Underwater video clip of degraded NH oyster reef. Note sediment-covered oyster shells lying on their sides.
Underwater video clip of natural, undisturbed NH reef. Note that most oyster shells extend vertically into the water column.
Eyed oyster larvae ready for remote setting
Spat settled on concrete substrate
Species Information The American oyster, or more commonly called the eastern oyster (Crassostrea virginica) is a sessile, reef-forming bivalve that lives in estuarine habitats to a water depth of about 40 feet (12 m) along the east coast of North America from the Gulf of St. Lawrence, Canada, to Key Biscayne, Florida. It is also found in the Gulf of Mexico and the West Indies to Venezuela. The largest populations are now in the Gulf of Mexico, Chesapeake Bay, and Long Island Sound. The presence of huge shell middens (mounds of shells discarded by native Americans) in many coastal areas testify to their importance as a food source for humans for thousands of years, but they also have been a very important component of coastal ecosystems as well. Eastern oysters are sometimes called a "keystone" species because they provide important ecological services such as water filtration, and the reefs they build provide habitat for many other species. These "services" are in addition to their role in supporting important commercial and recreational fisheries.
Morphology of the Eastern Oyster
The left valve of the oyster shell is always thicker and heavier than the right. The left valve is where the oyster is cemented to the substrate. Oyster shell shape and thickness is variable and depends on environmental conditions. For example, oysters living in areas where food supply is limited or siltation is excessive typically have long, thin shells as they attempt to reach higher into the water column for more food or to escape accumulating sediments. Regardless of their overall shape, however, most oysters have very sharp shell edges that can quickly inflict a deep cut. Spawning and Larvae
Spawning is stimulated by water temperature (~16-20oC). One or more males release sperm and a pheromone into the water to initiate females to release their eggs. Females can produce 20 to 80 million eggs per spawning, with the number of eggs being proportional to the size of the individual. After fertilization, oyster larvae remain in the water column for 2 to 3 weeks feeding on phytoplankton and other organic particles. Larvae are vigorous swimmers, and have a straight-hinged shell and locomotory cilia. Larvae settle by attaching to hard substrate such as an adult oyster with a type of liquid cement that is extruded from a pore in their foot. At this point, the larvae are now called spat.
Adult Oysters
Adults are completely sessile and typically occur in reefs or beds formed by many oysters cemented together. Growth is influenced by temperature, salinity, intertidal exposure, turbidity and food. An adult oyster can filter approximately 20 gallons of water per day, removing phytoplankton and other organic particles, thereby improving overall water quality. Oysters are subjected to a variety of diseases (see Oyster Disease page) and predators. Oyster predators in the northeast include oyster drills (Urosalpinx cinerea and Eupleura caudata) and crabs (Cancer irroratus and Carcinus maenas). Major competitors of oysters for food and space are the ribbed mussel (Geukensia demissa), slipper limpets (Crepidula sp.), and barnacles.
Oyster reefs play a vital role in maintaining overall health/water quality of the sensitive ecosystems in which they live and provide important habitat for other marine organisms.
References
M.A. Sellers and J.G. Stanley. Species Profiles: Life Histories and Environmental Requirements of Coastal Fishes and Invertebrates (North Atlantic) – American Oyster. Department of the Interior: US Fish and Wildlife Service. 1984. (Click here to the download the .PDF)
