1.Many parasites with complex life cycles critically rely on trophic transmission to pass from an intermediate host to a final host. Parasite-induced behavioural alterations in intermediate hosts are often supposed to be adaptive through increasing the susceptibility of intermediate hosts to predation by final hosts. However, the evidence is so far only correlational, and direct evidence for a causal link between one single behavioural alteration and increased trophic transmission is still missing.
2.Here, we addressed, for the first time, the relationship between increased vulnerability to fish predation and altered photophobia in an amphipod, Gammarus pulex, infected with a fish acanthocephalan, Pomphorhynchus tereticollis.
3.In microcosms, naturally-infected amphipods were significantly more vulnerable to fish predation than uninfected ones at two different light intensities. However, although variation in illumination significantly affected the extent of difference in photophobia between infected and uninfected individuals, it had no effect on predation bias towards infected amphipods.
4.In addition, although an injection of a mixture of serotonin and fluoxetine in uninfected amphipods mimicked the parasite-induced decreased photophobia, injected individuals were not more vulnerable to fish predation that uninfected ones injected with a control solution.
5.Overall, our results indicate that the decreased photophobia in infected intermediate hosts does not play in itself a causal role in the trophic transmission of the parasite to its final host. The actual role a parasite-induced behavioural alteration plays in trophic transmission should be carefully assessed before an adaptive interpretation is given.