In Lake Fjellfrøsvatn, northern Norway, the larval helminths Cyathocephalus truncatus and Cystidicola farionis use Gammarus lacustris as intermediate hosts and Arctic charr (Salvelinus alpinus) as final hosts. There was sampled 1,433 live G. lacustris from the lake and 1,964 G. lacustris from stomach contents of the charr. Prevalence of infection were, respectively, 0.49% and 3.72% for C. truncatus, and 0.21% and 0.20% for C. farionis. Usually, only 1 parasite was present in each host, and the 2 parasite species never co-occurred. Gammarus lacustris amphipods parasitized by C. truncatus were positively selected by the Arctic charr and were consumed approximately 8 times as often as were the unparasitized amphipods or the amphipods infected with C. farionis. This suggests that G. lacustris amphipods infected with C. truncatus larvae are more susceptible to predation than noninfected specimens, probably because of parasite-induced alterations in behavior or visibility. Alternatively, this could also be explained by selection toward the largest G. lacustris specimens observed, which are also the most frequently parasitized amphipods. However, the data show clearly that this was not a result of size-selective predation by the charr. In contrast, the presence of C. farionis did not increase the susceptibility to predation of its intermediate host. The discrepancy between the 2 helminth species supports the hypothesis that parasite-increased susceptibility to predation is related to the life history strategies of the parasites.

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