COPEPOD REACTION TO ODOR STIMULI INFLUENCED BY CESTODE INFECTION

The cestode Schistocephalus solidus uses copepods as first and stickle backs as second intermediate hosts. For transmission, an infected cope pod has to be preyed upon by a stickleback. We used copepods of the sp ecies Macrocyclops albidus to test whether infected and uninfected cop epods differ in their reaction to two kind of simultaneously presented odors: odors of sticklebacks and odors of sticklebacks and conspecifi cs. By giving this choice, we attempted to force the copepods to make a trade-off between the benefit of risk dilution and possible predator confusion and the costs of food competition and other disadvantages i nduced by conspecifics. Within 1-8 h after last feeding, uninfected co pepods clearly preferred the odors of conspecifics under the chemicall y simulated threat of predation. This was in contrast to the infected copepods, who tended to avoid the odor of conspecifics. When the time between experiment and last feeding varied, infected copepods showed a n increased preference for fish water only (or avoided conspecifics) w ith increasing hunger level. This suggests that S. solidus benefits fr om hunger-induced behavioral changes of its copepod host by influencin g its microhabitat selection. The same effect could be found in both s exes; however, it was significantly more pronounced in male than in fe male copepods. We propose several hypotheses that could explain the di fference between the sexes in their infection-dependent microhabitat s election.