ANALYSES OF A DEBILITATING PARASITE (MICROPHALLUS-PAPILLOROBUSTUS, TREMATODA) AND ITS HITCHHIKER PARASITE (MARITREMA-SUBDOLUM, TREMATODA) ON SURVIVAL OF THEIR INTERMEDIATE HOST (GAMMARUS-INSENSIBILIS, AMPHIPODA)

Parasites that alter their intermediate host's behavior to favor its p redation by the definitive host are known from a wide range of host-pa rasite associations. Recently, we found a new category of parasites, s o-called ''hitchhikers,'' unable to modify the behavior of their inter mediate host but exploiting the same host spectrum, that gain benefits in transmission success from the infection of these behaviorally mani pulated hosts. Because the probability of successful transmission by ' 'hitchhiking'' depends on 1) the efficiency of the ''favorization'' pr ocess of the debilitating parasite and 2) the effects of infection by the hitchhiker parasite on the host survival, we investigated these tw o aspects. In the laboratory, we showed that, in the absence of a pred ator (definitive host), there is no significant difference between the mortality rates of Gammarus insensibilis (second intermediate host) u ninfected and infected by the debilitating trematode Microphallus papi llorobustus. In the field, we showed that the hitchhiker trematode Mar itrema subdolum does not significantly reduce host survival. These res ults suggest that the higher mortality rate of manipulated hosts in th e field could be explained by the predation by the definitive hosts an d that M. subdolum does not alter, through survival reduction, the eff iciency of the favorization process.