Successful larval growth and development ofbivalves depend on energy derived from internal (endotrophicphase) and external (exotrophic phase) sources. Thepresent paper studies survival, growth and biochemicalchanges in the early developmental stages (from egg topediveliger) of the clam Ruditapes decussatus in order tocharacterize the nutritional requirements and the transitionfrom the endotrophic to the exotrophic phase. Three differentfeeding regimes were applied: starvation and twomono-specific microalgal diets (Isochrysis aff galbana andChaetoceros calcitrans). A comparison between fed andunfed larvae highlighted the importance of egg lipidreserves, especially neutral lipids, during a brief endotrophicphase of embryonic development (first 2 days afterfertilization). Egg reserves, however, may energeticallycontribute to the maintenance of larvae beyond theembryonic development. In fed larvae, the endotrophicphase is followed by a mixotrophic phase extending to days5–8 after fertilization and a subsequent exotrophic phase.Metamorphosis starts around day 20. The intense embryonicactivities are supported by energy derived from lipids,mainly from neutral lipids, and the metamorphic activitiesare supported by energy derived essentially from proteinsaccumulated during the planktonic phase and depend onthe nutritional value of diets. The diet of I. aff galbanaproves to be more adequate to R. decussatus larval rearing.The results provide useful information for the successfulproduction of R. decussatus aquaculture.