Earthworm Powder Mitigates Soybean Meal-Induced Growth Inhibition in Rice Field Eel (Monopterus albus) by Regulating Appetite and Improving Intestinal Health

The substitution of fish meal with soybean meal (SBM) in aquafeeds aligns with sustainable development but often leads to depressed feed intake and growth in fish. This study aimed to investigate the mitigating effect of earthworm powder (EP) on these negative impacts in rice field eels (<i>Monopterus albus</i>), focusing on appetite regulation, intestinal health, and gut microbiota. Three isonitrogenous (~41% crude protein) and isolipidic (~6.4% crude lipid) diets (control [CON], high-SBM [SBM], and SBM + 2.5% EP [EP]) were tested in a 56-day trial. Juveniles (initial weight 18.00 ± 0.01 g) were stocked at 40 fish per net (0.5 m × 0.5 m× 0.5 m) and fed to visual satiety once daily. The results indicated that EP improved growth performance through a dual mechanism. Firstly, it was associated with significantly increased feed intake, correlated with the upregulated expression of orexigenic genes (<i>agrp</i>, <i>npy</i>) in the brain, and associated with reduced levels of anorexigenic hormones (Cholecystokinin, Leptin). Secondly, it correlated with enhanced intestinal health, evidenced by improved morphology (villus height, goblet cells), improved digestive enzyme activity, enhanced antioxidant capacity (increased Catalase and Superoxide Dismutase activities), repaired intestinal barrier function (upregulated <i>zo-1</i>, <i>cla-12</i>), and alleviated intestinal inflammation (downregulated <i>tnf-α</i>, <i>il-1β</i>). Furthermore, EP supplementation was associated with a shift in gut microbiota, including the suppression of the potential pathogen <i>g_Clostridium_T</i> and promotion of the beneficial bacterium <i>g_Lactococcus_A</i>, alongside increased concentrations of major short-chain fatty acids (acetate, propionate, and butyrate). These correlative observations suggest that EP may help mitigate the growth-inhibiting effects of SBM in <i>Monopterus albus</i>, offering a potential functional strategy for high-SBM aquafeeds.