Association des conchyliculteurs professionnels du Nouveau -Brunswick

The American oyster, Crassostrea virginica (Gmelin), also known as the Eastern oyster, the Atlantic oyster, and the Virginian oyster, is a native species of the eastern coastline of North America. In Atlantic Canada, the largest populations are located in the southwest portion of the Gulf of St. Lawrence bordering on New Brunswick, Prince Edward Island and Nova Scotia.

Physical Description

The American oyster is a bivalve mollusc filter feeder consisting of a soft body protected by two calcareous shells (or valves) joined together by a tough ligament along a narrow hinge line. The shell of this species is thick and rough. The two valves are asymmetrical. The lower valve is concave to accommodate the body, whereas the upper valve is flat and acts like a lid. A large adductor muscle, attached to both valves, controls the opening and closing of the valves. The thickness, the strength, and the shape of the shells are factors that can vary according to the growing conditions of the animal. These factors can also affect the financial returns of aquaculturists.

Habitat

The typical environment of American oysters is found in the warm waters of bays and estuaries where the oysters are exposed to the ebb and flow of salt water tides. The American oyster can tolerate temperatures between -2^oC and 32^oC and a water salinity ranging between 5‰ and 30‰, the preferred salinity lies between 20‰ and 27‰. In order to reproduce, the American oyster requires a temperature of about 20^oC (68^oF). A reduced salinity can prevent the development of gonads. This species is found primarily in the intertidal zone. The American oyster is very rarely found at a depth below 40 metres.

Food and Growth

Oysters are filter-feeders. They open their valves to let the water in so they can extract tiny food particles (plankton). By filtering water, the oyster also obtains oxygen that enables it to breathe.

The growth rate of the oyster depends on factors such as water temperature and the quantity and quality of the food. When the environmental conditions become favourable, oysters eat almost continuously. The warmer the water gets (eg., 26^oC), the more oysters eat. When the temperature drops to about 4^oC, oysters stop eating.

The growth period of the American oyster stretches from May to November with a break in July for reproduction. It takes 4 to 7 years for the oyster to reach the market length of 76 mm, and from 3 to 4 years to reach cocktail oyster size, which is about 50 mm.

The form and outward appearance of an oyster shell reflects the oyster’s growing conditions. Overcrowding is one cause of shell distortions. On muddy bottoms and overcrowded reefs, oysters grow long and narrow. On hard, clean bottoms with no overcrowding, oysters grow round, compact and deeply cupped; producing oysters of first-rate quality.

Reproduction Cycle and Early Development

The spawning period takes place in the summer and is triggered by a rise in the water temperature. Thus when the temperature reaches 20^oC, the gametes (eggs and sperms) are released into the water column. The spawning generally lasts for a period of 4 to 6 weeks. After fertilization, the fertilized egg develops into a microscopic larva capable of moving around in the water column.

For several weeks, the larvae swim and drift in the tidal currants, feeding on microscopic algae. After undergoing a series of morphological changes, the larvae become capable of attaching themselves to a hard surface where they change and become juveniles or “spat.” Shellfish growers monitor carefully the period during which the larvae attach themselves because they want to collect a maximum amount of spat on the collectors they have anchored a head of time. The spat will grow on the collectors until it is large enough to be transferred to the growing structures.

The juvenile oysters are removed from the collectors and transferred to floating bags which are installed in open water in aquaculture sites. These oysters are sorted several times during their growth cycle and the density per bag is adjusted each time in order to ensure uniform growth. In a temperate zone like New Brunswick, it is only after 4 years of intensive labour that the oyster will reach the consumer.

Mortality

The oyster constitutes the prey of numerous predators, including rock crabs, lobsters, boring sponges, star fish, and mud worms. During its larval phase, the oyster is preyed upon by a group of plankton-feeding organisms, including its own species. Growing oysters off-bottom (in suspension) protects them and limits the impact of predation during the early stages of growth.

Unfavourable environmental conditions can also cause oyster mortality (eg., intake of fresh water, exposure to cold temperatures at low tide, accumulation of silt, heavy marine plant growth, lack of space, lack of available nutrients, industrial and/or domestic pollution).

Oysters are also affected by the presence of various micro-organisms that can cause infectious diseases such as Malpèque disease.