|
|
|
|
Oyster
|
| |
|
| |
The common name oyster is used for a number of different groups of bivalve mollusks, most of which live in marine habitats or brackish water.
Nutrition Facts
Discussion
Ask a question about 'Oyster' Start a new discussion about 'Oyster' Answer questions from other users |
Encyclopedia
The common name oyster is used for a number of different groups of bivalve mollusks, most of which live in marine habitats or brackish water. The shell consists of two usually highly calcified valves which surround a soft body. Gills filter plankton from the water, and strong adductor muscles are used to hold the shell closed.
Some types of oysters are highly prized as food, both raw and cooked. Other types, such as pearl oysters, are not commonly eaten.
True oysters, belonging to the family Ostreidae, 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.
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).
Physical characteristics
- 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 faeces or pseudofaeces. 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 litres of water per hour. Scientists believe that the Chesapeake Bay's once-flourishing oyster populations historically filtered the estuary's entire water volume of excess nutrients in approximately three to four days. Today that process would take nearly 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 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.
- 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.
Habitat and life habits
As a keystone species, oysters provide habitat for an extensive array of marine life. There are three main groups of oysters, the Ostrea species, the Crassostrea species and the Saccostrea species.
Crassostrea and Saccostrea species live mainly in the intertidal zone while Ostrea species are 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 anemones, barnacles, and hooked mussels use oyster reefs as habitat. Many of these animals serve as food for larger animals, including fish such as 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, 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.
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. Many species of bivalve, oysters included, seem to be stimulated to settle by the proximity of adults of their species.
A group of oysters is commonly called a bed.
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."
The oyster's greatest predators include crabs, sea birds, sea stars, and humans. Some oysters contain live crabs, known as an Oyster crab.
Fighting marine pollution
Oysters are known for their role of filtering and removing nitrogen from water. In fact, nitrogen is the main food of phytoplanktons who gather in masses at the surface disallowing sunlight from reaching deeper waters. Oysters feed on plankton and, thus, expel solid pellets of waste which get decomposed into the atmosphere as nitrogen. In Maryland, the Chesapeake Bay Program plans --with the help of oysters-- to cut the amount of nitrogen entering the Chesapeake Bay by 19 million pounds a year by 2010.
Oysters as food
Although Jonathan Swift is often quoted as having said, "He was a bold man that first ate an oyster", evidence of oyster consumption goes back into prehistory, as evidenced by oyster middens found worldwide. 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 treat, celebrated in oyster festivals 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.
Oyster fishing
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.
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 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.
The Pacific (Japanese) oyster, Crassostrea gigas has also been grown in the outflow of mariculture ponds. When fish or prawns are grown in ponds, it takes, typically 10kg of feed to produce 1kg of product (dry-dry basis). The other 9kg goes into the pond and after mineralization, provides the food for phytoplankton. This phytoplankton is the food for the oyster. (ref coming with results)
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. 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 west coast oyster industry. Pendrell sound is now a reserve for the catching of spat for cultivation 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.
Preparation and storage
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.
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 kilocalories (460 kJ), and are rich in zinc, iron, calcium, and vitamin A.
Unlike most shellfish, oysters can have a fairly long shelf-life: up to around two weeks; however, they should be consumed when fresh, as their taste reflects their age. For maximum shelf life, oysters should be stored out of water in refrigeration but not frozen and in 100% humidity. Oysters stored in water under refrigeration will open, utilize the small reserves of oxygen and die. Precautions should be taken when consuming them (see below). 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. 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.
It was once assumed that oysters were unsafe to eat in any month that did not contain an R in it (English). This meant that oysters were unsafe to eat in May, June, July, and August. This is a myth, but it has some basis in truth. Before refrigeration, oysters were much more likely to spoil in these months.
Oysters must be eaten while alive, or cooked alive. When raw, the shell should be tightly closed. If the shell is open, a slight tap should cause it to snap shut. If the shell remains open, the oyster is dead, and cannot be eaten .
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. Ones that don't open were dead before cooking, and should not be consumed.
In addition, depending on the region of the oysters, they can contain harmful bacteria. Oysters are filter feeders and will naturally concentrate anything present in the surrounding sea water. Gulf Coast oysters, 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 immuno-compromised individuals, who are unable to fight off infection and are likely to succumb to septicemia leading to death. Vibrio vulnificus is the most deadly seafood-borne pathogen, with a higher case-to-death ratio than even Salmonella enterica and Escherichia coli.
Oysters are sometimes cited as an aphrodisiac. It is disputed whether this is true. According to the The Daily Telegraph a team of "American and Italian researchers analysed 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." The oyster's erotic reputation may only be due to its soft, moist texture and appearance. However, it may also be due to its high zinc content, a mineral that aids in the production of testosterone.
How to open an oyster
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.
The blade needs to be inserted (with some moderate force and vibration if necessary) at the hinge in the rear of the shell with the blade inserted slightly you need to twist until a slight pop is heard/felt. 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.
A good demonstration of the opening technique is available . 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.
History
Middens 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 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. Many breweries produce Oyster Stout, a beer intended to be drunk with oysters, which sometimes even includes oysters in the brewing process.
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 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.
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, and with brown soda bread and butter on the side, and a squeeze of lemon over each oyster.
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. 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. This scarcity increased prices leading to their current status as a delicacy.
In the United Kingdom, the native variety is still held to be the finest, taking five years to mature and protected by an Act of Parliament during the May-August spawning season. The current market is dominated by the larger Pacific oyster and rock oyster varieties which are farmed all year round.
Pearl oysters
All types of oysters (and, indeed, almost all other shelled molluscs) can secrete concretions that are known by biologists as pearls, but those which sometimes form in edible oysters are unattractive and have no market value at all.
Pearl oysters however are not closely related to true oysters. They are in a totally different family, the Pteriidae (Feathered Oysters). Both cultured pearls and natural pearls can be obtained from these oysters, though other molluscs, such as the freshwater mussels, 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 naturally. In fact, in a haul of three tons of oysters, only around three or four oysters produce perfect pearls.
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 colours 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.
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 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.
Oyster diseases
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
"Dermo" (Perkinsus marinus) is caused by a protozoan parasite. It is a prevalent pathogen of oysters, causing massive mortality in oyster populations and poses a significant economic threat to the oyster industry. The disease is of no direct threat to any humans consuming infected oysters.
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.
MSX
MSX (Multinucleated Sphere 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.
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.
Other mollusks named "oyster"
A number of bivalve mollusks other than edible oysters and pearl oysters also 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)
Aquaculture
In 2005, China accounted for 80 per cent of the global oyster catch according to a FAO study. Within Europe, France remained the industry leader.
See also
External links
-
- Simulate Oyster Growth in real-time.
- University of Maryland Center for Environmental Science
-
-
|
| |
|
|
