Nonmanipulative parasites in manipulated hosts: 'hitch-hikers' or simply 'lucky passengers'?

Manipulation of intermediate host behavior to favor parasitic transmission has been demonstrated in a wide of range of parasitic taxa. Recent advances in parasitology have suggested that nonmanipulative parasite species can obtain a high probability of transmission simply by infecting hosts already manipulated ('hitch-hiker' parasites). In this study, 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 nonmanipulative trematodes Microphallus hoffmanni and Levinseniella tridigitata. Although these 2 nonmanipulative parasites should be a priori advantaged when infecting manipulated gammarids, there was no significant ecological association between parasite species. We discuss the possible reasons why these 2 nonmanipulative parasites are only 'lucky passengers' rather than 'hitch-hikers.'