Nitrogen flux in giant clams: size-dependency and relationship to zooxanthellae density and clam biomass in the uptake of dissolved inorganic nitrogen

Abstract

Tridacnid clams live on coral reefs in the Indo-Pacific, in waters containing low concentrations of inorganic nitrogen. This study examined nitrogen flux in the giant clam Tridacna gigas. Adults and juveniles of this species typically occur with symbiotic dinoflagellates of the genus 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 dependent on clam size. Non-symbiotic larvae and newly-settled juveniles with few zooxanthellae released ammonium. Larger juveniles and adults depleted ammonium or nitrate from seawater, when offered separately. Rates of uptake of DIN, standardized to either clam wet weight or number of zooxanthellae, were highest in small clams (1 to 5 cm in length), and decreased with increasing clam size (>5 cm). Clams maintained in seawater containing high concentrations of ammonium (ca. 20 μM) for ≥ 1 wk generally released ammonium in the dark and exhibited net uptake in the light. Freshly isolated zooxanthellae (FIZ) from small clams had higher uptake rates than FIZ from larger clams, implying that the latter may be more nitrogen-sufficient than the former. The gradient of nitrogen sufficiency in giant clams is related to zooxanthellae density, with peaks of both algal density and rates of uptake of DIN occurring in small sizes of clams, whose growth also appears to be limited by nitrogen availability.