Effects of plasma from bivalve mollusk species on the in vitro proliferation of the protistan parasite Perkinsus marinus

The in vitro culture of the Eastern oyster parasite Perkinsus marinus has provided a unique opportunity to examine its susceptibility to putative recognition and effector defense mechanisms operative in refractory bivalve species. In this study, we report the effect of supplementing the culture medium with plasma from: (1) uninfected to heavily infected Eastern oysters; (2) oyster species considered to be disease-resistant; and (3) bivalve mollusk species that are naturally exposed to the parasite but show no signs of disease. We also examined in vitro the interaction between hemocytes from Crassostrea virginica and C. gigas and P. marinus trophozoites. Our results revealed a significant decrease (32%) in proliferation of P. marinus in the presence of plasma from heavily infected C. virginica oysters. The inhibitory effects were less pronounced with plasma from moderately infected and uninfected oysters. In contrast, plasma from C. rivularis and C. gigas enhanced P. marinus proliferation. Proliferation was significantly reduced in media supplemented with plasma from Mytilus edulis, Mercenaria mercenaria, and Anadara ovalis. The highest inhibitory activity was apparent in M. edulis, for which 5% plasma-supplemented medium reduced growth by 35% relative to the controls. M. edulis active component(s) was heat-stable, yet pronase-sensitive. The significantly higher uptake of live P. marinus trophozoites by hemocytes from C. virginica, relative to those from C. gigas, suggests a certain level of specificity in the recognition/endocytosis of the parasite by its natural bivalve host species.