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.