NUTRITION OF THE GIANT CLAM TRIDACNA-GIGAS (L) .2. RELATIVE CONTRIBUTIONS OF FILTER-FEEDING AND THE AMMONIUM-NITROGEN ACQUIRED AND RECYCLEDBY SYMBIOTIC ALGA TOWARDS TOTAL NITROGEN REQUIREMENTS FOR TISSUE-GROWTH AND METABOLISM

We compare the relative contributions of filter-feeding and ammonium-n itrogen translocated from algal symbionts (zooxanthellae) towards the total nitrogen requirements for tissue growth and metabolism in the gi ant clam Tridacna gigas. Isotope enrichments suggest that zooxanthella e effected most if not all primary assimilation from high concentratio ns (less than or equal to 15 mu M) of N-15-ammonium. There was a net d aily uptake of ammonium from natural seawater, which was modulated acc ording to nutritional history, light history, ammonium concentration, and possible biological rhythmicity. Zooxanthellae also assimilated ni trogen excreted by host tissues, including N-15 absorbed from ingested alga. Nearly 100% of the N-15 subsequently released from zooxanthella e was incorporated in host tissues, with no significant loss from the clam over at least 10 days. Zooxanthellae therefore conserve and recyc le essentially all nitrogenous end-products within T. gigas, affording giant clams a nutritional advantage over non-symbiotic bivalves. In f urther contrast with heterotrophic bivalves from particle-rich environ ments, T. gigas show longer gut passage times and high net and gross e fficiencies (78.1 +/- 9.4% and 99.2 +/- 0.4%, respectively) with which organic nitrogen is absorbed from ingested particles. Filter-feeding and/or other non-autotrophic processes were essential as a source of n itrogen to maintain observed rates of juvenile growth, but declined in significance with increasing clam size. Even under cloudy conditions of limited irradiance, the total contribution of nitrogen acquired as ammonium by zooxanthellae exceeded the contribution from ingested part icles in T. gigas larger than about 0.1 g, increasing to at least 45% of total nitrogen requirements in clams of 10 g. Ammonium-nitrogen tra nslocated from zooxanthellae also satisfied all basal requirements for nitrogen in particle-starved clams larger than 0.1 g. Irrespective of size, ingested particles and ammonium-nitrogen acquired by zooxanthel lae together supplied about 70% of the total observed nitrogen require ments. We stress the need to determine effects of irradiance on uptake of dissolved inorganic nitrogen, and to quantify dissolved organic ma tter as a potential additional significant source of nitrogen, that mi ght further explain the high rates of growth in this clam species.