Changes in selection on gastropod shell size and thickness with wave-exposure on Northeastern Pacific shores

Patterns of selection on gastropod shell morphology are generally believed to be different on wave-exposed and wave-sheltered shores. The heavy surf o n 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 sel ect for increased shell size and thickness. We compared the risk of shelf-b reaking predation to littorinid gastropods of different sizes and shell-thi cknesses 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 tw o wave-sheltered sites (8% d(-1) and 2% d(-1) respectively). At least 30% o f the shell-breaking predation resulted in diagnostic "peeled" shell breaka ge patterns that could be directly attributed to predatory crabs. Observati ons with SCUBA at high tide suggested that most of the remainder of the she ll-breaking predation was from the red rock crab, Cancer productus, and tha t only a small amount was from pile perch, Rhacochilus vacca. In contrast t o our expectations, the smallest size-class of L. sitkana suffered signific antly lower rates of predation than the largest size-class at one of the wa ve-sheltered sites. The effect of shell thickness on predation mortality wa s as predicted from previous laboratory experiments. The thin-shelled litto rinid 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 shel l-breaking predation on wave-sheltered shores of the Northeastern Pacific s elects for L. sitkana with thicker but not necessarily larger shells than t hose on wave-exposed shores. (C) 1999 Elsevier Science B.V. All rights rese rved.