OYSTER is the common name for a number of different families of salt-water bivalve molluscs that live in marine or brackish habitats. In some species the valves are highly calcified, and many are somewhat irregular in shape. Many, but not all, oysters are in the superfamily Ostreoidea .
Some kinds of oysters are commonly consumed by humans, cooked or raw, and are regarded as a delicacy . Some kinds of pearl oysters are harvested for the pearl produced within the mantle . Windowpane oysters are harvested for their translucent shells, which are used to make various kinds of decorative objects.
* 1 Etymology
* 2 Types
* 2.1 True oysters
* 7.1 Fishing from the wild * 7.2 Cultivating oysters * 7.3 Restoration and recovery
* 8 Depuration
* 9 As food
* 9.1 Nutrition * 9.2 Selection, preparation and storage * 9.3 Opening oysters
* 10 Diseases * 11 See also * 12 References * 13 External links
First attested in English during the 14th century, the word "oyster"
True oysters are members of the family Ostreidae . This family includes the edible oysters, which mainly belong to the genera Ostrea , Crassostrea , Ostreola , Magallana , and Saccostrea . Examples include the Belon oyster , eastern oyster , Olympia oyster , Pacific oyster , and the Sydney rock oyster .
Removing a pearl from a pearl oyster Main article: Pearl oyster
Almost all shell-bearing mollusks can secrete pearls, yet most are not very valuable.
The largest pearl-bearing oyster is the marine Pinctada maxima, which is roughly the size of a dinner plate. Not all individual oysters produce pearls naturally. In fact, in a harvest of two and a half tons of oysters, only three to four oysters produce what commercial buyers consider to be absolute perfect pearls.
In nature, pearl oysters produce pearls by covering a minute invasive object with nacre . 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 natural pigment of the nacre, and the shape of the original irritant.
OTHER TYPES OF OYSTERS
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:
* Thorny oysters in the genus Spondylus * Pilgrim oyster, another term for a scallop , in reference to the scallop shell of St. James * Saddle oysters , members of the Anomiidae family also known as jingle shells * Dimydarian oysters , members of the family Dimyidae * Windowpane oysters
Pacific oyster , opened
In the Philippines, a local thorny oyster species known as Tikod Amo is a favorite seafood source in the southern part of the country. Because of its good flavor, it commands high prices.
Oysters are filter feeders , drawing water in over their gills
through the beating of cilia . Suspended 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
Oysters feed most actively at temperatures above 10 °C
(50 °F). An oyster can filter up to 5 L (1.3 US gal) of water per
Chesapeake Bay 's once-flourishing oyster population
historically filtered excess nutrients from the estuary's entire water
volume every three to four days. Today, that would take nearly a year.
Excess sediment, nutrients, and algae can result in the
eutrophication of a body of water.
In addition to their gills, oysters can also exchange gases across their mantles, which are lined with many small, thin-walled blood vessels . A small, three-chambered heart , lying under the adductor muscle , pumps colorless blood to all parts of the body. At the same time, two kidneys , 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 of ganglia.
While some oysters have two sexes (European oyster and Olympia oyster), their reproductive organs contain both eggs and sperm. Because of this, it is technically possible for an oyster to fertilize its own eggs. The gonads surround the digestive organs, and are made up of sex cells, branching tubules, and connective tissue.
Once the female is fertilized, she discharges millions of eggs into the water. The larvae develop in about six hours and exist suspended in the water column as veliger larvae for two to three weeks before settling on a bed and maturing to sexual adulthood within a year.
HABITAT AND BEHAVIOUR
Oyster reef at about mid-tide off fishing pier at Hunting Island State Park , South Carolina
As a keystone species , oysters provide habitat for many marine species. Crassostrea and Saccostrea live mainly in the intertidal zone , while Ostrea is subtidal . 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 sea anemones , barnacles , and hooked mussels , inhabit oyster reefs . Many of these animals are prey to larger animals, including fish, such as striped bass , black drum and croakers .
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 are protandric ; during their first year, they spawn as males by releasing sperm into the water. As they grow over the next two or three years and develop greater energy reserves, they spawn as females by releasing eggs . Bay oysters usually 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 adult conspecifics . Pacific oyster Crassostrea gigas equipped with activity electrodes to follow their daily behavior 24/7
Oysters are considered to filter large amounts of water to feed and breathe (exchange O2 and CO2 with water) but they are not permanently open. They regularly shut their valves to enter a resting state, even when they are permanently submersed. In fact their behavior 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 The website MolluSCAN eye is largely devoted to the online study of their daily valve behavior in Europe (France).
Some tropical oysters, such as the mangrove oyster in the family
Ostreidae , grow best on mangrove roots. Low tide can expose them,
making them easy to collect. In
The largest oyster-producing body of water in the
Common oyster predators include crabs , seabirds , starfish , and humans . Some oysters contain live crabs, known as oyster crabs .
As an ecosystem engineer oysters provide "supporting" ecosystem services , along with "provisioning", "regulating" and "cultural" services. Oysters influence nutrient cycling , water filtration , habitat structure, biodiversity , and food web dynamics. Oyster feeding and nutrient cycling activities could "rebalance" shallow, coastal ecosystems if restoration of historic populations could be achieved. Furthermore, assimilation of nitrogen and phosphorus into shellfish tissues provides an opportunity to remove these nutrients from the environment, but this benefit has only recently been recognized. In California's Tomales Bay, native oyster presence is associated with higher species diversity of benthic invertebrates but other ecosystem services have not been studied. As the ecological and economic importance of oyster reefs has become more widely acknowledged, creation of oyster reef habitat through restoration efforts has become more important- often with the goal of restoring multiple ecosystem services associated with natural oyster reefs.
Middens testify to the prehistoric importance of oysters as food,
with some middens in
New South Wales
The French seaside resort of
In the early 19th century, oysters were cheap and mainly eaten by the
working class . Throughout the 19th century, oyster beds in New York
Harbor became 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
New York City
In the United Kingdom, the native variety (
Ostrea edulis) requires
five years to mature and is protected by an
Act of Parliament
FISHING FROM THE WILD
Oysters are harvested by simply gathering them from their beds. 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 that are too deep to reach directly. In all cases, the task is the same: the oysterman scrapes oysters into a pile, and then scoops them up with the rake or tongs.
In some areas, a scallop 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 the oysters in its path. While dredges collect
oysters more quickly, they heavily damage the beds, and their use is
highly restricted. Until 1965,
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. Hope is believed to be the last-built Connecticut oyster sloop, completed in 1948.
Oysters can also be collected by divers .
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 for well over a century. The Pacific 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 of mariculture ponds. When fish or prawns are grown in ponds, it takes typically 10 kg (22 lb) of feed to produce 1 kg (2.2 lb) of product (dry-dry basis). 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 crossbreeding tetraploid and diploid oysters. The resulting triploid oyster cannot propagate, which prevents introduced oysters from spreading into unwanted habitats.
RESTORATION AND RECOVERY
In many areas, non-native oysters have been introduced in attempts to prop up failing harvests of native varieties. For example, the eastern 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 in Pendrell 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 the Great Wicomico River in the Chesapeake Bay , five-year-old artificial reefs now harbor more than 180 million native Crassostrea 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 m3 (25,000,000 imp bsh) annually. The 2009 harvest was less than 7,300 m3 (200,000 imp bsh). Researchers claim the keys to the project were:
* using waste oyster shells to elevate the reef floor 25–45 cm (9.8–17.7 in) to keep the spat free of bottom sediments * building larger reefs, ranging up to 8.1 ha (20 acres) in size * disease-resistant broodstock
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
Depuration of 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.
Wikimedia Commons has media related to OYSTER DISHES .
Raw oysters presented on a plate Oysters served on ice and with a piece of lemon on the side
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 as Vibrio parahaemolyticus have caused outbreaks in several harvesting areas of the eastern United States during the summer months, lending further credence to this belief.
Oysters are an excellent source of zinc , iron , calcium , and selenium , as well as vitamin A and vitamin B12 . Oysters are low in food energy ; one dozen raw oysters contains 110 kilocalories (460 kJ). They are rich in protein (approximately 9g in 100g of pacific oysters).
Traditionally, oysters are considered to be an aphrodisiac , 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 of sex hormones . Their high zinc content aids the production of testosterone.
Dietary supplements may contain calcium carbonate from oyster shells, though no evidence shows this offers any benefits beyond what calcium may offer.
SELECTION, PREPARATION AND STORAGE
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. Oysters should be refrigerated out of water, not frozen, and in 100% humidity. Oysters stored in water under refrigeration will open, consume available oxygen, and die. Freshly opened pearl oysters
Oysters must be eaten alive, or cooked alive. The shells of live
oysters are usually tightly closed or snap shut given a slight tap. If
the shell is open, the oyster is dead, and cannot be eaten safely.
Cooking oysters in the shell kills the oysters and causes them to open
by themselves. Traditionally, oysters that do not open have been
assumed to be dead before cooking and therefore unsafe. However,
according to at least one marine biologist, Nick Ruello, this advice
may have arisen from an old, poorly researched cookbook's advice
regarding mussels, which has now become an assumed truism for all
shellfish. Ruello found 11.5% of all mussels failed to open during
cooking, but when forced open, 100% were "both adequately cooked and
safe to eat."
Giant oyster in southern
Oysters can be eaten on the half shell, raw, smoked , boiled , baked
, fried , roasted , stewed , canned , pickled , steamed , or broiled ,
or used in a variety of drinks. Eating can be as simple as opening the
shell and eating the contents, including juice.
Care should be taken when consuming oysters. Purists insist on eating
them raw, with no dressing save perhaps lemon juice, vinegar (most
commonly shallot vinegar), or cocktail sauce . Upscale restaurants
pair raw oysters with a home-made
Mignonette sauce , which consists
primarily of fresh chopped shallot , mixed peppercorn , dry white wine
and lemon juice or sherry vinegar . Like fine wine, raw oysters have
complex flavors that vary greatly among varieties and regions: salty,
briny, buttery, metallic, or even fruity. The texture is soft and
fleshy, but crisp on the palate. North American varieties include:
Yaquina Bay from Oregon,
Duxbury and Wellfleet from
Oysters can contain harmful bacteria . Oysters are filter feeders, so will naturally concentrate anything present in the surrounding water. Oysters from the Gulf Coast of the United States, for example, contain high bacterial loads of human pathogens in the warm months, most notably Vibrio vulnificus and Vibrio parahaemolyticus . In these cases, the main danger is for immunocompromised individuals, who are unable to fight off infection and can succumb to septicemia , leading to death. Vibrio vulnificus is the most deadly seafood-borne pathogen .
Fresh oysters must be alive just before consumption or cooking. 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'.
Opening oysters, referred to as oyster-shucking, requires skill. The preferred method is to use a special knife (called an oyster knife , a variant of a shucking knife ), with a short and thick blade about 5 cm (2.0 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.
* Insert the blade, with moderate force and vibration if necessary, at the hinge between the two valves. * Twist the blade until there is a slight pop. * Slide the blade upward to cut the adductor muscle which holds the shell closed.
Inexperienced shuckers can apply too much force, which can result in injury if the blade slips. Heavy gloves are necessary; 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.
Opening or "shucking" oysters has become a competitive sport.
Oyster-shucking competitions are staged around the world. Widely
acknowledged to be the premiere event, the Guinness World Oyster
Opening Championship is held in September at the Galway Oyster
Festival. The annual Clarenbridge
Media related to Oyster-shucking at Wikimedia Commons
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.
* "Dermo" is caused by a protozoan parasite (
Perkinsus marinus ). It
is a prevalent pathogen , causes massive mortality, and poses a
significant economic threat to the oyster industry. The disease is not
a direct threat to humans consuming infected oysters. Dermo first
appeared in the
Gulf of Mexico
* Multinucleated sphere X (MSX) is caused by the protozoan Haplosporidium nelsoni , generally seen as a multinucleated Plasmodium . It is infectious and causes heavy mortality in the eastern oyster ; survivors, however, develop resistance and can help propagate resistant populations. MSX is associated with high salinity and water temperatures. MSX was first noted in Delaware Bay in 1957, and is now found all up and down the East Coast of the United States. Evidence suggests it was brought to the US when Crassostrea gigas , a Japanese oyster variety, was introduced to Delaware Bay.
Some oysters also harbor bacterial species which can cause human disease; of importance is Vibrio vulnificus , which causes gastroenteritis , which is usually self-limiting, and cellulitis . Cellulitis can be so severe and rapidly spreading, often it requires amputation. It is usually acquired when the contents of the oyster come in contact with a cut skin lesion, as when shucking an oyster.
Wikibooks Cookbook has a recipe/module on
* Food portal
Angels on horseback (classic recipe)
* List of delicacies
List of smoked foods
Ostrea angasi (Australian southern mud or native flat oyster)
Oysters in Cynee, Recipe for
Oysters in Bread Sauce (
Cynee) from the 1390 English text, The Forme of Cury, from Celtnet
* ^ ostrea, ostreum, Charlton T. Lewis, Charles Short, A Latin
Dictionary, on Perseus
* ^ ὄστρεον, Henry George Liddell, Robert Scott, A
Greek-English Lexicon, on Perseus
* ^ ὀστέον, Henry George Liddell, Robert Scott, A
Greek-English Lexicon, on Perseus
* ^ A B "A dozen ocean-cleaners and a pint of Guinness, please".
The Economist. 2008-12-18. Retrieved 2008-12-26.
* ^ "
* ^ http://dirt.asla.org/2011/02/24/oyster-tecture-in-action/
* ^ Welcome to BayKeeper
* ^ http://www.scapestudio.com/projects/oyster-tecture/
* ^ (
Wikimedia Commons has