Sept/Oct 2017For Wildlife Watchers: Eastern Oysterby Rob Simbeck
The Eastern oyster's vital role in South Carolina estuaries is underscored by SCDNR Biologist Nancy Hadley, "Oysters are ecosystem engineers - they build habitat; they control water quality; they modify their environment. They are keystone species, like coral reefs.
It's not hard to overlook some of the species that most impact our lives. Honeybees, pollinators imported from Europe, are at the heart of much of our food production. Bats are invaluable pest-control devices. Then there is the Eastern oyster, which shapes its environment as few species do.
"South Carolina estuaries are an oyster reef-based ecosystem," says DNR biologist Nancy Hadley. "Oysters are ecosystem engineers - they build habitat; they control water quality; they modify their environment. They are keystone species, like coral reefs."
Eastern oysters, found from Canada to Central America along the Atlantic and Gulf shores, were once staggeringly abundant. In the 1880s, we pulled twenty million bushels a year out of Chesapeake Bay. Now, thanks to the overharvesting, pollution, habitat loss and dredging affecting them worldwide, the Bay yields one-hundredth that amount.
"If oysters are removed," says Hadley, "the entire ecosystem will change. This has happened in the Chesapeake Bay."
As filter feeders, oysters draw in seawater over their gills with beating, hair-like cilia. Non-digestible particles are bound up in mucus and expelled as "pseudofeces," nutrient-rich strings that are an important part of the food chain for bottom-dwellers like polychaete worms. Desirable particles, predominantly single-celled algae called phytoplankton, are transported to the mouth, through the esophagus and stomach to the anus. The liver and heart do about what ours do, the latter pumping clear, colorless blood through the body and through two waste-removing kidneys. Two pairs of nerve cords and three pairs of ganglia make up the nervous system. All of that is lumped inside the mantle, two fleshy folds secreting the shells and playing a role in respiration and excretion.
Eating all that raw may not sound appetizing, but humans have been ingesting oysters, to judge from archeological finds, for well over 100,000 years, with Americans consuming 2.5 billion of them a year. For the past fifteen years, South Carolina's intertidal zone, the area between high and low tide where most oysters are found here, has yielded a commercial crop of 100,000 bushels a year, worth $2 million, making it the fourth-largest fishery in the state. The state imports another 100,000 bushels, and the recreational harvest is unknown.
In assessing the status of the state's reefs, Hadley cites the lack of solid historical data for comparison, but says oysters "may be holding their own." A Nature Conservancy study that paints a grim picture worldwide offers a somewhat brighter one in South Carolina - while reefs near Georgetown are at less than ten percent of their historic levels, Charleston's lie between 10 and 50 percent, and Beaufort's at more than fifty percent.
All of the state's estuarine areas contain oysters. "There are currently 83 oyster grounds that can be harvested by the public with a saltwater fishing license," says Hadley. "These represent more than 2,000 acres of actual shellfish beds." There is a harvest limit of two bushels per person per day for no more than two days in a seven-day period, during the harvest season, typically from October 1 through May 15.
A century ago, canneries and shucking houses dumped shells back into the beds each year. Now, with oysters sold to restaurants or individuals, "the need for restocking is as great or greater than ever, but the shells are harder to come by." As a result, the DNR has a shell recycling program, with drop-offs in more than two dozen places from Hilton Head to Myrtle Beach and as far inland as Columbia.
Less than 20 percent of what is consumed in the state is recaptured through the shell-recycling program. The state purchases up to 60,000 bushels per year from interstate sources for bed maintenance, drying them for six months to kill any remaining tissue.
Description: Calcium carbonate shells, dirty white to gray outside, pearly white nacre sometimes tinged with blue inside.
Habitat and Range: Atlantic and Gulf of Mexico. Subtidal estuarine areas through most of the range, but mostly intertidal along South Carolina and Georgia coasts.
Reproduction: In warm water, males and females release millions of sperm and eggs. Fertilized eggs develop and attach to hard surface, often the shells of living adult oysters.
Viewing Tips: Many oyster beds along the coast. Handle with care: those who work with them call them "nature's razors."
The Eastern oyster is one of five oyster species consumed in the U.S. and one of many worldwide that fall into the edible, pearl and windowpane categories, the latter often used for decorative purposes. The Eastern has a variably shaped shell with three layers formed largely of calcium carbonate extracted from seawater and combined with protein. The inner layer is the pearly nacre, though pearl oysters are from another family. The rough, stony outer layer ranges from dirty white to dark gray. The carbon oysters extract for their shells helps reduce the amount that can return to the atmosphere as the greenhouse gas carbon dioxide, and calcium carbonate helps fight acidification of the ocean, something helping to devastate coral reefs.
Oyster reefs are formed as larvae cement themselves to the shells of adult oysters, building a bed providing habitat for many small creatures and the creatures that feed on them, making oyster reefs excellent fishing spots. They are also natural breakwaters that help protect against erosion.
Eastern oysters are sexually dimorphic - there are separate sexes - but each winter the reproductive organs wither, to regenerate in the spring, at which point oysters can change from one sex to the other. During their first year, most oysters spawn as male. With growth, most change into females. In late spring, males release sperm, stimulating females to release eggs by the millions, with spawning triggered by water temperature - 68 degrees for reproductive development in South Carolina, and 75 for spawning.
"It is a privileged observer indeed who happens to witness this phenomenon," says Hadley. "I have only seen it once. It took me a few minutes to realize what was happening. The entire water column over the oyster bed turned milky white with millions, perhaps billions, of eggs and sperm."
Fertilized eggs develop into fully planktonic larvae, feeding on phytoplankton, in 12 to 24 hours. These larvae drift with the currents for several weeks, most becoming food for fish. After two or three weeks, they have doubled in size, have developed a foot and an "eye spot," and are ready to find a hard surface to which they will cement.
"The young oyster can never relocate," says Hadley, "so selecting a good site is critical. For this reason, oyster larvae actively seek out adult oyster shells for attachment, providing reasonable assurance they are in a suitable location and will be near other oysters, a necessity for broadcast spawners."
Oysters can reach a harvestable size of three inches in two years. They can live twenty years and reach eight inches in height, although given harvesting pressure they seldom live past five years. They might seem impervious to predators, but are eaten by starfish, crabs, a snail called an oyster drill, boring sponges and seabirds like oystercatchers.
A growing trend in the oyster industry is mariculture, or oyster farming, which can produce high-quality oysters in 12 to 18 months. Larvae are started in hatcheries, introduced to ground-up shells for attachment, and grown to market size in cages in suitable tidal creeks and estuaries. They can be bred for fast growth, desirable shell shape, disease resistance, even sterility, allowing them to devote all their energy to growth rather than reproduction.
This year, the South Carolina state legislature approved summer harvesting for such oysters in principal, with special handling precautions. With summer harvest, though, comes the increased risk of consumption-related illnesses, as oysters concentrate bacteria and viruses up to 100 times levels found in surrounding water.
With vigilance ideally as the watchword, we continue to eat Eastern oysters by the billions, with the Southeast increasingly seen as having great potential for larger-scale oyster farming. A growing movement compares oysters to wines, with taste differences due to regional variations.
As for the notion of oysters as "watchable wildlife," Hadley points us in the right direction by speaking, oddly enough, about the process of expelling the seawater they've drawn in to extract nutrients.
"Oysters spit when the tide goes out," she says, "and again when it comes in. As far as I know, nobody knows why, but if you sit by an estuary or tidal creek on a quiet night, you can hear them. It's like a symphony."
If that doesn't qualify them, I don't know what would.