AN ENVIRONMENTAL EXPLANATION FOR THE CHARACTER DISPLACEMENT PATTERN IN HYDROBIA SNAILS

Size differences between Hydrobia snails showed a clear character displacement pattern over a wide area along the Finnish coast of the Baltic Sea. H. ulvae mean size was higher in sympatric sites than in single-species sites, and in allopatric sites H. ulvae means were always higher than H. ventrosa means. Mean size of H. ventrosa showed little variation in the study area. The proportion and abundance of H. ventrosa had a high positive correlation with H. ulvae mean size. High correlations with H. ulvae size disappeared when adjusted by the type of the habitat which ranged from a sheltered lagoon to an open site; H. ulvae grows larger in the subset of H. ulvae environments where H. ventrosa is also able to live. A weaker negative effect of habitat type was observed in H. ventrosa. This result gives an environment-based explanation for the character displacement pattern and urges the need for critical evaluation of the evidence for character displacement in Hydrobia snails in other populations as well.