Associational resistance mediates predator--prey interactions in a marine subtidal system

There is a growing awareness of the role that indirect interactions play in influencing food webs and ecosystem structure. In this study, the hypothesis that crustose algal epibionts provide gastropods associational resistance from predation was investigated through field surveys and laboratory feeding assays. In rocky low intertidal/shallow subtidal systems in the northeast Pacific, several species of crustose algae (the red alga Peyssonnelia meridionalis and crustose corallines) can colonize the shells of living Tegula brunnea snails. The growth patterns of these epibiontic crustose algae allow them to cover their host’s surface completely, which may, in turn, protect their hosts from predation. A multi‐site field survey of T. brunnea revealed that >60% of snails were at least 75% covered with one or more species of crustose algae, with 35% fully covered, indicating that this is common in the field. Laboratory feeding assays revealed that sea stars, a primary predator of T. brunnea, distinguished among snails with different shell coverings; Pisaster consumed nearly three times as many bare (i.e. no crustose algae) snails as those covered with Peyssonnelia, while Pycnopodia consumed four times as many bare snails as those covered with crustose corallines. These results suggest that epibiont crustose algae can benefit their hosts via associational resistance; this finding may have implications for the role of associational resistance in trophic interactions.