Before 1990, the oyster parasite Perkinsus marinus, cause of Dermo disease in the eastern oyster, Crassostrea virginica was restricted to locations south of Delaware Bay. Since then, it has appeared over a new range extending from New Jersey to Maine. This sudden and rapid range expansion coincided with an extreme warming trend, more pronounced in winter than in summer, between 1990 and 1992. It was preceded by a more gradual trend beginning in the mid-1980s.

In some locations the parasite has caused heavy oyster mortalities; in others it appears to have resulted in few, if any, problems. The phenomenon has raised questions of both practical and fundamental importance:

1) Is Dermo disease continuing to "spread" into the Northeast?
2) How is the disease process affected by colder temperature regimes in the north?
3) Is the range extension associated with the development of a more cold tolerant strain of the parasite, P marinus?
4) What is the role of Dermo vs. other infectious diseases in recurrent losses of cultured oysters in the northeast?

   We are answering these questions in a project sponsored by the Northeastern Regional Aquaculture Center. It involves researchers at HSRL, the University of Pennsylvania Laboratory for Aquatic Animal Health and Pathology, Fairfield University, and Boston College, and 13 oyster growers in 18 different locations from New Jersey to Maine. The project involves frequent sampling over a two-year period, using standardized methods at all sites, of oysters of different ages and from different sources. The following parameters are being measured: P marinus infection prevalence and intensity; oyster growth, condition index, and mortality; water temperature and salinity. One outcome of the project will be a simple predictive model (developed at Boston College) relating temperature and salinity to subsequent prevalence, intensity, and mortality.

   To help determine whether the range extension is associated with the development of a more cold tolerant strain of P marinus, we isolated parasites from oysters in three geographically separated regions (Massachusetts, New Jersey, and South Carolina), propogated them in culture, and are testing them for in vitro growth over a temperature range from 5 to 35 degrees Celsius.