Haemocytic and humoral activities of eastern and Pacific oysters following challenge by the protozoan Perkinsus marinus

Eastern oysters (Crassostrea virginica) and Pacific oysters (Crassostrea gigas) were exposed to the protozoan parasite Perkinsus marinus. Pacific oysters were less susceptible to P. marinus infection than eastern oysters. The concentration of circulating haemocytes, percentage of granulocytes and plasma haemagglutinin titre increased in Pacific oysters exposed to P. marinus as compared to unexposed Pacific oysters. Similar increases were not observed in eastern oysters exposed to P. marinus. The percentage of granulocytes, percentage of phagocytic haemocytes, number of zymosan per phagocytic haemocytes and plasma haemagglutinin titre were greater in Pacific oysters than in eastern oysters heavily infected with P. marinus. However, the concentration of circulating haemocytes and the concentrations of plasma protein and lysozyme were lower in Pacific oysters. The data suggest that Pacific oysters may offer a less favourable environment for the development of P. marinus as compared to eastern oysters for at least two possible reasons; i.e. the elevated cellular and humoral activities may degrade the parasite more effectively, and/or the lower plasma protein levels may limit parasite growth.