In vitro interaction of Perkinsus marinus merozoites with eastern and Pacific oyster hemocytes

This study compared hemocyte responses of eastern and Pacific oysters to Perkinsus marinus, in vitro. Except for the percentage of hemocytes associated with P. marinus there was little or no significant difference between eastern and Pacific oysters with regard to their hemocytic response to P. marinus. In phagocytosis assays, merozoites were bound to all hemocyte types but in unequal proportions, unlike zymosan which was found predominantly associated with granulocytes. The number of merozoites enlarging in Ray's fluid thioglycollate medium after incubation with hemocytes in plasma for one day was significantly lower than after incubation in plasma alone in both oyster species. Electron microscopy or merozoites indicated that the parasites were rapidly phagocytosed and that some of the merozoites showed signs of degeneration in less than 12 h. The results suggest that limited intracellular killing of P. marinus had occurred, but was probably not mediated by oxygen metabolites, since no increase in chemiluminescence was observed when hemocytes of either eastern or Pacific oysters were exposed to merozoites.