A parasite spatially structures its host population


  • Todd Wellnitz,

  • Luisa Giari,

  • Barbara Maynard,

  • Bahram S. Dezfuli

T. Wellnitz and B. Maynard, Dept of Biology, Colorado, State Univ., Fort Collins, CO 80523, USA (wellnitz@lamar.colostate.edu). – L. Giari and B. S. Dezfuli, Dipto di Biolgia Evolutiva, Univ. Di Ferrara, Via L. Borsari 46, IT-44100 Ferrara, Italy.


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.