Caractérisation du rôle du stade non-infectieux du parasite acanthocéphale Pomphorhynchus laevis dans la manipulation comportementale de son hôte intermédiaire amphipode

Many trophically-transmitted parasite larvae actively manipulate their intermediate host behaviour in ways increasing its transmission probabilities to the next host in their life cycle. Only stages infective for the next host alter host behaviour in such a way. Before reaching infectivity, parasite larvae grow into several larval stages in the intermediate host, all being non-infective for the next (or definitive) host. Since early predation of the infected intermediate host results in parasite death, non-infective larval stages are predicted to increase their host survival, thus their own survival until infectivity, by enhancing hosts anti-predatory defences. Such a protective strategy has been demonstrated in an acanthocephalan parasite. However, the host may also suffer this increase of anti-predatory defences. For instance, this could divert infected hosts from their optimal balance between survival and seeking for food or mating opportunities. In this study, we tested the hypothesis of such trade-off in freshwater amphipods (Gammarus pulex), intermediate hosts of the acanthocephalan parasite Pomphorhynchus laevis. P. laevis acanthella (the non-infective stage of the parasite) is known to enhance refuge use of G. pulex, which decrease amphipods risks of being predated. During behavioural experiments, we measured refuge use and food intake of acanthella-infected hosts, in presence or absence of predator cues in the water. Food was deposited inside or outside the refuge, mimicking different intensity of host food foraging. Energetic reserves of amphipods were then quantified. We found that acanthella-infected amphipods did use more refuges and eat less than uninfected hosts. Whether the important refuge use or the acanthella infection was the cause of this reduced food intake was however unclear according to our results. Despite a reduced food intake, acanthella-infected hosts were not depleted in energetic reserves. Therefore, amphipods did not suffer from acanthella infection or refuge use, and could even benefit from these behavioural changes. We discuss these results from an te l-0 08 66 99 5, v er si on 1 27 S ep 2 01 3