Eastern Oyster (Crassostrea virginica)

Other Names
American Oyster, Atlantic Oyster, Common Oyster, Virginia Oyster
Valve (shell) length of the eastern oyster reaches up to 8 inches (20 cm). Its two shells (called "valves," hence the name bivalve) attach together at one end by a natural hinge and by a single large muscle. The pale white to gray shell is rough with ridges or bumps.
Life History
The eastern oyster feeds on plankton and algae. It has numerous predators, including birds such as the American oystercatcher, ocean dwellers such as sea anemones, sea stars, sea nettles, some parasites, and humans. About seven weeks after hatching, the eastern oyster reaches sexual maturity. Spawning season is from late spring to early fall during warm weather. Females may release more than 100 million eggs during a season. Only about one percent of the fertilized eggs reach the next stage of maturity.

Within hours of mixing with sperm, the fertilized eggs develop a shell and begin to move on their own. Oyster larvae, each about the size of a grain of pepper, use tiny, probing feet to find a suitable place to attach. Once settled, the foot excretes a cement-like glue. The oyster glues itself in place and spends the rest of its life there. Its lifespan varies, depending on freshwater inflow and predators.

Oysters are protandric-in the first year, they spawn as males, but as they grow larger and develop more energy reserves, they spawn as females. Oysters are also filter-feeders. They feed by using their gills to filter tiny food particles out of the water. Oysters have been found attached to bricks, boats, cans, tires, bottles, crabs, and turtles, but they prefer to attach to other oysters. When a large number of oysters join together, it's called an "oyster reef".

Some oyster reefs are so large they are included on topographic maps. In the eighteenth and nineteenth centuries, several oyster reefs were so big they posed navigational hazards to ships. A purple stain or muscle "scar" is left on the inside of the shell when the adductor muscle (the muscle that holds the bivalve together) is removed. If the water gets too cold, below 40 degrees F, oysters close their shells and don't open them again until the water warms up.
Eastern oysters are abundant in shallow saltwater bays, lagoons and estuaries, in water 8 to 25 feet (2.5 to 7.5 m) deep and between 28 and 90 degrees F.
Ocean waters from Canada to Mexico are where the eastern oyster is found.
How To Catch
If oysters lived singly, scattered over the bay bottom, they would be difficult to obtain in quantity and have less commercial value. Many edible mollusks live in such fashion and are seldom, if ever, found in the market. Oysters are gregarious reef builders, however, and occur in concentrations necessary for a commercial fishery.

The principal gear used in harvesting is the oyster dredge, essentially a basket attached to a toothed bar. When dragged by an oyster boat over a reef, oysters are scraped off the bottom by the bar and caught in the basket. The dredge periodically is hauled aboard and the catch dumped on the deck. Small oysters and shells are culled from the market oysters and discarded or thrown overboard. If culling is done properly, small oysters can be separated and returned to the water without damage.

Oysters are sold either in the shell or shucked. Most dealers employ experienced shuckers to open the oysters, using special knives. After cleaning and processing, the shucked oysters are packed in jars or cans and placed in cold storage until sold. All oysters sold in Texas must be certified, which means they must be harvested, handled, processed, and stored in accordance state and federal standards.
Where To Catch
Oysters are one of the few animals eaten entirely raw, but they are more than just a popular seafood: oyster shells are used in calcium supplements and in industrial processes. Because they are filter-feeders, oysters may ingest pollutants out of the water. The Texas Department of Health determines which areas are safe to collect oysters. If areas are too polluted, oysters become contaminated and unsafe to eat. The Texas Department of Health will then close the area(s) to oyster harvesting. Collecting or selling oysters taken from these closed areas is illegal.

During winter northers, low tides expose shallow reefs where individuals may wade or walk along the flats picking up oysters with no special equipment except sturdy gloves for protection against sharp bills.

Several laws that govern oyster harvest should be reviewed before heading out to the reef. A digest of saltwater fishing laws and maps of the approved and polluted oystering areas may be obtained from some coastal Parks and Wildlife offices.
How To Eat
The adage "Never eat oysters in months without an 'R' in them" was based partly on the difficulty in keeping oysters from spoiling in warm weather (May, June, July and August) before efficient methods of refrigeration had been developed. Actually, oysters are good to eat all year long. After spawning, however, they may become thin and watery. Oysters are in best condition in winter and early spring.

As filter-feeders, oysters can concentrate bacteria and viruses within their bodies. These may be harmful to man when oysters are eaten raw or insufficiently cooked. Such diseases as typhoid fever and hepatitis have been traced to contaminated shellfish taken from polluted waters in other parts of the country.

The Texas Department of Health determines the sanitary quality of the oystering areas and closes those that do not meet state and federal standards. It publishes maps of the Texas coast, designating approved oystering areas and closed or polluted areas. By law, no one may take oysters from the polluted areas either for sale or for personal use.

Frequently, the novice oyster shucker will encounter a golden-brown "worm" within the oyster's body. The initial reaction is to throw the oyster away, thinking it is parasitized. However, the "worm" is an enzyme complex formed by the oyster itself when it has been actively feeding. Thus, it could be considered a mark of freshness. Red and green worms that live outside the oyster's shell sometimes wander across the shucked oyster meat. They may be startling, but should not interfere with the enjoyment of the meal.
Threats and Reasons for Decline


Disease-causing parasites may reach epidemic proportions, killing large number of oysters within a short time. Such epidemic losses have been recorded wherever oysters are found. In Texas, and throughout the Gulf Coast, a parasitic fungus regularly causes moderate to severe losses among market oysters. A related parasite was responsible for the nearly total kill of oysters in Aransas Bay in the 1960's.

Living conditions in the estuary or bay undergo continual and often harsh changes, but the oyster is highly adaptable. It tolerates siltation, wide temperature ranges, near-fresh to very salty water, extreme tidal fluctuations and many other environmental changes. By tightly closing its shell, it can avoid contact with the harmful environment for some time. However, when its muscle tires, the shell must open and, if conditions have not improved, the oyster will die.

The oyster must also contend with many predators and parasites. Several types of crabs can crack the shell and feed upon the oyster. The oyster drill, a predatory snail, can rasp a hole through the shell and insert a tubular proboscis to reach the flesh. Certain sponges and mollusks burrow into the oyster valves for their own protection but may riddle the valves with extensive burrows. This weakens the shell and makes the oyster more vulnerable to predation. Organisms such as mussels and other encrusting colonial animals may crowd the oysters, interfere with feeding, smother young oysters and hinder spat from setting.
The eastern oyster, also referred to as the American oyster or the Gulf Coast oyster, is one of the most sought-after coastal mollusks. Not only do people enjoy eating them, live oyster reefs help clean the water and provide habitat for all sorts of other animals such as sponges, small crabs, and fishes.


Upon setting, the tiny spat begins to secrete new shell in successive layers, expanding in all dimensions. Special cells in the mantle covering secrete most of the shell material, although it can also be deposited by mantle cells within the valves. The oyster can, in effect, patch up its old home while building an addition to it.

Sometimes, sand grains, shell fragments or other particles become lodged within the mantle tissue. Mantle cells stick to such a particle and become lodged within the mantle tissue, continuing to secrete around it - eventually forming a "pearl." Because the commercial oyster lacks the ingredient in its secretion to form the mother-of-pearl coating that gives the luster and beauty of the true pearl, its pearls, although interesting, are not valuable.

Several pearls may occur in one oyster. A world record has not been established, but a Galveston Bay oyster containing 356 pearls must be a leading contender.

Growth of oysters in Texas waters is relatively fast and occurs throughout the year. Under ideal conditions, spat may reach one inch in three months, two inches in seven months, and three inches in 15 months. Nevertheless, growth can be variable and oysters of identical age may differ markedly in size. Probably most Texas oysters reach the legal market size of three inches in 18 to 20 months.