Adaptive Differences in Morphology of the Periwinkle Littorina sitkana between Two Habitats Separated by an Artificial Structure

Abstract

Morphological and ecological differences of the periwinkle Littorina sitkana (Philippi, 1845) were investigated between two adjacent habitats (inner and outer habitats) of a small harbor in southern Hokkaido, Japan. The inner side of the harbor was well sheltered, and many shore crabs Hemigrapsus sanguineus, a potential predator for gastropods, were present, whereas the outer side of the harbor was very exposed, with few shore crabs. In the inner habitat, most periwinkles possess a uniformly dark-colored, large and solid shell, which is sculptured only at the base of the last whorl, and a comparatively small foot. In the outer habitat, periwinkles possess a small, thin, and light-colored shell with a dark apex and spiral sculpture throughout the surface, and a comparatively large foot. Predation and flume experiments were conducted in the laboratory using snails from both habitats. Only snails from the outer habitat were predated by the shore crabs during the 120-h experiments, and snails from the outer habitat showed significantly stronger resistance to water flow. These morphological and ecological differences of L. sitkana exhibited in the present study may suggest a local adaptation of the same species to different habitats, created by an artificial structure.