The optimal foraging strategy of its stickleback host constrains a parasite's complex life cycle

The cestode parasite Schistocephalus solidus' growth is limited by the size of its second intermediate host, the three-spined stickleback, Gasterosteus aculeatus. S. solidus should thus prefer a large stickleback as host. Since the stickleback is a predator of the parasite's previous intermediate host, a small copepod, the stickleback that consumes the infected copepod will probably be of a size for which this copepod has the optimal prey size. The optimal foraging decision of the stickleback may or may not be compatible with the parasite's preference. Infected copepods are present in early summer when both many size classes of young of the year and adult sticklebacks are potential predators. We offered laboratory bred three-spined sticklebacks of four size classes individually the choice among five prey types: two size classes of copepods, two classes of Daphnia of corresponding size as alternative prey and a third Daphnia size class that was larger than the larger copepod. We found that small copepods, the potential hosts of S. solidus, were most accepted by the smallest sticklebacks of about 1.5 cm of length, larger fish consumed a decreasing proportion; fish larger than 3.8 cm did not consume them at all. Experience with copepods over several weeks increased the acceptance for this prey to some extend but hardly in the largest fish. This suggests that S. solidus will end up usually in sticklebacks that are too small for the parasite so that it has to allow its host's further growth after infection to reach its definitive size.