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Still-Life with Oysters by Alexander Adriaenssen

from Wikipedia

The word oyster is used as a common name for a number of different families of bivalve molluscs that live in marine or brackish habitats.

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, and the Sydney rock oyster.

Pearl oysters are not closely related to true oysters, being members of a distinct family, the feathered oysters (Pteriidae).

 

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. 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 hour. Excess sediment, nutrients, and algae can result in the eutrophication of a body of water. Oyster filtration can mitigate these pollutants.  Oysters consume nitrogen-containing compounds (nitrates and ammonia), 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.

In addition to their gills, oysters can also exchange gases across their mantles, which are lined with many small, thin-walled blood vessels.

 

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 egg. 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 swim around for about two to three weeks. After that, they settle on a bed and mature within a year.  A group of oysters is commonly called a bed or oyster reef.

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. 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 (0.98 in) long. Many species of bivalves, oysters included, seem to be stimulated to settle near adult conspecifics.

Oysters 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).

In the United Kingdom, the native variety (Ostrea edulis) requires five years to mature and is protected by an Act of Parliament during the May to 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 parts of the world, dredges are used. 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 you do not get the best oysters. Hand picking guarantees that only the plumpest and best quality are used.

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.

 

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 May, June, July, and August.

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.

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 Oyster Festival 'Oyster Opening Competition' is also held in Galway, Ireland.

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