Copepod reaction to odor stimuli influenced by cestode infection

The cestode Schistocephalus solidus uses copepods as first andsticklebacks as second intermediate hosts. For transmission,an infected copepod has to be preyed upon by a stickleback.We used copepods of the species Macrocy albidus to test whetherinfected and uninfected copepods differ in their reaction totwo kind of simultaneously presented odors: odors of sticklebacksand odors of sticklebacks and conspecificz. By giving this choice,we attempted to force the copepods to make a trade-off betweenthe benefit of risk dilution and possible predator confusionand the costs of food competition and other disadvantages inducedby conspecifics. Within 1–8 h after last feeding, uninfectedcopepods clearly preferred the odors of conspeciflcs under thechemically simulated threat of predation. This was in contrastto the infected copepods, who tended to avoid the odor of conspecifics.When the time between experiment and last feeding varied, infectedcopepods showed an increas preference for fish water only (oravoided conspecthcs) with increasing hunger level This suggeststhat S. solidus benefits from hunger-induced behavioral changesof its copepod host by influencing its microhabitat selection.The same effect could be found in both sexes; however, it wassignificantly more pronounced in male than in female copepods.We propose several hypotheses that could explain the differencebetween the sexes in their infection-dependent microhabitatselection.