Molecular aspects of lipid metabolism, digestibility and antioxidant status of Atlantic bluefin tuna (T. thynnus L.) larvae during first feeding

Atlantic bluefin tuna (Thunnus thynnus L.; ABT) larvae were fed on enriched rotifers Brachionus rotundiformis and copepod nauplii Acartia tonsa from first feeding to 15 days post hatching. Rotifers were enriched with five different commercial products: OG, MG, AG and RA plus selenium and vitamin E. Copepods (COP) were cultured with the algae Rhodomonas baltica. Metabolic processes were studied by determining the expression of 30 genes related to lipid metabolism (transcription factors, fatty acid metabolism and lipid homeostasis), antioxidant enzymes, myogenesis and digestive enzymes. Growth and development parameters and high expression of myogenesis genes myhc2 and tropo indicated that COP were better than enriched rotifers as live prey for first feeding ABT. COP and AG-fed larvae showed the lowest values for the transcription factors pparγ and srebp2. The expression of fas showed differences among treatments, with highest relative expression in COP-fed larvae and those fed with RA rotifers. In relation to fatty acid catabolism, larvae fed RA had the highest aco expression levels, with the lowest observed in those fed COP. The expression profiles of lipid homeostasis genes showed that larvae fed COP had higher fabp2 and 4 expressions. Larvae fed AG showed the lowest lpl expression levels, with highest values observed in larvae fed OG. Regarding antioxidant enzyme gene expression, sod showed highest values in larvae fed COP and RA, with larvae fed MG rotifers showing lowest expression levels. A similar pattern was observed for the expression of cat and gpx1 and 4. The expression of genes for digestive enzymes showed that tryp expression levels were highest in COP-fed larvae but, in contrast, COP-fed larvae showed the lowest anpep and alp levels. ABT larvae fed AG displayed the lowest expression level of pla2. bal1 and bal2 presented similar expression patterns, with highest values in COP-fed ABT and lowest expression in larvae fed AG rotifers. Copepods were a superior live prey for first feeding ABT larvae compared to enriched rotifers, as indicated by the higher growth and flexion index achieved by COP-fed larvae, possibly reflecting the higher protein content of the copepods.