PAIRING SUCCESS OF MALE GAMMARUS-LACUSTRIS INFECTED BY 2 ACANTHOCEPHALANS - A COMPARATIVE-STUDY

The costs of parasitism to host reproduction can be best assessed usin g field studies to determine overall mating success and experimental s tudies to examine how parasites may affect mating behavior. We compare d the influence of two parasites, Polymorphus paradoxus and P. marilis (Acanthocephala), on the pairing success of their intermediate host ( Gammarus lacustris, Crustacea) in both the field and laboratory. Paras itism significantly lowered the pairing success of male gammarids. In the field, P. paradoxus-infected males paired significantly less often than P. marilis-infected or uninfected males. Those infected by P. ma rilis were also found in precopula significantly less often than uninf ected ones. In the laboratory the pairing success of males infected by either parasite was significantly reduced in both competitive and non competitive situations. As in the field studies, the pairing success o f P. paradoxus-infected males was significantly lower than that of P. marilis-infected and uninfected males. Polymorphus marilis-infected ma les were also outcompeted by uninfected individuals; however, their pa iring success improved when alone with a female (noncompetitive experi ments). We relate the differential influence of the two parasites on t he pairing success of male gammarids to their effects on the physiolog y and behavior of G. lacustris.