Effects of natural and field-simulated blooms of the dinoflagellate Prorocentrum minimum upon hemocytes of eastern oysters, Crassostrea virginica, from two different populations

Oysters, Crassostrea virginica, from two populations, one from a coastal pond experiencing repeated dinoflagellate blooms (native), and the other from another site where blooms have not been observed (non-native), were analyzed for cellular immune system profiles before and during natural and simulated (by adding cultured algae to natural plankton) blooms of the dinoflagellate Prorocentrum minimum. Significant differences in hemocytes between the two oyster populations, before and after the blooms, were found with ANOVA, principal components analysis (PCA) and ANOVA applied to PCA components. Stress associated with blooms of P. minimum included an increase in hemocyte number, especially granulocytes and small granulocytes, and an increase in phagocytosis associated with a decrease in aggregation and mortality of the hemocytes, as compared with oysters in pre-bloom analyses. Non-native oysters constitutively had a hemocyte profile more similar to that induced by P. minimum than that of native oysters, but this profile did not impart increased resistance. The effect of P. minimum on respiratory burst was different according to the origin of the oysters, with the dinoflagellate causing a 35% increase in the respiratory burst of the native oysters but having no effect on that of the non-native oysters. Increased respiratory burst in hemocytes of native oysters exposed to P. minimum in both simulated and natural blooms may represent an adaptation to annual blooms whereby surviving native oysters protect themselves against tissue damage from ingested P. minimum.