The effects of temperature and light on two algal populations in the temperate sea anemone Anthopleura elegantissima (Brandt, 1835)

The effects of temperature (13 degrees C and 20 degrees C) and irradiance (low light (LL) = 10 mu mol/m(2)/s; high light (HL) = 137 mu mol/m(2)/s) on the population density of two symbiotic algae in the tentacle region of the intertidal sea anemone Anthopleura elegantissima (Brandt) were examined in this study. Anemones contained zooxanthellae (brown anemones), zoochlorellae (green anemones), or a mixture of both algae (mixed anemones). Temperature and light have different effects on zooxanthellae and zoochlorellae, resulting in changes in the algal complement of A. elegantissima. High temperature caused a significant decrease in the density of zoochlorellae over 25 days (in green anemones: - 32 and - 20 algae/mu g tentacle protein per day under HL and LL, respectively) and a reduction in algal mitotic index (from 15% to <5% cells dividing after 6 days). Low light caused a significant increase in the density of zooxanthellae over 25 days (in brown anemones: +15 and +20 algae/mu g tentacle protein per day at 13 degrees C and 20 degrees C, respectively). Overall, temperature had a greater effect than light on the population density of algae in the tentacles of A. elegantissima; the percent change in zoochlorellae density in tentacles of anemones held at 20 degrees C was significantly less (in green anemones, -69%) than changes obtained with all other symbiont-temperature interactions. Zooxanthellae and zoochlorellae in mixed anemone tentacles showed the same population density response to the treatments as zooxanthellae and zoochlorellae in brown and green anemones, respectively. Population densities of both algal symbionts are regulated by temperature and light, and the relative abundance of each alga in a host anemone may be shifted with changes in these environmental factors. (C) 1997 Elsevier Science B.V.