The complement system is composed of over 30 plasma proteins that collaborate to distinguish and eliminate pathogens. C3 is the central component in this system. In vertebrates, it is proteolytically activated by a C3 convertase through both the classic, lectininduced and alternative routes [35] . Although the complement pathway has not been extensively described in bivalves, there is evidence that supports the presence of this defense mechanism. ESTs with homology to the C1q domain have been detected in the American oyster, C. virginica [36] , the tropical clam Codakia orbicularis [37] , the Zhikong scallop Chlamys farreri [38] and the mussel M. galloprovincialis [39] [40] . More recently, a novel C1q adiponectin-like, a C3 and a factor B-like proteins have been identified in the carpet shell clam R. decussatus [41] [42] . These data support the putative presence of the complement system in bivalves.