Embryonic development and influence of egg density on early veliger larvae and effects of dietary microalgae on growth of brown mussel Perna perna (L. 1758) larvae under laboratory conditions

The effect of egg density on embryonic development and larval quality as well as the lipid and fatty acid contents (eicosapentaenoic acid, EPA; docosahexaenoic acid, DHA) of cultured microalgae fed to Perna perna larvae was studied under controlled conditions to provide information needed for development of an experimental hatchery. Embryonic development followed the common sequence exhibited by other bivalves. d-larva stage was attained 40-44 h post-fertilisation at 21 +/- A 1 A degrees C. The umbo-stage was reached in 11 days, and pediveliger larvae were observed 26 days post-fertilisation. Low egg density (range 20-100 eggs cm(-2)) produced high proportions of normal d-larvae. Larval development showed two growth phases: 1st-the mixotrophic stage and 2nd-the exotrophic stage where the composition of diets had significant effects on larval growth with higher rates in larvae fed with the mixed microalgae (Isochrysis galbana + Chaetoceros calcitrans, I. galbana + Phaeodactylum tricornutum and I. galbana + Skeletonema costatum) in comparison with the monospecific diet (I. galbana). Fatty acid analysis showed that larval growth and survival were strongly influenced by proportions of dietary DHA and EPA. These results indicate that DHA and EPA are the key factors in determining larval performance, considerably more than the total amount of other fatty acids.