Immunological competence of eastern oysters, Crassostrea virginica, fed different microalgal diets and challenged with a temperature elevation

The influence of nutrition, in terms of both quantity and quality, upon innate immune system competence (cellular defense mechanisms) was explored experimentally in eastern oysters, Crassostrea virginica. Oysters (ca. 50 mm shell height) were fed diets of two cultured microalgal strains, Tetraselmis chui strain PLY429 and Skeletonema costatum strain LB1077/1B, known to differ in biochemical composition—both alone and as a 50:50 mix—at two daily rations. Unfed controls also were included. After 5 weeks of feeding at 20 °C, the temperature was increased to 28 °C over 2 days, and the oysters were fed the same algal diets for one additional week. Dependent variables measured by flow cytometry, both before and after temperature increase, were hematological characteristics (relative percentages, sizes and granularities of four hemocyte types) and hemocyte defense-related activities (viability, aggregation and adherence, phagocytosis, and respiratory burst). Main effects and interactions of temperature increase, food quantity, and food quality upon each immunological variable were determined by ANOVA. Furthermore, Discriminant Analysis (DA) was applied to identify a suite of immunological parameters capable of differentiating oysters from the separate dietary treatments, and Principal Component Analysis (PCA) was used to characterize relationships between immunological variables. ANOVA of individual immunological variables revealed few significant main effects or interactions of food quantity, quality, and temperature increase; however, trends were apparent. A DA model, incorporating several immune function variables, differentiated oysters according to the food quantity and quality treatments from which they were taken. PCA revealed positive relationships between hemocyte phagocytosis and aggregation, which had a negative relationship with respiratory burst and mortality. Accordingly, we characterized “healthy” oyster immune capability as (1) highly aggregating and (2) phagocytic hemocytes with (3) low mortality and (4) low respiratory burst, and “unhealthy” immune function as the opposite. Temperature increase had a significant (p