The effect of zooxanthellae availability on the rates of skeletal growth in the Red Sea coral Acropora hemprichii

Zooxanthellae density affects growth rate of Acropora hemprichii at reef flat and 10 m depth, where the correlations were significantly moderate at reef flat (r = 0.461 & P < 0.01) and significantly high at 10 m depth (r = 0.636 & P = 0.424). While non interactive effects were obtained at 20 and 25 m depth, where the correlations were non significant (r = 0.346 & P < 0.19 and r = 0.103 & P < 0.706, respectively). Either zooxanthellae density, hosted by A. hemprichii, or growth rate was decreased with depth increase. Zooxanthellae density at reef flat (1.55 ± 0.303 × 106 cells/cm2) was twice higher than at 25 m depth (0.706 ± 0.253 × 106 cells/cm2). However, growth rate at reef flat was approximately three times higher than 25 m depth (0.013 ± 0.0024 mm/day). The maximum growth rate (0.0335 mm/day) and zooxanthellae density (1.32 × 106 cells/cm2) were recorded during summer season, and the minimum growth rate (0.01769 mm/day) and zooxanthellae density (0.931106 cells/cm2) were recorded during autumn.