PHOTOBIOLOGY OF THE SYMBIOTIC SEA ANEMONE ANTHOPLEURA ELEGANTISSIMA: PHOTOSYNTHESIS, RESPIRATION, AND BEHAVIOR UNDER INTERTIDAL CONDITIONS

¹ Department of Zoology, University of Maine, Orono, Maine 04469

Continually immersed specimens of Anthopleura elegantissima freshly collected from the shore receive at least 34—42% of their respiratory carbon requirement from their endosymbiotic zooxanthellae. This value decreases to 17% when intertidal sea anemones are exposed to air for 15 h during daytime spring low tides, primarily owing to a reduction in photosynthesis by the algae, which are shaded when the anemone retracts its tentacles and contracts its marginal sphincter. Contraction of continually immersed anemones during peak irradiance likewise decreases their photosynthesis. This behavior is primarily in response to high levels of intracellular oxygen generated by the symbiotic algae, although ultraviolet radiation also causes contraction, especially in aposymbiotic individuals which have lower concentrations of UV-B absorbing substances than do symbiotic anemones. Although A. elegantissima may vary its photosynthetic capacity according to long-term average light conditions, behavioral adaptations such as contraction and attachment of debris to the column verrucae protect the animal and its algal symbionts from deleterious photodynamic effects during short-term exposure to peak levels of irradiance.

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