Ontogenetic variations in lipid class and fatty acid composition of haddock larvae Melanogrammus aeglefinus in relation to changes in diet and microbial environment

Abstract

This study examined the ontogenetic variations in lipid class composition and fatty acid partitioning between neutral and polar lipids of haddock ( Melanogrammus aeglefinus) larvae in relation to changes in diet and microbial environment. Three groups of larvae were exposed to the candidate probiotic bacterium Arthrobacter sp. (RSXII) and three equivalent control groups were left unexposed. Larvae were fed rotifers between 2 and 31 days post hatch (dph), which were gradually replaced by Artemia until 45 dph and progressively weaned from live feed to a dry diet between 45 and 56 dph. Overall, the larvae grew exponentially during the course of the experiment as reflected by rates of increase in dry mass and accumulation of lipid. Proportions of arachidonic acid (20:4 n-6, ARA) in haddock larvae were 3–7× higher than observed in the diets, suggesting the selective incorporation of this fatty acid in both lipid reserves and membranes. The initial proportion of docosahexaenoic acid (22:6 n-3, DHA) in haddock larvae was 4.6× higher than the dietary values. However, DHA in the neutral lipids decreased markedly during larval ontogeny to proportions that reflected the dietary values. Regarding polar lipids, the proportions of DHA were markedly less variable than those recorded for neutral lipids, which supports the notion that phospholipids in fish are generally higher in polyunsaturated fatty acids (PUFA) and more resistant to changes in diet than are neutral lipids. During the transition from a diet of rotifer to a diet of Artemia, haddock larvae showed an arrested accumulation of triacylglycerols and a marked increased in ARA, presumably attributable to stimulation of immune functions due to an increase in the bacterial load in the rearing system. Finally, larvae exposed to RSXII showed a superior rate of increase in dry mass and lipid accumulation compared to control groups, suggesting that RSXII enhanced the general health of haddock larvae by modifying their bacterial microflora.