Changes in selection on gastropod shell size and thickness with wave-exposure on northeastern pacific shores.

Abstract

Patterns of selection on gastropod shell morphology are generally believed to be different on wave-exposed and wave-sheltered shores. The heavy surf on wave-exposed shores is thought to select for small size whereas the high risk of shell-breaking predation on wave-sheltered shores is thought to select for increased shell size and thickness. We compared the risk of shell-breaking predation to littorinid gastropods of different sizes and shell-thicknesses by tethering them on wave-exposed and wave-sheltered shores of the Northeastern Pacific. Over 2 years we found that the predation rate on the direct-developing gastropod Littorina sitkana was consistently much lower at two moderately wave-exposed sites (less than 0.01% d −1) than on the two wave-sheltered sites (8% d −1 and 2% d −1 respectively). At least 30% of the shell-breaking predation resulted in diagnostic “peeled” shell breakage patterns that could be directly attributed to predatory crabs. Observations with SCUBA at high tide suggested that most of the remainder of the shell-breaking predation was from the red rock crab, Cancer productus, and that only a small amount was from pile perch, Rhacochilus vacca. In contrast to our expectations, the smallest size-class of L. sitkana suffered significantly lower rates of predation than the largest size-class at one of the wave-sheltered sites. The effect of shell thickness on predation mortality was as predicted from previous laboratory experiments. The thin-shelled littorinid species, Littorina subrotundata, suffered significantly higher rates of predation than two thicker-shelled species, L. sitkana and L. scutulata s.l., at three of our four sites. We conclude that the higher rates of shell-breaking predation on wave-sheltered shores of the Northeastern Pacific selects for L. sitkana with thicker but not necessarily larger shells than those on wave-exposed shores.