The American oyster is an iconic species of the Chesapeake Bay and other coastal waters of eastern North America. Historically, this species was so common and made such large reefs that it was a documented navigation hazard in some areas. Unfortunately, a history of overexploitation, pollution, and physical damage has reduced the American oyster population size to as low as one percent of its original abundance in some areas.
American oysters, like all true oysters, live a sessile lifestyle. After a short mobile phase in the plankton, juveniles settle on dead oyster shells or some other hard surface and permanently cement themselves in place. From then on, they build strong, calcium carbonate shells. In this manner, they form very large reefs that provide habitat for several other species of invertebrates and fishes. American oysters are filter feeders that strain plankton and other organic matter from the water above the surface of their reefs. In areas with high oyster densities, they can keep the water nearly free of floating matter. Each American oyster filters approximately 50 gallons of water per day.
American oysters reproduce via broadcast spawning, where several females release their eggs and several males release their sperm into the water column at the same time. This behavior increases the likelihood that eggs will be fertilized and decreases the likelihood that the eggs will be eaten by predators near the reef’s surface. These oysters are extremely productive, and a single female may release as many as 150 million eggs throughout her lifetime. When they hatch, all American oysters are male. Most of them reproduce at least once as male, but as they mature, some individuals change to female. This sex change is a result of the difference in energy required to make sperm and eggs. Small individuals can make sperm, while continuing to grow. Larger individuals that have stopped actively growing can afford to utilize their energy to make the larger eggs. Reproduction is triggered by temperature in this species.
The American oyster has been overfished and depleted for more than 120 years. The Chesapeake Bay population has been exploited particularly hard, and catches peaked as early as the 1880s. By the early 1900s, it was already depleted. Catch now is approximately one percent of what it was at its 1880s peak. The overexploitation of American oysters has changed the Bay significantly. While the oysters formerly filtered the entire contents of the Bay in short periods of time, that is no longer the case. High levels of pollution in the Chesapeake are exacerbated by the fact that its natural scrubber system has been removed. Though it is likely not at risk of full extinction – thanks to its large range and very high reproductive output – the American oyster is severely depleted throughout its range and no longer provides the habitat and filtration services that it once did.