Oysters, like other bivalves, have two shells that are hinged at one end. The shape of the shell and its weight vary according to the oysters habitat. For instance, oysters that live in subtidal areas do not form clusters and tend to have regular, heavy shells. Cluster-forming, intertidal oysters have shells that are typically thin, elongated, and irregularly shaped. All oysters are attached to the substrate or one another by their left valve. This valve tends to be thicker and more deeply curved than the right one. Inside an oysters shell, the internal organs are covered by a thick fold of tissue called the mantle, whose primary function is to secrete the shell. Unattached parts of the mantle enclose a space known as the mantle cavity which, in a living oyster, is always full of seawater. This keeps the oysters organs constantly bathed in water even when it is exposed to air at low tide.
Habitat and Biology
Although they occur at depth down to about 30 meters (100 feet), oysters' primary habitat is in shallow water. In South Carolina, oyster reefs develop in intertidal areas and subtidal areas (less abundant in South Carolina) to about a depth of 2-3 meters (6-9 feet). Oysters typically form intertidal beds that become established on firm substrates where salinities are moderately high, food supplies are sufficient, and siltation is not excessive. In contrast, the oyster beds of Chesapeake Bay and Apalachicola Bay are primarily subtidal. Most of the extensive oyster beds in South Carolina develop within a short distance of high salinity sources of water.
The spawning activity of oysters is influenced by temperature and salinity; its onset varies throughout the oysters range. In South Carolina, the spawning season extends from April through October, with a peak during the summer (Burrell 1986) when water temperatures are above 20 °C. Oysters are broadcast spawners, ejecting sperm and eggs into the water, where fertilization occurs. Female oysters are induced to spawn by the presence of male sperm in the water, and they may spawn several times during one season. Oysters may spawn multiple times during the season, with each female releasing millions of eggs during each spawning event. Oysters are protandric because they develop first as males and then become females due to regression of the testes and subsequent development of the ovaries.
Free-swimming oyster larvae hatch approximately 24 hours after fertilization. The fertilized eggs develop in a free-floating or planktonic environment, and a trochophore larva hatches from the egg 6-9 hours after fertilization. The larvae remain planktonic for about three weeks and progress through a number of larval stages. Larvae are distributed throughout the estuarine environment by tidal currents, rising to the surface on flooding tides, and sinking to the bottom on ebbing tides. This vertical migration behavior helps keep the larvae from being washed out to sea. The last larval stage, the pediveliger, has a muscular foot that it uses to crawl on the bottom, searching for a suitable place to attach. Attachment sites include almost any hard surface such as other living oysters, oyster shell, rocks, docks, pilings, and glass bottles. When they attach, the larvae are called spat, and the process of settling and changing from a free-swimming to an immobile lifestyle is called spatfall. Larvae often settle on top of other oyster shells, forming large beds or oyster reefs. Once attached, the newly settled oysters stay in the same location throughout their life. Most of the oyster larvae settling do not find suitable habitats, but rather they end up on muddy or sandy areas which lack the solid surfaces required for survival. Others may find solid substrates to settle on but may encounter poor water quality or inadequate food resources that prevent larval growth. Only those larvae that settle in areas with adequate substrate, water quality, and food resources have a chance of surviving. Because the presence of oysters and oyster shell suggests that there are suitable resources for growth and survival, larvae may select oyster shell as a substrate. This behavior results in the formation of oyster reefs throughout many intertidal areas in South Carolina. In addition, commercial oyster harvesters often seed areas with oyster shell (called culch) to promote spat settlement.
Oysters feed by filtering bits of food out of water that is drawn in by the beating of small hair-like structures (cilia) that are located on the gills. Cilia create water currents that supply oxygen to the gills and remove carbon dioxide. These currents also carry food particles and other small pieces of material to the gills. The particles are sorted by ciliary action and food particles are carried to the mouth while other material that is not ingested gathers on mucus on the gills, which is then expelled. The primary component of the diet of oysters and clams is phytoplankton, which are small, single to multicellular algae.
The American oyster, also called the eastern oyster, is the only commercially important oyster species on the East Coast of the United States. In South Carolina, it is among the most popular local seafoods. Harvesting is done by handpicking clusters of oysters at low tide in authorized areas. In addition to providing commercial and recreational benefits, oysters also fulfill several important ecological functions. For instance, their filtering action serves to remove suspended sediments from the water as well as certain pollutants. Oyster reefs provide valuable shelter and habitat for many other species, such as stone crabs, and prevent erosion by stabilizing marsh edges. Several marine species--some of commercial importance--seek habitats where the bottom is covered with loose oyster shell.
The oyster population in South Carolina is currently stable. However, oyster harvesting in the United States has declined more than 50 percent since the 1930s, while the demand for oysters has continued to increase. The population decline is primarily due to over-harvesting, pollution, and disease. Diseases such as Perkinsus marinus, MSX, and Seaside disease all have similar effects on oyster populations. These diseases wipe out populations before they are able to reach a harvestable size. Many shellfish beds in the United States have been closed to reduce health risks from consumption of contaminated shellfish.
Burrell, V. G., Jr. 1986. Species profiles: life histories and environmental requirements of coastal fishes and invertebrates (South Atlantic)--American oyster. United States Fish and Wildlife Service Biological Report 82(11.57). Washington, DC.
Keith, W. J. and W. D. Anderson. 1995. Oysters and clams. Information & Education Series. South Carolina Department of Natural Resources, Charleston, SC.
Kennedy, V. S., R. I. E. Newell, and A. F. Eble. 1996. The eastern oyster, Crassostrea virginica. Maryland Sea Grant College, College Park, MD.