Effect of nutrition on Crassostrea gigas larval development and the evolution of physiological indices. Part A: Quantitative and qualitative diet effects

Several quality indices have been defined for marine bivalve larvae, including the coloration of whole larvae and the Triacylglycerols/Sterols ratio (TAG/ST), putting emphasis on the role of lipids in larval development. Using an image analysis system, epifluorescence microscopy and Nile Red, to specifically colour the reserves, we improved and quantified the overall physiological index based on the ratio of the lipid surface to the total larval surface. This Overall Lipid Index (OLI) was then tested in two experiments on the influence of nutritional parameters on larval development and metamorphosis in Crassostrea gigas, following the parallel evolution of the indices. The higher the input of microalgae, the quicker larvae developed, leading to a maximum development rate of 16.5 µm d− 1. This rate was accompanied by the highest lipid reserves. At the end of rearing, the lipid reserves occupied 30% of the larval surface in the batches with the highest food ration, while they only covered 12% in the larvae on the lowest ration. In the same way, the TAG/ST ratio increased over the rearing period and reached its highest levels in larvae with the highest food ration. We also noted that the decrease in lipids over the first week of rearing was higher when the nutritional input was lower. Finally, deficient reserves and poor performances were recorded in larvae fed only the flagellates Pavlova lutheri (P), Isochrysis affinis galbana (T) or a mixture of these species (PT). In contrast, a high lipid accumulation and better performances were shown when the diet was based on the diatom C. calcitrans f. pumilum (Cp), whether alone or in combination with other species.