The effect of wave action on growth in three species of intertidal gastropods

Populations of the limpets Collisella digitalis and C. scabra, as well as the thaidid whelk Nucella (Thais) emarginata, had greater mean shell lengths at a protected site (Tomales Bay) than at an exposed site (Mussel Point) on the California coast near the Bodega Marine Laboratory. To determine the relative importance of wave action as well as genetic differentiation among populations in explaining this pattern, tagged snails of all three species were reciprocally transferred between the two sites. For C. digitalis, total wet mass (tissue plus shell) increased by 34.4% at the protected site, but decreased by 2% at the exposed site over a two and one-half month period. For C. scabra, growth was 43.1% at the protected, and 2.7% at the exposed site, and for Nucella, 9.5% and 1%, respectively. Although some evidence of population differentiation was found, particularly for the direct-developing whelk, source differences in growth were not as large as the site effect. At least for the whelk, absolute differences in barnacle prey abundances did not occur between sites. However, all three gastropods had higher abundances at the exposed site. While factors such as genetic differentiation and competition may partially explain why gastropods are on the average smaller at exposed sites, we suggest that wave action may also play a role, possibly by limiting time available for feeding, and therefore energy available for growth. Although wave action, acting via size-specific mortality, has been suggested to limit the size that consumers can reach on exposed shores, it may also indirectly affect intertidal gastropod populations by altering foraging behavior, growth and life histories.