Experimental investigation of tidal and freshwater influence on Symbiodinium muscatinei abundance in its host, Anthopleura elegantissima

Controlled experiments testing effects of temperature, salinity, and aerial exposure were paired with field observations to investigate symbiont expulsion in the abundant intertidal anemone, Anthopleura elegantissima. In the study region, A. elegantissima hosts a single symbiont, the dinoflagellate Symbiodinium muscatinei. The San Francisco Bay outflow creates a tidally influenced low-salinity plume that impacts adjacent coastal sites. Salinity, temperature, and aerial stress induce a bleaching response similar to corals where symbionts are expelled, causing further energetic stress. Using field observations of environmental conditions and symbiont abundance at sites on a gradient of exposure to estuarine outflow, along with fully crossed multifactorial lab experiments, we tested for changes in symbiont abundance in response to various combinations of three stressors. Lab experiments were designed to mimic short term outflow events with low salinity, high temperature, and aerial exposure treatments. The aerial exposure treatment was a statistically significant factor in suppressing symbiont repopulation (ANOVA, p=0.017). Symbiont density decreased with increasing tidal height (ANOVA, p=0.036), suggesting that aerial exposure may affect symbiont density more than sea surface temperature and salinity. Unanticipated documentation of survival in 9 months of sand burial and subsequent repopulation of symbionts is reported in comparison to results from past observations. The study of this symbiosis is useful in examining predicted changes in ocean conditions in tidepool communities.