Illumination enhances the protein abundance of sarcoplasmic reticulum Ca-ATPases-like transporter in the ctenidium and whitish inner mantle of the giant clam, Tridacna squamosa, to augment exogenous Ca uptake and shell formation, respectively.

The fluted giant clam, Tridacna squamosa, can perform light-enhanced shell formation, aided by its symbiotic dinoflagellates (Symbiodinium, Cladocopium, Durusdinium), which are able to donate organic nutrients to the host. During light-enhanced shell formation, increased Ca transport from the hemolymph through the shell-facing epithelium of the inner mantle to the extrapallial fluid, where calcification occurs, is necessary. Additionally, there must be increased absorption of exogenous Ca from the surrounding seawater, across the epithelial cells of the ctenidium (gill) into the hemolymph, to supply sufficient Ca for light-enhanced shell formation. When Ca moves across these epithelial cells, the low intracellular Ca concentration must be maintained. Sarco(endo)plasmic reticulum Ca-ATPase (SERCA) regulates the intracellular Ca concentration by pumping Ca into the sarcoplasmic/endoplasmic reticulum (SR/ER) and Golgi apparatus. Indeed, the ctenidium and inner mantle of T. squamosa, expressed a homolog of SERCA (SERCA-like transporter) that consists of 3009 bp, encoding 1002 amino acids of 110.6 kDa. SERCA-like-immunolabeling was non-uniform in the cytoplasm of epithelial cells of ctenidial filaments, and that of the shell-facing epithelial cells of the inner mantle. Importantly, the protein abundance of SERCA-like increased significantly in the ctenidium and the inner mantle of T. squamosa after 12 h and 6 h, respectively, of light exposure. This would increase the capacity of pumping Ca into the endoplasmic reticulum and avert a possible surge in the cytosolic Ca concentration in epithelial cells of the ctenidial filaments during light-enhanced Ca absorption, and in cells of the shell-facing epithelium of the inner mantle during light-enhanced shell formation.