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New Species Related to Oyster Parasite Found in Chesapeake Bay Clams

By Don Comis
May 30, 2000

A new Perkinsus species has been found living with the Dermo parasite that has decimated oysters over the past several decades in the Chesapeake Bay.

Scientists were surprised not only to find the new species, but to find it on softshell clams--and to find it co-existing on the same clams with Dermo, Perkinsus marinus. Such a double infection is rare for Dermo.

Dermo had always been found only on the Bay's eastern oysters. It and the parasite MSX affect about 80 percent of the Bay's oysters. Dermo kills oysters by eating away at their digestive lining.

Shaban I. Kotob and Mohamed Faisal, with Virginia Institute of Marine Science (VIMS), the College of William and Mary, in Gloucester Point, Va., made the discovery and molecular identification along with S.M. McLaughlin, with the National Marine Fisheries Service, Oxford, Md., and Peter van Berkum, a microbiologist with the Agricultural Research Service in Beltsville, Md. ARS is the chief scientific agency in the U.S. Department of Agriculture. Kotob, a research assistant professor at VIMS, asked van Berkum for assistance in identifying the new Perkinsus species, which has not yet been named. ARS scientist van Berkum is an expert in classifying soil bacteria that help legume crops such as soybeans make their own nitrogen fertilizer.

In 1964, Bay watermen harvested 3.2 million bushels of oysters in Maryland and Virginia waters and 650,000 bushels of softshell clams in Maryland waters. In 1996, the combined oyster harvest totaled only about 220,000 bushels, and Maryland’s clam harvest was down to 26,931 bushels. But the clam decline was thought to be the result of overharvesting and predation, as well as high salinity, high water temperatures, pollution, and other parasites.

A few reports suggest that water pollutants may affect the spread of Dermo and other parasites, but further research is required.

Scientific contact: Shaban I. Kotob, VIMS, the College of William and Mary, Gloucester Point, Va., phone (804) 684-7413,; or Peter van Berkum, ARS Soybean and Alfalfa Research Laboratory, Beltsville, Md., phone (301) 504-7280, or 504-6612, fax (301) 504-5728,