Abstract
The prevalence of infection with trematode parthenitae was studied in Littorina obtusata littoral periwinkles with different shell color genotypes. Activities of genes responsible for a purple or orange single-pigment background shell coloration was not found to affect the prevalence of trematode infection in periwinkles. In L. obtusata with a yellow-purple background shell color and a pattern of white pigment spots on the shell, the prevalence of infection with Microphallus piriformes and M. pygmaeus was lower than the theoretical expectation. The prevalence of infection in periwinkles with purple, orange, or white stripes on the shell did not differ from that of unstripped periwinkles. The differences found were presumably associated with genetically determined susceptibility to infection. Possible consequences of differential infection with trematodes in periwinkles with different shell color genotypes are discussed in terms of the stability of parasitic systems and possible changes in the phenotypic structure of the host population.