The presence of zooxanthellae in tissues of the cold-temperate water coral Plesiastrea urvillei (Milne Edwards and Haime) has been confirmed histologically. Numbers of zooxanthellae per unit surface area and increases in submerged wet weight as a measure of calcification have been followed for 150 days under four different conditions: light-fed, light-starved, dark-fed, and dark-starved. No significant difference was found in density of zooxanthellae or calcification rates between light-fed and light-starved, and between dark-fed and dark-starved. After Day 48 the calcification rate in the dark dropped, however, by a factor of ≈4 to a constant lower rate and was correlated with a decrease in density of zooxanthellae. Zooxanthellae thus enhance calcification about 4 times during photosynthesis. Measurements of oxygen consumption and production indicated that even at the low light intensities experienced on a cloudy winter day by the coral in its natural environment more energy was fixed during photosynthesis than was required by the host. The retention of zooxanthellae and continued calcification in the dark for upwards of 48 days is considered to be an adaptation to the lower light levels experienced by P. urvillei compared with tropical corals. © 1979.
