some severe cases amputation. It is usually acquired when the contents of the oyster come in contact with a cut skin lesion, as when shucking an oyster.