Diurnal polyp expansion behavior in stony corals may enhance carbon availability for symbionts photosynthesis

Photosynthesis of zooxanthellate stony corals may be limited by inorganic carbon at high irradiances. We demonstrated that oxygen consumption of expanded corals is higher than that of contracted corals in both night-expanding and day-expanding corals. It is assumed that at the single-polyp level, the expansion of tentacles increases the surface area for solute exchange with the surrounding water, which may alleviate potential carbon limitation and excess oxygen levels in the tissue under high irradiance. We investigated this hypothesis using stable carbon isotope (δ13C) analysis of coral species from the Red Sea exhibiting different morphologies. δ13C ratios in zooxanthellae of branched coral colonies with small polyp size that extend their tentacles during daytime (diurnal morphs) showed lower δ13C values in their zooxanthellae − 13.83 ± 1.45 ‰, compared to corals from the same depth with large polyps, which are usually massive and expand their tentacles only at night (nocturnal morphs). Their algae δ13C was significantly higher, averaging − 11.33 ± 0.59‰. Carbon isotope budget of the coral tissue suggests that branched corals are more autotrophic, i.e., that they depend on their symbionts for nutrition compared to massive species, which are more heterotrophic and depend on plankton predation.