NONMANIPULATIVE PARASITES IN MANIPULATED HOSTS - HITCHHIKERS OR SIMPLY LUCKY PASSENGERS

Manipulation of intermediate host behavior to favor parasitic transmis sion has been demonstrated in a wide of range of parasitic taxa. Recen t advances in parasitology have suggested that nonmanipulative parasit e species can obtain a high probability of transmission simply by infe cting hosts already manipulated ('hitch-hiker' parasites). In this stu dy, from a field collection of Gammarus aequicauda (Amphipoda, second intermediate host), we analyzed the ecological association between the manipulative trematode Microphallus papillorobustus and the 2 nonmani pulative trematodes Microphallus hoffmanni and Levinseniella tridigita ta. Although these 2 nonmanipulative parasites should be a priori adva ntaged when infecting manipulated gammarids, there was no significant ecological association between parasite species. We discuss the possib le reasons why these 2 nonmanipulative parasites are only 'lucky passe ngers' rather than 'hitch-hikers.'