A parasite spatially structures its host population

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 indiv...