AMMONIUM CONTRIBUTION FROM BARING BIVALVES TO THEIR CORAL HOST - A MUTUALISTIC SYMBIOSIS

The mytilid bivalve Lithophaga simplex is found to inhabit the sclerac tinian coral Astreopora myriophthaima in high densities. This boring b ivalve, living inside the CaCO3 skeleton of the coral, produces consid erable amounts of ammonium as a nitrogenous waste product. Ammonium pr oduction rate by the bivalves and consumption rate by the coral (via t he symbiotic algae) were measured in laboratory experiments. The popul ation density of L. simplex bivalves in A, myriophthalma corals was su rveyed in he Nature Reserve Reef, Eilat, Red Sea, Israel. Ammonium pro duction rate by the bivalves, inhabiting the coral at a density of 0.2 2 +/- 0.11 bivalves cm(-2), is calculated to be 8.2 +/- 3.8 and 3.5 +/ - 1.6 nmol (cm(2) coral)(-1) h(-1) during daytime and nighttime, respe ctively. Under conditions of low ammonium concentration (0.2 to 1.2 mu M) the consumption rate of the coral ranged between 5 and 22 nmol cm( -2) h(-1). Thus, under naturally occurring levels of ammonium (<0.15 m u M), recycling of nitrogenous waste produced by the bivalves (ammoniu m) may account for a significant portion of the needs of the coral/zoo xanthellae. In contrast to the generally accepted view of boring bival ves as parasites of their coral hosts, it is hypothesized that the ass ociation between L, simplex and A. myriophthalma may also be an exampl e of mutualistic symbiosis. The results indicate a possible pathway in the biogeochemical cycle of nitrogen in the coral reef environment.