Summary1. 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.

9 pagesInternational audience1. Many parasites with complex life cycles alter the phenotype of their intermediate hosts inways that seem to favour transmission to a final host. Although there is a large literature onhost manipulation, how parasites alter the phenotype of their hosts remains poorly known.2. The bird acanthocephalan Polymorphus minutus is known to alter geotaxis in its amphipodhost, Gammarus roeseli. Here, we examine the potential roles of low oxygen availability andthe excretion, by the parasite, of two products from its own anaerobic metabolism (lactate andsuccinate) in altered geotaxis.3. Under hypoxia, uninfected G. roeseli showed negative geotaxis and lower metabolic rate,two traits also altered by infection with P. minutus, albeit with different intensities. Theinjection of a mixture of lactate and succinate in uninfected amphipods mimicked theparasite-induced reversion of geotaxis, without affecting the metabolic rate.4. In addition, both P. minutus-infected gammarids and uninfected ones conditioned tohypoxia for 2 days showed elevated levels of lactate in the brain, but not in the haemolymph.5. Overall, our results indicate that the pathways involved in anaerobic metabolism andhypoxia signalling might be responsible for the changes in geotaxis and metabolic rate inducedby P. minutus infection.6. Our study emphasizes the need to consider the tight and complex connections between physiologicalprocesses and behavioural adjustments, in particular at the brain level, in the understandingof parasitic manipulation, and more broadly of behavioural changes in infected hosts