Pathogen proteases are important virulence factors that facilitate infection, diminish the activity of lysozymes and quench the agglutination capacity of hemocytes. Because protease inhibitors play important roles in invertebrate immunity by protecting hosts through the direct inactivation of pathogen proteases, many bivalves have developed protease inhibitors to regulate the activities of pathogen proteases [1] . Some genes encoding protease inhibitors were identified in C. gigas [57] , A. irradians [58] , C. farreri [59] and C. virginica; in the latter a novel family of serine protease inhibitors was also characterized [60] [61] [62] .