Perkinsus marinus causes devastating losses in populations of the eastern oyster (Crassostrea virginica). Our studies have demonstrated that P. marinus secretes extracellular serine proteases which enhance parasite propagation and compromise host defences. Crassostrea virginica. however, possesses several inhibitors of these proteases. The Pacific oyster (C. gigas) is resistant to P. marinus and possesses protease inhibitors with significantly higher specific activities than those in C. virginica. Interestingly, Crassostrea spp. themselves, elaborate metalloprotease activities which can be detected in their plasma, and are increased during P. marinus infections. Together our work suggests that there may be a broad spectrum of humoral host defences that is brought to bear on P. marinus infections by these two Crassostrea species
