Abstract: This study aimed to evaluate the effects of Ca²⁺ concentration on physiological response of three hermatypic corals, including Seriatopora caliendrum, Galaxea fascicularis and Montipora digitata. The calcification rate, maximum quantum yield (F_v/F_m), and effective quantum yield (Φ_PSⅡ) were measured at six Ca²⁺ concentrations (180, 250, 320, 390, 460 and 530 mg·L^-1). The results showed that the calcification rate of three corals significantly reduced at low Ca²⁺ concentrations (180 mg·L^-1). With increasing Ca²⁺ concentration, the calcification rates of S. caliendrum and M. digitata displayed inverted parabolic growth and peaked at 320-390 mg·L^-1, while the calcification rate of G. fascicularis increased with increasing Ca²⁺ concentration at low Ca²⁺ concentration (180-320 mg·L^-1) and then did not change afterwards. Fourteen days after treatment initiation, the F_v/F_m and Φ_PSⅡ of S. caliendrum associated zooxanthellae showed a trend of first increasing and then decreasing with increasing Ca²⁺ concentration, probably because the corals suffered from a certain stress at too low or too high Ca²⁺ concentration. G. fascicularis associated zooxanthellae displayed no significant difference in F_v/F_m but a significant increase in Φ_PSⅡ at fourteen days after treatment initiation compared to that at day one of treatment initiation. These results indicate speciesspecific physiological response of corals to different Ca²⁺ concentrations. Therefore, it is necessary to pay attention to and adjust Ca²⁺ concentration in closed circulating aquacultural systems for hermatypic corals to maintain the optimum growth.

Key words: hermatypic coral, calcium ion, calcification rate, maximum quantum yield, effective quantum yield.