Fast Facts: Eastern Oysters
Eastern oysters, Crassostrea virginica, are bivalve mollusks that feed by filtering phytoplankton from the water column while submerged. They have ridged shells, of which the bottom valve is cupshaped and the top valve is relatively flat.
The range of eastern oysters extends across the Western Atlantic coast, from Canada to the Gulf of Mexico and south to Argentina. Oysters can tolerate a wide range of temperatures, oxygen levels, and salinities. They are found both in intertidal and in subtidal zones. Intertidal oysters tend to form clusters of elongated, irregularly-shaped shells due to their exposure at low tides, while subtidal oysters tend to have larger more rounded shells and tend not to cluster. Aggregations of oysters are known as oyster beds or oyster reefs.
Oysters are broadcast spawners, distributing eggs and sperm freely in the water column, where fertilization takes place. Spawning typically occurs in the summer. Oysters are protandrous hermaphrodites, spawning first as males and then after 2-3 years, changing sex to spawn as females.
Role in Ecosystem
Oysters are a keystone species because they provide architectural complexity that serves as essential habitat in which many other marine species find refuge for their young and also from predators. They also provide important ecosystem services. As filterfeeders, they consume or filter out nitrogencontaining compounds and other particulate matter, including sediments, from the water. This increases water clarity, thereby enhancing the ability of sea grass to receive sunlight to photosynthesize. Finally, oysters are important prey in the diet of other invertebrates, fish, and humans. For humans, oysters not only provide food, but also support an important fishing industry that provides jobs for thousands of people in the United States.
Oysters face many threats that range from overfishing and climate change to habitat degradation and invasive species. Due to these threats, oyster populations have declined worldwide, including areas throughout Chesapeake Bay and the Gulf of Mexico. The Nature Conservancy reports that over the last century, about 85% of oyster reefs have been lost worldwide.
State regulations, including seasonal closures, catch limits, and gear restrictions, have been implemented to limit the harvest of oysters. Some areas have built artificial reefs, using materials such as oyster shells, concrete, or crushed rock, in order to attract oyster larvae to attach to the substrate.