Cryptic coloration, microhabitat choice and polymorphism in Idotea baltica (Isopoda)

Abstract: In this thesis I have studied cryptic coloration, microhabitat use and colour polymorphism in the marine isopod Idotea baltica. In the Baltic Sea it lives and feeds mainly on the seaweed Fucus vesiculosus, which in sheltered sites usually is whitespotted on its lower parts due to epizoites. The white-spotted morph of I. baltica was found to suffer a lower predation risk on white-spotted background and the uniform morph had a lower predation risk on a uniform background. However, the morphs of I. baltica did not prefer visually matching microhabitat. Instead, males more than females used the exposed upper parts of F. vesiculosus. This probably results from males and females differently trading feeding against protection from visual predators, and may explain the differences in morph frequencies between the sexes.A comparison of colour pattern elements of the isopods and on the seaweed showed that the white-spotted morph achieves crypsis not only through visual resemblance to its background, but also by concealment of the body outline through disruptive coloration. A model of optimisation of crypsis in heterogeneous habitats suggested that maximal protection can sometimes be achieved by a coloration compromising the requirements of several microhabitats.A comparison among ten I. baltica populations inhabiting sites with highly variable densities of white epizoites on F. vesiculosus revealed that variation in morph frequencies was related to the density of epizoites, but not to the small variation in allozyme frequencies. These results suggest that the colour polymorphism in I. baltica is maintained by divergent local selection for crypsis counteracted by gene flow as well as by opposing selection on coloration in males and females.