Defense-related hemocyte activities of American oysters Crassostrea virginica Gmelin from an oceanic and an estuarine habitat were monitored from spring to winter of 1987 at a constant temperature (15 °C). The ability of cells to spread to an ameboid shape in vitro at ambient salinity and after an acute change in salinity was much reduced from late spring to early fall. This summer-stress period was also indicated by laboratory experiments. Stress may have been due to high water temperatures but spawning efforts may have also contributed. Reduced hemocyte activity occurred at a time when infections by two major oyster parasites are at a peak in enzootic areas. Hemocyte locomotion rates fluctuated during the study period but did not show a significant cyclic pattern. At all times of the year, hemocytes responded to acute in vitro changes in salinity according to a pattern previously described. Oysters inhabit a wide range of salinities and temperatures and these data indicate that natural environmental fluctuations may influence their defensive capacity.



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