Dietary black soldier fly meal remodels hepatic lipid metabolism and fillet fatty acids profile of flathead grey mullet Mugil cephalus (Linnaeus, 1758)

The effect of dietary black soldier fly (Hermetia illucens) meal on flathead grey mullet (Mugil cephalus) liver transcriptomic response and fillet quality was assessed. To this end, 360 juvenile flathead grey mullets (weight = 40.2 ± 0.5 g) were randomly divided into four experimental groups. Fish were fed a reference diet (BSF0) and three experimental diets in which graded levels of partially defatted black soldier fly meal were used as protein source (BSF10, BSF15, and BSF20). A dose-dependent transcriptional response to insect meal inclusion was evident on the modulation of differentially expressed genes (DEGs). While BSF10 did not result in significant transcriptomic changes, functional analyses of the hepatic transcriptome highlighted strong impacts at metabolic, endocrine, and immune levels at 20% BSF inclusion. A metabolic switch from glycolytic flux toward oxidative fatty acid catabolism, as a consequence of dietary BSF meal, was found, altogether indicating a remodeling of the hepatic circadian–metabolic axis. Moreover, the consistent upregulation of the pathways involved in the control of protein folding and oxidative phosphorylation suggested an attempt to maintain cellular homeostasis, particularly in the BSF20 group. Chemical analyses revealed a comparable protein and lipid content of the final edible product across the groups, indicating that up to 20% dietary protein replacement with BSF is nutritionally sustainable. However, lauric acid (12:0) represented the most indicative FA in fillet of fish fed BSF-based diets, and an inverse proportion of palmitic acid (16:0) content between the diet and fillet composition was observed in fish fed experimental diets. The increase in n3 FAs, including docosahexaenoic acid (22:6n3, DHA) in fillets from the BSF20 group, indicates metabolic adaptations in response to a dietary decrease of such FAs. Together, the results obtained herein indicate that M. cephalus can accommodate partial replacement of dietary conventional protein sources with BSF meal, with no adverse effects on the quality of the final edible product. However, longer-term feeding trials are needed to assess a long-term use of BSF-based diets on cellular metabolic imbalances, as finely unveiled by the transcriptomic shift induced by 20% replacement.