Effects of Decreased Salinity on Expulsion of Zooxanthellae in the Symbiotic Sea Anemone Anthopleura elegantissima

Abstract

Many natural conditions cause expulsion of zooxanthellae from corals and sea anemones. Recent studies have focused on causes and mechanisms of this release. We examined an incidence of bleaching in a field population of the sea anemone Anthopleura elegantissima (Brandt). Our data suggest that expulsion of zooxanthellae was caused by reduced salinity from freshwater runoff after heavy rainfall. In the laboratory, A. elegantissima expelled zooxanthellae in quantities directly correlated with strength and duration of exposure to hyposalinity. The mechanism of release appears to be rupture of the host cell, followed by accumulation of clumps of zooxanthellae that are then expelled from the coelenteron. A. elegantissima has little or no ability to osmoregulate the water in its coelenteron, and internal salinity drops rapidly with external salinity reduction.