Phenotypic selection in an intertidal snail: effects of a catastrophic storm

Littorina obtusata exhibits clear morphological variation (e.g. shell height, shell length, and aperture area) among shores differentially exposed to wave energies. Selection imposed by the hydrodynamic environment is often invoked to explain the correlation between morphology and wave exposure in intertidal organisms, but rarely is this hypothesis tested. I examined the effects of a catastrophic storm on the shell length and relative shell height and aperture area of L. obtusata populations on 2 protected and 1 wave-exposed share in New England (USA) to test this hypothesis. Snails sampled after the storm had relatively squatter shells than those sampled before the storm, which is consistent with the pattern in shell height found in natural populations. The rapid shift supports the hypothesis that shell morphology on wave-exposed shores reflects, in part, selection imposed by hydrodynamic stress. Compared to morphological differences found between natural populations, shifts in relative shell height after the storm were small. Hence, despite its magnitude, the impact of this storm may be limited by available variation in relative shell height, especially if selection pressures operating during \u27typical\u27 conditions have depleted variation in this trait. There were 2 surprising results. First, relative aperture area decreased after the storm. This result is counter-intuitive because one would expect a shift to larger apertures, which can accomodate a larger adhesive foot. Second, a decrease in shell length after the storm was found only at the wave-exposed site, suggesting that the effects of the storm were more pronounced on this population. Because a shift in relative shell height and aperture area occurred at all 3 sites, these traits appear to be more sensitive to large storms than is shell length. The shifts in both relative shell height and aperture area may reflect differences in the hydrodynamic properties of shells collected before and after the storm, but I suggest that the shifts were mediated by the ability of snails to avoid free-stream flows by hiding in sheltered crevices. Both reduced shell height and aperture area may be advantageous when trying to fit into sheltered microhabitats during periods of increased hydrodynamic stress. In particular, the shift in aperture area may reflect the premium on successful crevice use over conventional adhesion during periods of extreme wave energies. Finally, while recent work on intertidal snails has emphasized the importance of phenotypic plasticity to morphological differentiation among populations, my results suggest that selection can assume a more prominent role during unusual events Like this storm