Impact of olfactory non-host predator cues on aggregation behaviour and activity in Polymorphus minutus infected Gammarus pulex

Parasites with a complex life cycle are supposed to influence the behaviour of their intermediate host in such a way that the transmission to the final host is enhanced, but reduced to non-hosts. Here, we examined whether the trophically transmitted bird parasite Polymorphus minutus increases the antipredator response of its intermediate host, the freshwater amphipod Gammarus pulex to fish cues, i.e. non-host cues (‘increased host abilities hypothesis’). Aggregation behaviour and reduced activity are assumed to decrease the predation risk of gammarids by fishes. Uninfected G. pulex are known to aggregate in the presence of a fish predator. In the present study, gammarids were allowed to choose either to join a group of conspecifics or to stay solitary (experiment 1) or between two groups differing in infection status (experiment 2), both in the presence or absence of fish odour. The perception of the groups was limited to mainly olfactory cues. Contrary to the ‘increased host abilities hypothesis’, in infected gammarids of experiment 1, fish cues induced similar aggregation behaviour as in their uninfected conspecifics. In experiment 2, uninfected as well as infected gammarids did not significantly discriminate between infected and uninfected groups. Although only uninfected gammarids reduced their activity in the presence of predator cues, infected G. pulex were generally less active than uninfected conspecifics. This might suggest that P. minutus manipulates rather the general anti-predator behaviour than the plastic response to predation risk.