Larval biology of tridacnid clams

Previous attempts at the mariculture of tridacnid bivalves have emphasized parameters such as growth rates of metamorphosed spat and adults. Many of these studies agree that larval mortality in these clams is high, but there have been few attempts made to define culture conditions that would enhance larval survival. We have attempted to address aspects of the larval biology of Hippopus hippopus and Tridacna gigas with an aim toward improving larval survival. We emphasize the effects of larval density and exogenous food (dissolved and particulate) supply on survival and growth of tridacnid larvae. The brief trochophore stage showed high mortality, but optimal survival was found at densities between 0.2 and 5 larvae/ml. Survival and growth rate of veligers were found to be optimal at densities of 0.2-10 larvae/ml. Control (unfed) veligers demonstrated high growth rates during the first half of the veliger stage, after which growth rates declined to zero, and mortality increased markedly. In contrast, veligers provided with particulate food (Isochrysis galbana, Tahitian strain) had significantly higher growth rates (p < .05) and lower mortality than the controls. Phaeodactylum tricornatum is not an effective food for tridacnid larvae. Optimal growth and survival of veligers fed I. galbana occurred with algal densities of 10⁴-10⁵ cells/ml. Dissolved organic nutrients in the form of vitamins and yeast extract significantly enhanced veliger growth and survival. Although all adult tridacnids possess symbiotic algae which contribute significantly to their nutrition, these algae are not passed from one clam generation to another. Under experimental conditions, veligers take all strains of Symbiodinium microadriaticum provided into their stomachs, but symbiosis is not established until after metamorphosis. A larval history of adequate nutrition may well enhance veliger growth, shorten the developmental time to metamorphosis, and improve growth after the establishment of symbiosis.