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

Daniel Killam, Shibajyoti Das, Rowan C. Martindale, Katelyn E. Gray, Adina Paytan, Christopher K. Junium

Research output: Contribution to journalArticlepeer-review

1 Scopus citations


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.

Original languageEnglish (US)
Pages (from-to)165-175
Number of pages11
JournalGeochimica et Cosmochimica Acta
StatePublished - Oct 15 2023


  • Coral reefs
  • Nano-EA-IRMS
  • Red Sea
  • Tridacna
  • δN

ASJC Scopus subject areas

  • Geochemistry and Petrology


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