When two parasite species are manipulators and have different definitive hosts, there is a potential for conflict between
them. Selection may then exist for either avoiding hosts infected with conflicting parasites, or for hijacking, i.e.
competitive processes to gain control of the intermediate host. The evidence for both phenomena depends largely on the
study of the relative competitive abilities of parasites within their common intermediate host. We studied the effects of
simultaneous infection by a fish acanthocephalan parasite, Pomphorhynchus laevis, and a bird acanthocephalan parasite,
Polymorphus minutus, on the behaviour of their common intermediate host, the amphipod Gammarus pulex. We compared
the reaction to light and vertical distribution of individuals infected with both parasites to those of individuals harbouring
a single parasite species and uninfected ones under controlled conditions. Compared to uninfected gammarids that were
photophobic and tended to remain at the bottom of the water column, P. laevis-infected gammarids were attracted to light,
whereas P. minutus-infected individuals showed a modified vertical distribution and were swimming closer to the water
surface. The effects of both P. laevis and P. minutus appeared to be dependent only on their presence, not on their intensity.
Depending on the behavioural trait under study, however, the outcome of the antagonism between P. laevis and P. minutus
differed. The vertical distribution of gammarids harbouring both parasites was half-way between those of P. laevis- and
P. minutus-infected individuals, whereas P. laevis was able to induce altered reaction to light even in the presence
of P. minutus. We discuss our results in relation to the occurrence of active avoidance or hijacking between conflicting
manipulative parasites and provide some recommendations for future research.