The not so humble Oyster
Bivalve molluscs belonging to the familises Ostreidae (true oysters) and Pteriidae (Pearl Oysters), edible oysters of commercial importance belong to two genera, Ostrea and Crassostrea. There are over 100 species of oyster, widely distributed throughout all oceans with warm and temperate waters.
In cultivation the spat, young oysters, are trapped on milled spats which are set out on timber racks in groups (or trapped on bundles of mangrove tree branches). These frames are then restacked facing inwards in 'depots' for up to 18 months while the spats mature. The depots are broken up and the young oysters spaced apart on frames before they are sufficiently grown to touch each other, but mature enough to resist predators. Generally it takes another 18 months for oysters to reach marketable size.
Photo credit: Taken by me
Anatomy of an Oyster
The two valves or shells of the oyster are irregular and quite unequal. The left or lower valve is hollowed to accommodate the body; the right or upper valve is thinner, more flattened, and in some specimens concave. These valves, though lined with lustrous nacre, become greatly thickened with age, when their outer surfaces appear rough and shaggy.
When free growing, they present a pear-shaped outline, broadening out from the hinge, which is devoid of teeth. Although a specimen of Crassostrea virginica five inches long is relatively large, ancient shells a foot in length have been dredged up from tidal rivers.
The living oyster deviates somewhat from the bilateral symmetry characteristic of most bivalves, as the right side is less developed than the left. The single adductor muscle which passes through the body to connect the shells is capable of a contractual pull seemingly quite out of proportion to its size. The gills are nearly equal; the mantle edges free and bearing a marginal fringe. In a freshly opened oyster the heart may be observed still pulsating, the blood a watery blue in color. As the oyster is a sedentary type and once located never voluntarily moves again, the foot or organ of locomotion is small or almost entirely lacking.
Some Serving Suggestions... - If you must add anything!
- Sprinkle with an aged sweet vinegar
- A squeeze of lime juice
- Add a drop of green tabasco
- Serve atop a nori roll
- Scatter onto an omelette just before turning, and serve with a sprinkle of soy sauce.
- Toss into your favourite stir fry or noodle dish just before serving
During the spawning season, which occurs in spring or summer, the generative organs, comprising a number of branching cavities, occupy a considerable portion of the body volume. The genus is prolific, as specimens have been reported bearing as many as 125,000,000 ova.
Inmost species, including the important Ostrea virginica and Ostrea angulata, the sexes are separate, and the prospect that the ova extruded into the water by the female may come in contact with the well-nigh numberless sperm ejected by the male is governed entirely by chance. From the fertilized egg a tiny organism emerges which develops a ciliated velum or foot in the veliger or free-swimming stage. As it may be carried many miles by tides and currents, the vast majority fall a prey to various forms of marine life, including adult oysters, or succumb to other hazards. After a period of some days the survivors develop true shells and sink to the bottom.
In some species, however, notably the Ostrea edulis of Europe, the oyster passes through a curious cycle of sex transition. The adult. upon reaching maturity, is a male, but changes to a female and may, if the length of the season permits. repeat the process. As ova and sperm do not seem to meet in the same individual. such sex variation may be regarded as an alternating hermaphroditism. When the ova become fertilized within the body cavity, the female oyster is said to be "white sick." A fortnight later when the spawn resembles a cloud of dust, she is said to be "black sick." The extruded spawn, known as trochospheres, about Vr50 of an inch in length, soon develop into veligers and after a brief free-swimming stage also sink to the bottom.
Food and Growth
Oysters of all species attach themselves by a limy secretion from the left mantle fold to any substantial support-a stone, an empty shell, a bottle or even an old boot. This object may become peppered with tiny oysters. At first whitish discs about 1/20 of an inch in diameter, they turn brown as rapid growth produces more visible shells and are then known to the trade as "spat". When these in turn attain a length of an inch or more, they become "seed oysters" and are ready to be transplanted to some more favorable locality.
Three things largely determine the rate of growth-the nature and amount of the food supply, the temperature of the water, and its relative salinity. Food consists of minute organisms, both plant and animal, which float in the water protozoans, the eggs and larvae of marine worms and mollusks, and the spores of algae. But the bulk of oyster food comprises diatoms, tiny plants usually enclosed in glassy cases which are often etched with intricate designs. In polar seas diatoms swarm in such myriads that they impart a greenish tinge to the water and even a gritty feeling when rubbed between the fingers.
Though less numerous where oysters grow, they st ill abound in the shallows, and even more so on mud surfaces exposed at low tide, where their gardens sometimes color the surface with microscopic verdure.
Water temperatures are also a consideration. Although various species of oysters can survive in temperatures ranging from above 90F to near the freezing point. the common or edible varieties find their optimum warmth somewhere between 50 and 70 F. In northerly waters four or five years are required to develop a mature oyster. In Chesapeake Bay, the favorite breeding ground of Ostrea virginica, a similar growth may occur in a year and a half. Temperatures also determine reproduction. In the warm Gulf of Mexico oysters may spawn when four months old; in Long Island Sound, when a year old; in Cape Cod waters, not until the second season; in the chill seas of northern latitudes they may not spawn at all.
The salinity of the water is also a variable but important factor. Although oysters are marine mollusks, they prefer waters that are slightly brackish. Hence, they thrive in inlets fed by streams or shallows. where the salinity is somewhat tempered by rain overflows from the land. Too much fresh water, however, is detrimental, and oysters are sometimes killed by inundations from spring freshets. Hence, in the mouths of French rivers they would die out if not frequently replenished, just as they have largely disappeared from the ever-freshening Baltic Sea. They require water which contains at least three per cent salt.
Oysters are low in food energy; one dozen raw oysters contain approximately 110 calories (0.460 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. Precautions should be taken when consuming them. Purists insist on eating oysters raw, with no dressing save perhaps lemon juice, 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, such as Christmas. 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 along the East Coast of the US and are still easily found in states bordering the ocean. Oysters are nearly always eaten raw in France.
Fresh oysters must be alive just before consumption. There is a simple criterion: oysters must be tightly closed; oysters that are already open are dead and must be discarded.
To confirm if an open oyster is dead, tap the shell. A live oyster will close and is safe to eat, a dead oyster can also be closed however it will make a distinct noise when tapped and are called "clackers".
Opening oysters requires skill, for live oysters, outside of the water, shut themselves tightly with a powerful muscle sealing 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, inserting the blade (with some moderate force and vibration if necessary) at the hinge in the rear of the shell, and sliding it upward to cut the adductor muscle (which holds the shell closed).
Inexperienced shuckers tend to apply excessive force, which may result in injuries if they slip. Always use a heavy glove; 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 this technique is available here.
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, is tricky when an oyster's shell is in such a condition.
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. Cooked oysters are savory and slightly sweet-tasting, and the varieties are mostly equivalent.
In The Kitchen
Harvesting consists of removing the frames and dislodging the oysters. They are then sorted, the smaller ones being returned to the frames for further growth. Because a large proportion of oysters are eaten raw the risk of contamination by bacteria is high. Since the late 70's the industry introduced stringent purification measures to have the risk reduced to a minimum.
In pearl cultivation, a pearl is formed by the gradual buildup of a type of calcium material exuded by an oyster around a foreign object inside the shell. Although pearls can be grown in edible oysters they do not have the luster of those produced by true pearl oysters.
How do you like them? Raw? Or do you have a favorite recipe?