Do three-spined sticklebacks avoid consuming copepods, the first intermediate host of Schistocephalus solidus ? — an experimental analysis of behavioural resistance

Many parasites that use intermediate hosts are transmitted to the next host through predation. If the next host's fitness is strongly reduced by the parasite, it is under selection either to recognize and avoid infected intermediate hosts or to exclude that prey species from its diet when alternative prey are available. We investigated the predator-prey interaction between laboratory bred three-spined sticklebacks (Gasterosteus aculeatus), the second intermediate host of the cestode Schistocephalus solidus, from 2 parasitized and 1 unparasitized population, and different prey types: infected and uninfected copepods and size-matched Daphnia as alternative prey. Copepods with infective procercoids were more active, had a lower swimming ability and were easier to catch than uninfected controls. The sticklebacks preferred moving copepods. Therefore parasitized copepods were preferentially attacked and consumed. There was no effect of the sticklebacks' parent population being parasitized or not. The sticklebacks switched from Daphnia to (uninfected) copepods in the course of a hunting sequence; this switch occurred earlier in smaller fish. With this strategy the fish maximized their feeding rate: Daphnia were easier to catch than copepods but increasingly difficult to swallow when the stomach was filling up especially for smaller fish. However, there was no indication that sticklebacks from infected populations either consumed Daphnia rather than copepods or switched later in the hunting sequence to consuming copepods than fish from an uninfected population. Thus, sticklebacks did not avoid parasitized prey although S. solidus usually has a high prevalence and causes a strong fitness reduction in its stickleback host.