Habitat heterogeneity, predation and gene flow: colour polymorphism in the isopod, Idotea baltica

Abstract

The colour polymorphic isopod Idotea baltica inhabits the brown alga Fucus vesiculosus which is often colonised by the white epizoite Electra crustulenta (Bryozoa). In an experiment the predation risk for the different colour morphs of I. baltica was highly dependent on background colouration. Morph frequencies and Electra density varied substantially among 10 collecting sites but correlated poorly with each other, suggesting that local selection for cryptic colouration may be counteracted by gene flow. Indeed, estimates based on four polymorphic allozymes suggested rate of gene flow to be high. These results support the hypothesis that locally varying selection for cryptic colouration counteracted by gene flow contributes to the maintenance of colour polymorphism in I. baltica. The visual differences between the microhabitats and the differential microhabitat use between males and females seem to result in different patterns of selection on males and females for cryptic colouration. Also this is likely to play an important role for the polymorphism.