Some kinds of oyster are commonly consumed by humans, cooked or raw. Other kinds, such as pearl oysters, are not.
True oysters are members of the family Ostreidae. This family includes the edible oysters, which mainly belong to the genera Ostrea, Crassostrea, Ostreola and Saccostrea. Examples include the Belon oyster, eastern oyster, Olympia oyster, Pacific oyster, Sydney rock oyster and the Wellfleet oyster.
Almost all shell-bearing molluscs can secrete pearls, yet most are not very valuable.
Pearl oysters are not closely related to true oysters, being members of a distinct family, the feathered oysters (Pteriidae). Both cultured pearls and natural pearls can be obtained from pearl oysters, though other molluscs, such as the freshwater mussels, also yield pearls of commercial value.
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 three tons of oysters, only three to four oysters produce perfect pearls.
In nature, pearl oysters produce natural pearls by covering a minute invading parasite with nacre, not by ingesting a grain of sand. Over the years, the irritating object is covered with enough layers of nacre to form what is known as a pearl. There are many different types, colours and shapes of pearl; these qualities depend on the natural pigment of 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 six years, the oyster can 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 increasingly scarce, and a necklace with only natural pearls can easily cost several hundred thousand US dollars.
A number of bivalve molluscs 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:
Crassostrea gigas, opened
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 faeces or pseudofaeces. Oysters feed most actively at temperatures above 10 °C (50 °F). An oyster can filter up to 5 litres (1.3 US gal) of water per hour. 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. Oyster filtration can mitigate these pollutants.
In addition to their gills, oysters can also exchange gases across their mantle, which is 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.
While oysters have two sexes, they may change sex one or more times during their life span. Because of this, it is technically possible for an oyster to fertilize its own egg. The gonads surround the digestive organs, and are made up of sex cells, branching tubules and connective tissue. Oysters cannot be sexed by examining the shell.
A group of oysters is commonly called a bed or oyster reef.
As a keystone species, oysters provide habitat for many marine species. Crassostrea and Saccostrea live mainly in the intertidal zone, while Ostrea 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 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 by the end of June. 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 millimetres (0.98 in) long. Many species of bivalve, oysters included, seem to be stimulated to settle near adult conspecifics.
Some tropical oysters in the family Isognomonidae grow best on mangrove roots. Low tide can expose them, 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 consume nitrogen-containing compounds (nitrates and ammonia), removing them from the water. Nitrogen compounds are important phytoplankton nutrients. Phytoplankton increase water turbidity. Limiting the amount of phytoplankton in the water improves water quality and other marine life by reducing competition for dissolved oxygen. Oysters feed on plankton, incidentally consuming nitrogen compounds as well. They then expel solid waste pellets which decompose into the atmosphere as nitrogen. In Maryland, the Chesapeake Bay Program plans to use oysters to reduce the amount of nitrogen compounds entering the Chesapeake Bay by 19,000,000 pounds (8,600,000 kg) per year by 2010.
Middens testify to the prehistoric importance of oysters as food. In the United Kingdom, the town of Whitstable is noted for oyster farming from beds on the Kentish Flats that have been used since Roman times. The borough of Colchester holds an annual Oyster Feast each 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 Festival is held in September. Many breweries produce Oyster Stout, a beer intended to be drunk with oysters that sometimes includes oysters in the brewing process.
The French seaside resort of Cancale is noted for its oysters, which also date from Roman times. Sergius Orata of the Roman Republic is considered the first major merchant and cultivator of oysters. Using his considerable knowledge of hydraulics, he built a sophisticated cultivation system, including channels and locks, to control the tides. He was so famous for this that the Romans used to say he could breed oysters on the roof of his house.
In the early 1800s, 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. Oysters were naturally quite popular in New 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, when combined with effluent and increasing sedimentation from erosion, which destroyed most of the beds by the early 20th century. Oysters’ popularity has put by 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 the United Kingdom, the native variety is still held to be the finest, requiring 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 year round.
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 Maryland limited dredging to sailboats, and even since that date motor boats can be used only 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 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. 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 or Japanese oyster, Crassostrea gigas has been grown in the outflow of mariculture ponds. When fish or prawn are grown in ponds, it takes, typically 10 kilograms (22 lb) of feed to produce 1 kilogram (2.2 lb) of product (dry-dry basis). The other 9 kilograms (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. Because the resulting triploid oyster cannot propagate, spawning does not occur preventing 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, 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 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 still a far cry from the late 1880s, when the Bay’s population was in the billions, and watermen harvested about 25 million imperial bushels (910,000 m3) annually. The 2009 harvest was less than 200,000 imperial bushels (7,300 m3). Researchers claim that the keys to the project were:
In 2005, China accounted for 80% of the global oyster harvest. Within Europe, France remained the industry leader.
Jonathan Swift is quoted as having said, "He was a bold man that first ate an oyster", but evidence of oyster consumption goes back into prehistory, 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, especially 'wild', are excellent sources of several minerals, including iron, zinc and selenium, which are often low in the modern diet. They are also an excellent source of Vitamin B12. Oysters are considered the healthiest when eaten raw on the half shell.
Oysters can be eaten on the half shell, raw, smoked, boiled, baked, fried, roasted, stewed, canned, pickled, steamed, broiled or used in a variety of drinks. Preparation widely varies. It can be as simple as opening the shell and eating the contents, including juice. Butter and salt are often added. In the case of oysters Rockefeller, preparation can be very elaborate. They are sometimes served on edible seaweed, such as brown algae.
Unlike most shellfish, oysters can have a fairly long shelf-life: up to two weeks; however, their (decreasingly pleasing) taste reflects their 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. 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: sweet, salty, earthy, or even melon. The texture is soft and fleshy, but crisp on the palate. North American varieties include: Kumamoto and Yaquina Bay from Washington State, Malpeque from Prince Edward Island, Canada, Blue Point from Long Island, New York, and Cape May oysters from New Jersey. Salinity, mineral, and nutrient variations in the water that nurtures them influence their flavor profile.
Non-local oysters are generally expensive. In the United States, oysters are most often cooked, but there is also a high demand for raw oysters on the half-shell (shooters) at oyster bars. Canned smoked oysters are also widely available as preserves with a long shelf life. Raw oysters are still found in many temperate 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 only safe to eat in months with the letter ‘r’ in their English and French names. This is a myth whose basis in truth is that in the northern hemisphere oysters are much more likely to spoil in May, June, July, and August.
Oysters must be eaten alive, or cooked alive. The shells of live oysters are normally 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. Oysters that don't open were dead before cooking and are unsafe.
Oysters can contain harmful bacteria. Oysters are filter feeders and 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 immuno-compromised 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, 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. A team of American and Italian researchers analyzed bivalves and found they were rich in rare amino acids that trigger increased levels of sex hormones. Also oysters have a high zinc content, a mineral that aids in the production of testosterone. Alternatively, the oyster's erotic reputation may only be due to its soft, moist texture and appearance.
Fresh oysters must be alive just before consumption or cooking. There is a simple 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 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 centimetres (2.0 in) long.
Insert the blade, with moderate force and vibration if necessary, at the hinge between the two valves. Then twist the blade until there is a slight pop. Then 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 less than 3 seconds to do the deed.
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 and there is 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 Guiness World Oyster Opening Championship is held in September at the Galway Oyster Festival, in Galway Ireland.
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.