The most abundant bivalve in southern estuaries is the American, or eastern, oyster, Crassostrea virginica. Its gray shell is easily recognized by its rough, irregular shape and by the purple scar on the inside marking the attachment site of the tough muscle that holds the two shells together.
An oyster reef at low tide in the estuary around Sapelo Island. By E. Sherr.
Oysters are found everywhere in coastal habitats. They encrust pilings and docks and form large colonies, or oyster reefs, in salt marsh creeks. Even so, conditions along the Georgia coast are not optimal for oysters. Suitable territory for the attachment of oyster larvae is scarce, so oysters crowd together and form tall slender shells, unlike the plump oval shells of solitary Chesapeake Bay oysters. Also, the large tidal differences along the Georgia coast mean that oysters here spend a lot of their day out of water, shut up tightly to avoid drying out, while oysters in other estuaries are able to feed continually underwater. The resulting small slender oysters (as in photo above) which eke out a precarious existence in the creek beds are known as "clutch oysters."
Food: Oysters are a prime seafood, nutritious and easy to collect. On Sapelo Island, as on other sea islands, there are many old oyster-shell middens left by previous inhabitants. When I lived on Sapelo, we often had oyster roasts under the oak trees and made some small shell heaps of our own.
This delectable shellfish was the basis of a great fishery along the Atlantic and Gulf Coasts of North America from the first settlement by European colonists until World War II. (For an amusing and fact-filled account of the nineteenth-century U.S. oyster industry, try Mark Kurlansky's best seller 'The Big Oyster: History on the Half Shell.')
Despite the suboptimal growing conditions in muddy salt marsh estuaries, oystering was a main source of income for many families along the southeastern Atlantic Coast, more so in South Carolina than in Georgia. Collecting, shucking, and canning oysters are hands-on jobs that require cheap, plentiful labor. Harold Coffin had a small canning operation for shrimp and oysters on Sapelo Island in the 1920s. Oyster production in southeastern estuaries peaked during the years between the World Wars. Although there was a bump up in oyster production after introduction of mechanical shuckers in the mid-1940s, oyster fisheries declined sharply after 1950 everywhere along the Atlantic and Gulf Coasts. There were several reasons for the decrease. Coastal development led to the filling, polluting, and silting of prime oyster habitat; shucked oyster shells were not returned to estuaries for the resettlement and growth of oyster larvae; and better job opportunities in the booming postwar economy meant fewer people willing to work for low pay in oyster factories.
Adding to these trends, in the 1950s a major shellfish disease, dermo, was discovered infecting oysters in Chesapeake Bay, Virginia. The dermo disease is caused by a flagellated protist that is ingested by an oyster, whereupon the single-celled invader insinuates itself into the oyster's blood cells and starts reproducing. The parasites fill up the blood cell until it bursts, releasing more infective protists throughout the oyster's body. The oyster eventually fills with protists. The oyster becomes sick, stops growing, and may die. At death, the shell of the oyster gapes open and the oyster's body disintegrates, releasing millions of the dermo parasites to find other oysters to infect. No wonder that this disease is so highly contagious; after the original outbreak, it soon spread to oyster beds in estuaries from Virginia to Louisiana. A study of the disease in oysters in the Duplin River along Sapelo Island found 90 percent to 100 percent of the bivalves infected in 1999 and 2000. Most of the oysters don't die, but their growth and food quality are affected by the parasite.
Fortress: Oysters also contribute to living spaces for both men and marsh creatures. In colonial times, settlers along the Atlantic Coast used oyster shells when constructing walls of tabby concrete, a kind of cement made from lime, sand, water, and crushed shells left over from oyster harvests. Tabby ruins stand on Sapelo Island. The name of this old plantation, Chocolate, may be a corruption of the name of a local Native American village called Chucalate.
Oyster shells are visible in the original tabby concrete walls of slave housing at Kingsley Plantation on Fort George Island, Florida, USA, built in 1814.
2013-07-28 22-13" by User:Sartoriatrix - Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons -
Oyster reefs, built up as these bivalves attach to one another to gain a shell-hold in the muddy bottom of the estuary, create new habitats for multitudes of other animals. Oyster reefs are protective "cities" for creatures that could not easily survive without such a refuge from the sweep of tidal currents. Some species have adapted so well to their shelly haven that they are found nowhere else. The yellow boring sponges that infest oyster shells are preyed upon by a yellow-orange nudibranch (a shell-less mollusk that looks something like an exotic slug). Several types of bristle worms also bore into oyster shells. The pea crab, Zaops ostreum, uses oysters as both food and fortress. This tiny crab is a parasite living within the shells of oysters, where it nibbles off parts of the oyster's gills. A little pink pea crab can often be found inside the shell of a roasted oyster. Small mud crabs, shrimp, and brittle stars live among the oysters, which can become encrusted with algae, sponges, and bryozoans. Gobies and other small fish feed on the abundant denizens of the oyster reef when the tide is high, and cling to oyster shells to avoid being swept out of their refuge by outgoing currents.
Tiny parasitic pea crab with pink egg mass in opened oyster.
"Zaops ostreum" by NOAA http://oceanexplorer.noaa.gov/explorations/04etta/background/decapo/medi/zaops_ostreum_oyster.html
Licensed under Public Domain via Wikimedia Commons -
Often oyster reefs stretch completely across creeks, damming the channels. When the tide retreats, small pools remain behind the oyster dams, giving a watery shelter to shrimp, crabs, and killifish that had foraged in the marsh on the flood tide.
Oyster reefs as fortress has gained increasing attention as coastal managers have focused on restoring oyster reefs along the Northeastern and Southeastern Atlantic and Gulf Coasts of the U.S. to serve as living shoreline protection against erosion due to storm waves and surges.
Large oyster reefs such as this one in an estuary in South Carolina can protect coastlines from storm erosion. 2010-02-17 05:04 Jstuby 2048×1536 (1218500 bytes) [[Oyster]] reef at about mid-tide off fishing pier at [[Hunting Island State Park]], South Carolina - Wikipedia Commons