Photosymbiosis and nutrient utilization in giant clams revealed by nitrogen isotope sclerochronology

Giant clams are reef-dwelling bivalves that reach unusual sizes through a partnership with photosymbiotic algae. To date, no shell-based biogeochemical proxy has been found which directly records the photosymbiotic development and health of these animals. We present new results showing a size-related decline in nitrogen isotopic values of shell-bound organic matter from the hinge layers of giant clams from the Northern Red Sea. In three of four tested shells, δ¹⁵N values decline from >+4‰ at the juvenile stage to between 0 and −2.5‰ at maturity. These trends are consistent with a transition from heterotrophic nutrition early in the bivalve's life to receiving most of their nutrition from photosynthetic symbionts and external dissolved inorganic nitrogen at maturity. We find more muted declines or no change within the outer shell layer, with more inter-individual variability, which is likely related to the greater influence of the symbionts in the adjacent siphonal mantle of the animals. We use a von Bertalanffy-linked trophic model that uses δ¹⁵N of nitrate and particulate organic matter to corroborate and explain the trophic transition in the ontogeny of the clams, and propose that high-resolution δ¹⁵N measurements in bivalves could be used as a proxy for photosymbiosis and reef paleoenvironmental conditions in the fossil record.