Recent advances in lipid nutrition in fish larvae

Due to the importance of dietary lipid utilization for larval rearing success, increasing attention has been paid during the last years to different aspects of larval lipid nutrition such as digestion, absorption, transport and metabolism, which are frequently studied by different research groups. The present study reviews the published information on these aspects, including some recent results obtained in our laboratory, that contribute to a better understanding of larval lipid nutrition. Neutral lipase activity was found in the digesta of larval gilthead seabream as early as first feeding, followed by a significant increase which reached up 8 times the initial levels at day 15 and was clearly influenced by the fatty acid composition of dietary lipids. Accordingly, the capacity for lipid absorption by the intestinal epithelium has been also observed at the onset of exogenous feeding, although the specific location in the different digestive tract segments differ with species. Whereas the capacity to absorb lipid increases with development in live prey-fed larvae, this improvemment is delayed in larvae fed formulated diet. Increasing dietary phosphatidyl cholines levels enhanced lipid absorption regardless of whether it is of soybean or marine origin, but the latter improved hepatic lipid utilization. Enzymatic, histological and biochemical evidences suggest that marine fish larvae are able to effectively digest and absorb n-3 HUFA-rich triacylglycerols, but feeding with phosphoacylglycerols, particularly if they are rich in n-3 HUFA, would enhance phosphoacylglycerols digestion and specially lipid transport alowing a better n-3 HUFA incorporation into larval membrane lipids and promoting fish growth. Although the essentiality of n-3 HUFA for larval marine fish has been studied extensively, only recently has the importance of dietary arachidonic acid in the larvae of few species been recognised. Evidences for competitive interactions among these essential fatty acids suggest that besides a minimum dietary requirement for each essential fatty acid, their relative ratios must also be considered.