- The influence of the elevation of seawater temperature on the relationship between growth and viability of corals and various physiological parameters of their symbiotic zooxanthellar populations was examined for two hermatypic species, the scleractinian coral Stylophora pistillata (Esper) and the hydrocoral Millepora dichotoma (Forskål). Elucidation of this linkage between seawater temperature and the physiological status of these reef-building organisms was facilitated by a combination of our unique, high-resolution, computer-controlled set up and direct monitoring techniques. At 34°C, zooxanthellar densities in M. dichotoma colonies decreased significantly; whereas, those in S. pistillata colonies increased. These different responses of the two species are probably an expression of their different tolerance mechanisms to elevated temperatures. For both species, chlorophyll a concentrations increased at 34°C, while those of chlorophyll c decreased. Elevation of seawater temperature above 26°C caused decrease in calcification rates. Our study indicates that 31.1°C is the critical point for the zooxanthellae-host association, and further temperature elevation may bring the animal host into the lethal range of survival.