Ontogenèse des fonctions digestives et besoins nutritionnels chez les larves de poissons marins

The production of fish larvae by hatchery shows an intensive increase for several species, but the specific nutritional requirements of larvae were poorly known until some years. We have conducted several experiments aimed to understand the specificities in larvae digestion, and to formulate a compound diet taking into account the differences compared to juveniles. The chosen species was the European sea bass, Dicentrarchus labrax, and the data were validated on different species. Larvae were fed live preys or experimental compound diet, especially formulated for the experiment. It appeared that pancreatic digestive enzymes, such as trypsin and amylase could be assayed in early ontogenesis, before mouth opening, and increase dramatically during the first days of life. The intestinal digestion goes through a maturation process: the cytosolic enzymes, mainly peptidases, decrease during the first days, when concurrently the brush border membrane enzymes, such as alkaline phosphatase, increase. The activity pattern during development is genetically determinated, but the enzyme expression can be modulated by diet composition. The first experiments were aimed to determine the level of protein nutritional requirement during larval development. The best growth and survival were obtained with diet incorporating 50 and 60% protein / diet dry matter. Nevertheless, a poor regulation of the enzyme hydrolyzing protein, trypsin, was evidenced in young stages. Following experiments showed that dietary peptides induce better digestive enzyme regulation than native proteins: incorporation of peptides in diet fed to larvae induced a proportional enhancement of peptidases. This higher activity was associated to better growth and survival, suggesting a specific requirement in larval stages for peptides. Phospholipids appeared to be another specific larval requirement in larvae. Indeed, in sea bass larvae phospholipase A2 was largely regulated by phospholipid concentration in the diet. This regulation occurs at transcriptional level, as shown by enzyme activity and quantity of ARNm coding for phospholipase. Besides, larval development, assayed by growth, survival and skeletal quality, was directly related to dietary phospholipid amount. This suggests that the high regulation of phospholipase is associated to a specific dietary phospholipid requirement in larvae. The study of the effect of the different nutrients on digestive enzyme expression allowed the formulation of a diet especially adapted to larval requirements.