NITROGEN FLUX IN GIANT CLAMS - SIZE-DEPENDENCY AND RELATIONSHIP TO ZOOXANTHELLAE DENSITY AND CLAM BIOMASS IN THE UPTAKE OF DISSOLVED INORGANIC NITROGEN

Tridacnid clams live on coral reefs in the Indo-Pacific, in waters con taining low concentrations of inorganic nitrogen. This study examined nitrogen flux in the giant clam Tridacna gigas. Adults and juveniles o f this species typically occur with symbiotic dinoflagellates of the g enus Symbiodinium sp., often referred to as zooxanthellae, which live in their tissues. Intact clams took up or released dissolved inorganic nitrogen (DIN), with the direction and magnitude of the fluxes depend ent on clam size. Non-symbiotic larvae and newly-settled juveniles wit h few zooxanthellae released ammonium. Larger juveniles and adults dep leted ammonium or nitrate from seawater, when offered separately. Rate s of uptake of DIN, standardized to either clam wet weight or number o f zooxanthellae, were highest in small clams (1 to 5 cm in length), an d decreased with increasing clam size (> 5 cm). Clams maintained in se awater containing high concentrations of ammonium (ca. 20 mu M) for gr eater than or equal to 1 wk generally released ammonium in the dark an d exhibited net uptake in the light. Freshly isolated zooxanthellae (F IZ) from small clams had higher uptake rates than FIZ from larger clam s, implying that the latter may be more nitrogen-sufficient than the f ormer. The gradient of nitrogen sufficiency in giant clams is related to zooxanthellae density, with peaks of both algal density and rates o f uptake of DIN occurring in small sizes of clams, whose growth also a ppears to be limited by nitrogen availability.