Symbiodiniaceae-Derived Fatty Acids Are Stored Differentially Across Giant Clam Species and Organs

Abstract

Giant clams are invertebrates that form mutualistic associations with Symbiodiniaceae dinoflagellates. Despite their ecological significance, gaps persist regarding our understanding of their trophic ecology. Specifically, it is unknown whether Symbiodiniaceae-derived photosynthates are metabolized differently according to species and organ. Therefore, we maintained Tridacna derasa and T. noae for 3 months in a well-lit recirculated system without food supply. Samples were taken from eight organs and underwent lipid extraction and fatty acid esterification before analysis of three symbiont-derived fatty acids (stearidonic acid, docosapentaenoic acid, and docosahexaenoic acid—SDA, DPA, and DHA, respectively) using gas chromatography. Results show considerable variation in fatty acids concentration among species and organs. SDA was found in higher concentrations in T. noae, especially in the adductor muscle. DPA was detected in low concentrations across T. noae organs and absent for T. derasa. DHA did not vary significantly among species and organs. Our findings indicate that Symbiodiniaceae supply clams with fatty acids, which are stored differentially according to species and organs. This demonstrates that these compounds are translocated to multiple organs throughout the complex giant clam anatomical system, in contrast to simpler hosts like corals. These results advance our understanding of the physiological dynamics of the mollusk-algae association.