Parallel morphological evolution and habitat-dependent sexual dimorphism in cave- vs. surface populations of the Asellus aquaticus (Crustacea: Isopoda: Asellidae) species complex.

Studying parallel evolution (repeated, independent evolution of similar phenotypes in similar environments) is a powerful tool to understand environment-dependent selective forces. Surface-dwelling species that repeatedly and independently colonized caves provide unique models for such studies. The primarily surface-dwelling Asellus aquaticus species complex is a good candidate to carry out such research, because it colonized several caves in Europe. By comparing 17 functional morphological traits between six cave and nine surface populations of the A. aquaticus species complex, we investigated population divergence in morphology and sexual dimorphism. We found habitat-dependent population divergence in 10 out of 17 traits, likely reflecting habitat-driven changes in selection acting on sensory systems, feeding, grooming, and antipredator mechanisms. Sexual dimorphism was present in 15 traits, explained by sexual selection acting on male traits important in male-male agonistic behavior or mate guarding and fecundity selection acting on female traits affecting offspring number and nursing. In eight traits, the degree of sexual dimorphism was habitat dependent. We conclude that cave-related morphological changes are highly trait- and function-specific and that the strength of sexual/fecundity selection strongly differs between cave and surface habitats. The considerable population variation within habitat type warrants further studies to reveal cave-specific adaptations besides the parallel patterns.