The shell of Crassostrea gigas varies widely with the environment where it is attached. Its large, rounded, radial folds are often extremely rough and sharp. The two valves of the shell are slightly different in size and shape, the right valve being moderately concave. Shell colour is variable, usually pale white or off-white. Mature specimens can vary from 80 mm to 400 mm long
The flat oyster Ostrea Edulis, a native of Europe, has been part of the human diet for many centuries. The Romans built ponds to stock and sort oysters. In the 17th century, oyster spat were collected on rocks, separated from each other and deployed into ponds in salt marshes on the Atlantic coast of France. A decline in activity in salt marshes facilitated oyster culture development by expanding grow-out acreage availability. During the 18th and 19th centuries, fishing effort led to over-exploitation, failing recruitment, and destruction of European natural beds, which were also affected by extremely cold winters. Shortage in seed supply prompted the managers to develop cultural practices aimed to sustain a repletion and reseeding programme. Eventually, the 'leasing ground' system and the development of artificial spat collectors and their systematic use facilitated the development of the sector. Mostly in intertidal areas, wooden spat collectors were initially used followed by oyster shell strings and slates. Then, in 1865, the liming tile technique (roof tiles coated with a mixture of lime and fine sand), and wooden boxes to grow juveniles were developed in south-western France. Coated tiles became the main method of spat collection in France and the Netherlands: spat were removed by hand after 6-10 months, then reared in trays or re-laid in subtidal plots. On the Mediterranean coast, off-bottom culture was initiated in 1900, using oysters cemented onto steel poles. Growout facilities were developed in shallow waters (3-4 m), then oyster culture increased substantially by transferring spat from traditional spatfall areas (Brittany). Spat were cemented individually onto poles, which were hung from frameworks developed over mussel leasing grounds. This form of culture was replaced by cupped oyster (Crassostrea angulata) production in 1950.
The most obvious cultural changes during the 20th century occurred in two areas: spat collection techniques and the occurrence of disease problems in oyster populations. Spat collection techniques changed when seeding cockle (1904), then mussel (1939) shells in subtidal waters became the common practice in the Netherlands. This technique was more and more cost effective requiring far less labour. Since the 1980s, the use of tubular nets filled with mussel shell and deployed off-bottom has also proven more cost effective in southern Brittany, France. More recently, hatcheries have begun to produce cultchless flat oyster spat.
With regard to disease, a massive mortality widely struck European flat oyster populations in 1920. The population later recovered but was replaced by cupped oysters in several traditional rearing areas. Then, two diseases (Marteilia refringens and Bonamia ostrea) spread in the early 1970s and 1980s, drastically reducing the production of O. edulis in almost all European traditional rearing areas. Despite new management practices, and intensive repletion programmes, the production of O. edulis has remained low since that time.
The European flat oyster is found along the western European coast from Norway to Morocco in the north-eastern Atlantic and in the whole Mediterranean Basin. Natural populations are also observed in eastern North America from Maine to Rhode Island, following intentional introductions in the 1940s and 1950s.
Ostrea edulis is a protandric hermaphrodite, changing sexes generally twice during a single season. Oysters function as males early in the spawning season and later change to females and vice versa. The flat oyster is usually male in the fall following its settlement. O. edulis exists as a series of physiological races, and genetic differentiation has been demonstrated along the European coastline. One of the lowest temperature races occurs in Spain where 12-13 °C is required for spawning while 25 °C is the spawning temperature in Norwegian fjords. In France, gametogenesis occurs at 10 °C and spawning between 14 and 16 °C. Female gametes are liberated into the palleal cavity where they are fertilized by externally released sperm. In contrast to the large C. gigas reproductive effort, O. edulis produces between 500 000 and 1 million eggs per spawning. Following an incubation period of 8-10 days, depending on temperature, final release into environment occurs. Then larvae (160 µm in size) spend 8 to 10 days as a pelagic stage before settlement. Appropriate larval growth and survival rates are obtained in salinities as low as 20‰, although they can survive at salinities as low as 15‰.
Little Known Facts about Oysters...
How Oysters Breathe
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 a pair of kidneys located on the underside of the muscle purify the blood of any waste products it has collected.
Male or Female
There is no way of telling 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.
What is that tiny crab we see in an oyster?
It is a species of crab (Pinnotheres ostreum) that has evolved to live harmoniously inside an oyster's shell. These dime-sized crabs, much sought after by gourmands, are not abundant.
How do pearls end up inside of oysters?
An oyster produces a pearl when foreign material becomes trapped inside the shell. The oyster responds to the irritation by producing nacre, a combination of calcium and protein. The nacre coats the foreign material and over time produces a pearl.
The "R" Myth
Folklore says that oysters should be eaten only in months with (r's) in them like September, October, etc. however oysters can be eaten 12 months a year. The notion that oysters should not be eaten in 'r' less months, that is, months that occur during warm weather may have started in the days when oysters where shipped without adequate refrigeration and could spoil. But today all that has changed and we can enjoy oysters twelve months a year.
Oysters and Their Nutritional Value
Oysters are not only delicious, but they're also one of the most nutritionally well balanced of foods, containing protein, carbohydrates and lipids. The National Heart and Lung Institute suggest oysters as an ideal food for inclusion in low-cholesterol diets. Oysters are an excellent source of vitamins A, B1(thiamin), B2 (riboflavin), B3 (niacin), C (ascorbic acid) and D (calciferol). Four or five medium size oysters supply the recommended daily allowance of iron, copper, iodine, magnesium, calcium, zinc, manganese and phosphorus.
Oysters are bivalve (having two shells) mollusks that are related to clams, mussels, and scallops. They may be served raw, usually sitting on one shell, or smoked, baked, or as the featured ingredient in oyster stew. Oysters are harvested from both Pacific and Atlantic waters. Oysters might be a superfood that's good for your health and may even make you feel sexier (and some say it's a aphrodisiac). They're an excellent source of zinc, which your body needs for hundreds of different biochemical processes to occur. Zinc deficiency can be bad for your immune system and can inhibit growth (but don't worry - eating lots of oysters won't cause a zinc toxicity). A serving of oysters is also a good source of calcium, potassium, iron, magnesium (magnesium deficiency can cause fatigue and other problems), and vitamin B-12.