The intertidal sea anemone Anthopleura xanthogrammica is distributed widely along the Pacific coast, from Baja California Mexico (~30ºN) to southern Alaska (57ºN). In much of its range, A. xanthogrammica has the ability to co-host algal symbionts from two distinct taxa known as zoochlorellae (the chlorophyte Elliptochloris marina) and zooxanthellae (brown dinophytes in the genus Symbiodinium). Laboratory studies and field distributions have demonstrated that zoochlorellae and zooxanthellae represent "cool" and "warm" symbionts respectively, based on their...