Mechanisms of differential survival and growth of two species of Littorina on wave-exposed and on protected shores

Gastropods from wave-exposed shores are typically smaller and thinner-shelled with a larger foot than gastropods from protected shores. We investigated the value of these adaptations by comparing the survival and growth of one exposed shore species and one protected shore species of Littorina transplanted to wave-exposed and protected shores. Adult snails on exposed shores with heavy surf had reduced winter growth rates and low over-winter survival rates. Demographic studies of the exposed shore species showed that few adults survived the winter at the most exposed sites; recruitment to the following year's population was primarily from small juveniles that overwintered between barnacles and within algae. The protected shore species had slightly lower survival than the exposed shore species on exposed shores. The protected shore species was also more susceptible to dislodgement in a flow tank. The low winter growth rates and survival may explain why the large, late-maturing, protected shore species does not occur on exposed shores. Adult snails on protected shores had moderate winter growth rates and moderate over-winter survival. The exposed shore species survived very poorly on protected shores, in part because of predation by shore crabs. In laboratory experiments the thin-shelled exposed shore species was significantly preferred by shore crabs and withstood significantly less force. The small size, thin shell, and large shell aperture of the exposed shore species makes it vulnerable to the shore crabs and may explain why it does not occur on protected shores.