Evolution of sex differences in microhabitat choice and colour polymorphism in Idotea baltica

We studied microhabitat choice of colour morphs, causes of sex differences in microhabitat use and colour polymorphism in Idotea baltica, a marine isopod living mainly on the brown alga Fucus vesiculosus. The colour morphs differ in frequencies between the sexes and appear to be cryptic on the visually heterogeneous Fucus. In this study, no colour-morph-dependent preference for visually matching microhabitats was found. However, in all three experiments conducted, females were found more often on the lower parts of the Fucus than males. The microhabitat choice of the sexes was directed by some character of Fucus itself, not by preferred :height within the plant. However, the sexes did not choose differently between upper and lower parts of Fucus as food. The food choice and substrate choice correlated in males but not in females, implying that microhabitat and feeding preferences are more tightly associated in males. We propose that the stronger preference for the less exposed lower parts of Fucus as microhabitat and the lack of correlation between microhabitat and substrate choice in females can be explained in terms of a greater investment in anti-predator protection in females than in males. Thus, the sexual difference in microhabitat choice would ultimately result from different strategies maximizing reproductive success in males and females. We suggest that the sexual differences in coloration and colour morph frequency in I. baltica are explained as an adaptation to sex differences in patterns of habitat use. (C) 1997 The Association for the Study of Animal Behaviour.