Algal/invertebratee ndosymbioseas re common in the marinee nvironmenta nd appeart o be nutritional in nature. The giant Tridacnid clams form a mutualistic extracellular endosymbiosis with dinoflagellate algae (zooxanthellae) of the species Symbiodiniunt microadriaticum. The large size of the Tridacnids is generally attributed to the nutritional role of their endosymbionts. This thesis examines the nature and mechanisms by which important biochemicals, such as lipids, are translocated from the symbionts to the host. Methodologies have been developed to enable the determination of the carbon isotope composition of individual saturated and polyunsaturated fatty acids to enable natural abundance isotopic variations in these compounds to be investigated in the Tridacnid-algal symbiosis. Compound specific isotope analysis of fatty acids in different species of Tridacnids, their zooxanthellae and non-symbiotic species reveal that several fatty acids are apparently directly translocated from algae to host. Evidence is presented for the synthesis of fatty acids from acetate by the clam. Carbon isotopic data also reveal that carbon limitation may play a role in lipid metabolsim in giant clams. Compound specific isotope analysis has also been used to investigate the contribution of translocated compounds during a diel cycle in clam haemolymph and reveals that concentrations of key metabolites vary over the diel cycle as a function of irradiance. The cyclic sugar alcohol scyllo-inositol was detected in clam haemolymph, which may be related to osmoregulation in the clam or to a signalling role in cell proliferation. Oxygen and carbon stable isotope compositions and strontium abundances of shell carbonate are examined in several clams subjected to varying degrees of phosphate and ammonium nutrient supplementation to investigate the relationship between growth rate and stable isotope compositions and strontium contents.