δ13C as a Continuum: Tissue-Based, Ontogenetic, and Interspecific Variation in Carbon Sourcing of a Photosymbiotic Bivalve (Subfamily Fraginae).

Photosymbiotic marine invertebrates derive carbon from both symbiont photosynthesis and heterotrophic feeding, yet the relative contribution of each source is often inferred from bulk tissue δ13C values without accounting for intertissue and intraspecific variations. Here, we measured δ13C in three tissues (mantle, gill, and foot) of the photosymbiotic bivalve Fragum unedo (subfamily Fraginae) across a range of body sizes from Gathaagudu (Shark Bay), Western Australia, and compiled published δ13C data from marine mollusks and cnidarians spanning photosymbiotic, chemosymbiotic, and non-symbiotic nutritional modes. Within F. unedo, δ13C differed consistently among tissues: the symbiont-free foot was enriched by approximately 1.1‰ relative to the mantle and 1.6‰ relative to the gill, which is consistent with bulk mixing between host and symbiont biomass in symbiont-bearing tissue, and also may be due to post-photosynthetic fractionation during metabolite translocation from symbiont-bearing source tissues to heterotrophic sink tissues. δ13C values also increased with body size, with a total ontogenetic shift of approximately 3‰ between the smallest and largest individuals, suggesting a progressive increase in reliance on symbiont-derived carbon through growth. Both tissue and size effects are comparable in magnitude to the differences commonly used to distinguish nutritional modes among taxa, indicating that tissue selection and developmental stage can directly alter carbon source assignments. The compiled dataset reveals that chemosymbiotic, photosymbiotic, and non-symbiotic taxa do not occupy discrete isotopic categories but instead form a continuous δ13C gradient with substantial overlap between nutritional modes. The values for F. unedo fall within the photosymbiotic range but near its lower boundary, consistent with a mixed nutritional strategy. These results demonstrate that δ13C is a valuable tracer of carbon sourcing in symbiotic systems but should be interpreted as a continuous variable shaped by tissue identity, ontogeny, and environmental context rather than as a categorical marker of symbiotic state.