Copepod reaction to odor stimuli influenced by cestode infection

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Abstract
The cestode Schistocephalus solidus uses copepods as first and sticklebacks 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 whether infected and uninfected copepods differ in their reaction to two kind of simultaneously presented odors: odors of sticklebacks and odors of sticklebacks and conspecificz. 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 induced by conspecifics. Within 1–8 h after last feeding, uninfected copepods clearly preferred the odors of conspeciflcs under the chemically 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 an increas preference for fish water only (or avoided conspecthcs) with increasing hunger level This suggests that S. solidus benefits from hunger-induced behavioral changes of its copepod host by influencing its microhabitat selection. The same effect could be found in both sexes; however, it was significantly more pronounced in male than in female copepods. We propose several hypotheses that could explain the difference between the sexes in their infection-dependent microhabitat selection.

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