The difference between a rock and a biological hard place: epibionts in the rocky intertidal

Abstract

In dynamic environments such as the rocky marine intertidal zone, epibiosis may promote the survival of some organisms. We compared the species composition and prevalence of the epibiotic community on different species of molluscan shells to those on neighboring rock at three sites and two tidal heights per site on San Juan Island, Washington (48°N, 123°W; summer 2014). Across all three sites and both tidal heights examined, the algal species dominating the bedrock did not match the dominant epibiotic species. Most epibiotic species were from a subset of the less common bedrock species. Within the lower tidal height, many potential host shells lacked epibionts, but the bedrock was heavily colonized. Our findings suggest that shells do not represent a microhabitat that is more beneficial to settlers than the surrounding bedrock. Epibiosis is unlikely to elevate local species diversity in this ecosystem because the few epibionts were generally from a subset of the species on the bedrock. Smooth-shelled limpets had a significantly greater prevalence of epibiosis in the lower than in the higher tidal height, but the prevalence of epibiosis did not differ significantly between tidal heights for carnivorous, spired-shell species. This variation among basibionts in pattern of prevalence across tidal heights suggests that the carnivorous snails are more likely than the limpets to range across intertidal heights.