Illumination enhances the protein abundance of sarcoplasmic reticulum Ca2+-ATPases-like transporter in the ctenidium and whitish inner mantle of the giant clam, Tridacna squamosa, to augment exogenous Ca2+ uptake and shell formation, respectively

Abstract

The fluted giant clam, Tridacna squamosa, can perform light-enhanced shell formation, aided by its symbiotic dinoflagellates <i>(Symbiodinium, Cladocopium, Durusdinium)</i>, which are able to donate organic nutrients to the host. During light-enhanced shell formation, increased Ca²⁺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 Ca²⁺from the surrounding seawater, across the epithelial cells of the ctenidium (gill) into the hemolymph, to supply sufficient Ca²⁺for light-enhanced shell formation. When Ca²⁺moves across these epithelial cells, the low intracellular Ca²⁺concentration must be maintained. <i>Sarco</i> (endo)plasmic reticulum Ca²⁺-ATPase (SERCA) regulates the intracellular Ca²⁺concentration by pumping Ca²⁺into the sarcoplasmic/endoplasmic reticulum (SR/ER) and Golgi apparatus. Indeed, the ctenidium and inner mantle of <i>T. squamosa</i>, expressed a homolog of <i>SERCA (SERCA</i>-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 <i>T. squamosa</i> after 12 h and 6 h, respectively, of light exposure. This would increase the capacity of pumping Ca²⁺into the endoplasmic reticulum and avert a possible surge in the cytosolic Ca²⁺concentration in epithelial cells of the ctenidial filaments during light-enhanced Ca²⁺absorption, and in cells of the shell-facing epithelium of the inner mantle during light-enhanced shell formation.