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Oysters

Oysters in the Shell

The oyster is among the more valuable resources of the Texas coast, providing a livelihood for numbers of anglers and a popular food for Texas appetites. Unlike most other seafood, oyster meat is completely utilized - no "gutting and grilling" needed. It is one of the few of the animal foods eaten raw.

It is often grouped with shrimps and crabs under the term "shellfish," but the oyster is no relation to either. Specifically, it a bivalve mollusk with two shell valves hinged together at one end and closed by a single, large muscle attached to the valves near the other end.

The commercial oyster (Crassostrea virginica or eastern oyster) can be found in all bays along the upper and middle Texas coast. It is absent throughout most of the Laguna Madre but reappears near Port Isabel and in South Bay. Typically, it is most abundant in mid-bay areas, forming extensive reefs.

 

Life Cycle

Oysters spawn from the warm weather in late spring until early fall. Eggs discharged by females are fertilized in the water by sperm released by males. This is an uncertain event at best and only a fraction of the eggs becomes fertile. However, spawning of many oysters at the same time assure an abundance of embryonic oysters.

The fertilized egg quickly develops into a swimming, shell-bearing larval oyster or "veliger." During the veliger stage, lasting two or three weeks, larval losses are very high. While in this stage, larvae are distributed throughout a bay by tidal currents. Currents may carry them upward into water that is too fresh, or outward into the salty Gulf where they will perish. Many organisms feed upon the larvae or produce substances which poison them. Pollution from industrial waste may cause larval death. As a result, the percentage of larvae that develop to the next, "setting", stage is probably small (similar to the percentage of eggs that develop into larvae). However, that small percentage can result in tremendous numbers of tiny oysters.

Larvae that have settled to the bottom and cemented themselves to a suitable surface are called "spat," presumably because their golden-brown shells, just visible to the naked eye, look like specks of tobacco juice. Spat will set upon many different materials, such as bricks, bottles, cans, tires, even crabs and turtles. However, oyster shells (both in use and empty) provide the most abundant, naturally available setting material.

Oyster spat are gregarious, that is, they tend to settle where some spat are already present. When spat are plentiful, overcrowding may occur but this tendency helps to maintain the oyster population on established reefs and is a means by which new reefs may evolve.

 

Pearls

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.

Oyster Ecology

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.

 

Disease

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.

 

Oyster Harvest

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.

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.

 

Eating Oysters

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.

Adapted from "Oysters in Texas"
prepared by Robert P. Hofstetter, Coastal Fisheries, Texas Parks and Wildlife.




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