Divergent Responses of Cyprinus carpio to Hermetia illucens and Musca domestica Larval Oils: A Matter of Source and Dose in Growth Performance, Lipid Metabolism and Gut Microbiome

The search for sustainable alternatives to fish oil (FO) in aquafeeds is imperative for the long-term sustainability of aquaculture. This study evaluated the effects of partially replacing FO with oils derived from Hermetia illucens (RH) and Musca domestica (RY) larvae at 10%, 30%, and 50% levels on growth, lipid metabolism, hepatic antioxidant capacity, and gut microbiota in juvenile Cyprinus carpio. Results showed distinct source- and dose-dependent responses. RY produced a consistent dose-dependent trend in growth parameters, whereas RH exhibited a biphasic pattern, with growth impairment at 30% replacement and recovery at 50%. Both oils significantly suppressed hepatic fatty acid synthase (FAS) activity, yet this suppression was accompanied by dose-dependent up-regulation of fas expression, particularly at high RH and RY levels, indicating complex post-transcriptional regulation. RH at higher replacement levels reduced digestive enzyme activities and antioxidant capacity, while higher RY caused minimal disruption. Both oils reshaped gut microbiota toward oil-specific community structures. Two-way ANOVA revealed significant synergistic interactions between oil source and replacement level for most serum biochemical and hepatic antioxidant parameters (ηp2 > 0.90). These findings demonstrate that RY is a tolerable alternative across a wide replacement range, though all RY groups exhibited lower growth than the control, 30% is identified as the maximum tolerable level; whereas RH requires precise dosing at 10% to avoid the 30% impairment threshold.