Epibiosis on Gastropod Shells in the Rocky Intertidal: Effects of Zonation and Shell Morphology

Abstract

In a dynamic environment such as the rocky marine intertidal zone, the stresses experienced by epibionts and their hosts may differ when these animals are engaged in symbioses compared with when they are attached alone to the surrounding rock. We compared the species composition of the epibiotic community on gastropod shells to that of colonizers on neighboring rock surfaces to determine whether intertidal zone, wave action, or basibiont identity affected the epibionts present. We examined two tidal heights (a lower Saccharina sessilis zone and a higher Fucus gardneri/Semibalanus cariosus zone) at three sites of varying wave exposure (Westside Preserve, Cattle Point, and Colin’s Cove) on San Juan Island, WA. In the lower tidal zone (but not in the higher), the percent of unfouled snails was much greater than predicted based on the surrounding substratum. Fewer than four algal species dominated the substratum at both tidal heights, but the identity of the dominant algae differed between tidal heights and did not match the dominant epibiotic species. Instead, small individuals of the barnacles Semibalanus cariosus and Balanus glandula, spirorbid worms, an excavating alga, an excavating sponge, and diatoms were the most prevalent epibionts. While the basibiotic gastropods were less fouled than we expected overall, we found heavily-fouled individuals (especially limpets) interspersed among unfouled individuals. We deployed cleaned empty shells, either sanded (“weathered”) or intact, of the gastropod species Tectura scutum, Lottia digitalis, Nucella canaliculata, and Nucella lamellosa for 3 to 6 weeks in the higher intertidal zone and subtidally at two sites. Differences in settlement of epibionts seemed more due to microhabitat effects than to shell morphology, but it is hard to generalize from such short-term settlement studies.