DIFFERENTIAL PREDATION ON SEXES AFFECTS COLOR POLYMORPHISM OF THE ISOPOD IDOTEA-BALTICA (PALLAS)

Variable selection, including spatio-temporal variation, frequency-dep endent selection and differential selection due to habitat choice, may maintain polymorphism in heterogeneous environments. We studied preda tion as a selective agent on colour polymorphism of the aquatic isopod I. baltica. Variable predation on this species can arise from at leas t three sources. First, apostatic selection was studied by testing the formation of preferences on colour morphs in the perch, a common pred ator of I. baltica. Such acquired preferences should induce apostatic selection. While our results indicate some acquired preferences, there was significant heterogeneity in the behaviour of predator individual s. Second, temporal variation in selection can arise due to habitat sh ift from the green algae juvenile habitat to the bladderwrack adult ha bitat, and the consequent change in the crypsis of the morphs. Differe nt crypsis between sexes probably promoted high predation mortality am ong females in the juvenile habitat. The high rate of male mortality d uring the breeding period, on the other hand, was presumably due to th eir high mate-searching activity. Third, the sex-dependent habitat cho ice of I. baltica leads to sexual differences in the susceptibility of morphs to predation. Predators preferred the white-spotted morph over the uniform one in males but not in females, supporting the 'dimorphi c niche' hypothesis as an explanation of sexual differences in morph f requencies. Finally, no evidence was found that the colouration patter ns were under sexual selection. We therefore conclude that variable pr edation is the most promising explanation for the maintenance of polym orphism in I. baltica.