Pomphorhynchus laevis host manipulation regardless of presence of gammarid conspecifics or predator cues.

Pomphorhynchus laevis is a freshwater heteroxenous parasite that infects Gammarus pulex as intermediate host and Squalius cephalus as definitive host. This acanthocephalan parasite is able to manipulate the behavior of G. pulex to increase probability of transmission between G. pulex prey and S. cephalus predator. Manipulation results from complex interactions and could be triggered or enhanced by biotic parameters. This experimental study investigated the effect of the presence both of G. pulex conspecifics and of S. cephalus cues on the behavior of P. laevis-infected and uninfected G. pulex in flowing water. The presence of G. pulex conspecifics increased the distance covered by both uninfected and P. laevis-infected gammarids in the artificial stream, whereas S. cephalus cues did not influence the distance covered by either. In static water, S. cephalus cues significantly decreased G. pulex attraction to light, regardless of infection status. Our results show that P. laevis manipulates distance covered regardless of the presence of either G. pulex conspecifics or S. cephalus cues. We discuss how these two biological parameters affect uninfected G. pulex and other factors involved in P. laevis manipulation of its gammarid host.