Light-dependent calcification in Red Sea giant clam <i>Tridacna maxima</i>

Abstract. Tropical giant clams of the Tridacninae family, including the species Tridacna maxima, are unique among bivalves as they live in a symbiotic relationship with unicellular algae and generally function as net photoautotrophic. Light is therefore crucial for these species to thrive. Here we examine the light-dependency of calcification rates of T. maxima in the Central Red Sea as well as the patterns of its abundance with depth in the field. Red Sea T. maxima show highest densities in a depth of 3 m with 0.82 ± 0.21 and 0.11 ± 0.03 individuals m−2 (mean ± SE) at sheltered and exposed sites, respectively. Experimental assessment of net calcification (μmol CaCO3 cm−2 h−1) and gross primary production (μmol O2 cm−2 h−1) under seven light levels (1061, 959, 561, 530, 358, 244 and 197 μmol quanta m−2 s−1) showed net calcification rates to be significantly enhanced under light intensities corresponding to a water depth of 4 m (0.65 ± 0.03 μmol CaCO3 cm−2 h−1; mean ± SE), while gross primary production was 2.06 ± 0.24 μmol O2 cm−2 h−1 (mean ± SE). We found a quadratic relationship between net calcification and tissue dry-mass (DM in gram), with clams of an intermediate size (about 15 g DM), showing the highest calcification. Our results show that the Red Sea giant clam T. maxima stands out among bivalves as a remarkable calcifier, displaying calcification rates comparable to other tropical photosymbiotic reef organism, such as corals.