DO 3-SPINED STICKLEBACKS AVOID CONSUMING COPEPODS, THE FIRST INTERMEDIATE HOST OF SCHISTOCEPHALUS-SOLIDUS - AN EXPERIMENTAL-ANALYSIS OF BEHAVIORAL RESISTANCE

Many parasites that use intermediate hosts are transmitted to the next host through predation. If the next host's fitness is strongly reduce d 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 (G asterosteus aculeatus), the second intermediate host of the cestode Sc histocephalus solidus, from 2 parasitized and 1 unparasitized populati on, and different prey types: infected and uninfected copepods and siz e-matched Daphnia as alternative prey. Copepods with infective procerc oids were more active, had a lower swimming ability and were easier to catch than uninfected controls. The sticklebacks preferred moving cop epods. 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 (u ninfected) copepods in the course of a hunting sequence; this switch o ccurred earlier in smaller fish. With this strategy the fish maximized their feeding rate: Daphnia were easier to catch than copepods but in creasingly difficult to swallow when the stomach was filling up especi ally for smaller fish. However, there was no indication that stickleba cks from infected populations either consumed Daphnia rather than cope pods or switched later in the hunting sequence to consuming copepods t han fish from an uninfected population. Thus, sticklebacks did not avo id parasitized pre); although S. solidus usually has a high prevalence and causes a strong fitness reduction in its stickleback host.