Sexual and local divergence in host exploitation in the marine herbivore Idotea baltica (Isopoda)

We studied sexual and habitat-specific variation in herbivores' host exploitation patterns and tested for a hypothesis on sex-specific adaptation in replicated landscapes. The hypothesis has not to our knowledge been previously tested either in a marine environment or for the host exploitation traits of an herbivore. The hypothesis states, first, that populations may show different adaptations due to spatially differing selective environments, and second, that males and females that differ ecologically (e.g. in reproductive behavior, in host use patterns) will respond to selective environments in distinct ways. We investigated possible differences in the host exploitation patterns of the marine generalist crustacean grazer Idotea baltica between the sexes or among populations originating from different habitats: plant assemblages dominated by either a brown alga or an angiosperm species. We determined the preferences for both the structure and nutritive quality of five common host species and compared these to the actual performance on the hosts. We found that performance on the hosts differed between the sexes and differently so in the two habitat types, supporting the hypothesis of sex-specific adaptation between distinct selective environments. Habitat-specific preference for the structural host further supported ecological divergence between habitat types. The different approaches for testing host exploitation resulted in a very different rank order of the hosts, indicating the complexity of the factors involved. The herbivore differentiates for the chemical quality of the hosts, but this alone does not explain host exploitation; rather it supports the theory of enemy-free space as a determinant of host choice. We also discuss the chemical characteristics of the host associated with the exploitation patterns found.