MERZ, R.A.; HASHIMOTO, M.; PATEL, D.A.; Swarthmore College, Pennsylvania; Swarthmore College, Pennsylvania; Swarthmore College, Pennsylvania: Do intertidal barnacles (Semibalanus cariosus) and anemones (Anthopleura elegantissima) benefit from being neighbors?

To ask whether S. cariosus and A. elegantissima could benefit by close association, we tested an array of possible interactions. At Cattle Point, San Juan Island, WA we performed reciprocal removals. Barnacles showed no immediate decline when adjacent anemones were removed; however, when barnacles were removed, the population of remaining anemones dropped (14 – 66%), usually stabilized within 2 weeks and then often increased from migration and division. This recovery was significantly faster in the lower intertidal. We sampled summer daytime low-tide temperatures of rock surfaces, anemones and barnacles. Typically, solitary anemones (but not barnacles) were hotter than the adjacent exposed rock surface, whereas anemones and barnacles in groups were significantly cooler. We found that the shells of S. cariosus serve as habitat for a wide diversity of invertebrates compared to the adjacent rock (21 vs. 0.2 individuals per cm ²). In lab we observed behavior of barnacle predators (Pisaster ochraceus, Pycnopodia helianthoides, Nucella) as they attempted to move through a field of A. elegantissima towards barnacles. Starfish were deflected or delayed (P. ochraceus 4-fold and P. helianthoides 8-fold), and withdrew tube feet and arched away from the substratum when moving over a bed of anemones. Nucella moved through bunches of anemones by using their shells as shields against anemone tentacles. These data suggest that although sessile species in the rocky intertidal compete for space, anemones living amongst barnacles may benefit from temperature moderation during low tides and a ready source of prey items living on barnacle shells. Barnacles living next to anemones may have an advantage in avoiding crawling predators.