Many trophically-transmitted parasites induce behavioural alteration in their intermediate hosts that tend to increase host vulnerability to predation. Inter-population variability in parasite-induced alterations is expected to arise from variable local opportunities for trophic transmission. Yet, this hypothesis has not been investigated so far. We addressed the issue in four populations of the fish parasite Pomphorhynchus tereticollis (Acanthocephala), using variable fish biomass density as a proxy for transmission opportunities. We found variation in the intensity of parasite-induced changes in phototaxis and refuge use among populations. Two of the populations with the lowest predator biomass exhibited the highest levels of behavioural manipulation and prevalence, as expected at low transmission opportunities. They also exhibited micro-habitat segregation between infected and uninfected gammarids in the field. In addition, infection had variable effects on two physiological defence systems, immunity and antioxidant capacity, and on total protein content. Overall, our study brings partial support to the prediction that host manipulation and prevalence should be higher at low predator biomass. Although stronger evidence should be sought by increasing population replicates, our study points to the importance of the ecological context, specifically transmission opportunities brought about by predation pressure, for the evolution of parasite manipulation in trophically-transmitted parasites.
