A parasite spatially structures its host population

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Acanthocephalan parasites are capable of altering the behavior of their intermediate amphipod hosts by inducing hyperactivity and increasing downstream drift. We used a laboratory and field study to investigate how Echinogammarus stammerii amphipods infected by the acanthocephalan Pomphorhychus laevis compensated for downstream displacement and responded to a fish extract predator cue. Experiments conducted in laboratory streams showed infected amphipods drifted more and compensated for drift less than non‐infected individuals. When fish extract was added to the channels, non‐infected individuals decreased their activity while infected individuals did not. A field study examined the up‐ and down‐stream movements of a population of E. stammerii infected with P. laevis in the River Brenta, Italy. Drift by E. stammerii was positively influenced by acanthocephalan infection. Infected amphipods made up 40% of the drift but accounted for just 5% of the E. stammerii population. Infected individuals, however, were under‐represented among upstream‐moving amphipods collected from the main river channel and a near‐shore migration column, comprising only 3% of sampled individuals. These data suggest that infected individuals may be effectively displaced downstream, creating a mechanism for spatial separation of the non‐infected and infected sub‐populations.