Immersion affects intertidal invertebrate assemblages of coralline algae

Marine invertebrates living in the rocky intertidal are exposed to many periodic stressors including insolation and desiccation at low tide.  Motile taxa have the potential to follow the tides and thereby avoid exposure to air.  Alternatively, they could congregate in moist refugia during the period of exposure.  We examined assemblages of invertebrates living in coralline algal beds (Rhodophyta: Rhodophyceae) in Barkley Sound, Vancouver Island to determine whether they showed any differences during the exposed vs. immersed phases of the tide.  Samples of coralline algae consisting of a mixture of Corallina vancouveriensis and C. officinalis were collected from two transects at each of three sites.  The transects were sampled at both low tide and high tide (via SCUBA diving).  Results were analyzed using the multivariate technique analysis of similarity (ANOSIM) and ANOVA. Invertebrate assemblages occupying coralline algae sampled at low tide (exposed samples) were significantly different from those sampled at high tide (immersed samples).  Contrary to expectation and results from a previous study, exposed algae had significantly more individuals per sample than the immersed samples.  We hypothesize that invertebrates use the complex habitat provided by the coralline algae as a moist refuge from desiccation stress.