Research Product

Perkinsus marinus infection intensity was measured in eastern oysters (Crassostrea virginica) that were collected in October 1993, December 1993, March 1994, May 1994 and July 1994 from Apalachicola Bay (FL), Chesapeake Bay (VA), and Oyster Bay (NY). Gill, mantle, digestive gland, adductor muscle, hemolymph and remaining tissue (including gonadal material and rectum) were dissected from 20 oysters from each site at each collection time and were separately diagnosed for P. marinus infections by incubation in Ray's Fluid Thioglycollate Medium (RFTM) and subsequent microscopic quantification of purified enlarged hypnospores. At all sampling times and sites, average P. marinus infection intensity (gm wet weight tissue-1 or ml hemoloymph-1) was lowest in hemolymph samples, and generally highest in the digestive gland. P. marinus prevalence was 100% at both FL and NY sites for each of the five collection times, and, in only one month (May 1994), was less than 100% for the VA site. Seasonal intensity patterns and mean total body burdens differed among the sites. Average body burden was highest in VA during October and progressively declined to a minimum in May. This decline was probably due to mortality of heavily infected oysters and diminution of parasite activity typically associated with colder temperatures and reduced salinities. Intensities varied little during the months of October and December at both the FL and NY sites. Minimum average intensities were observed in March in FL oysters and May in NY oysters. The retention of relatively high P. marinus infection levels throughout the winter in NY oysters compared with VA oysters may have been due to constant high salinity in Long Island Sound which favors parasite activity, and also to a rapid decline in temperature in the fall that may have prevented epizootic oyster mortalities such as those associated with P. marinus.