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

## TL;DR

Earthworm powder helps rice field eels eat more and grow better when soybean meal replaces fish meal in their diet.

## Contribution

Earthworm powder is shown to improve appetite and intestinal health in rice field eels fed soybean meal.

## Key findings

- Earthworm powder increased feed intake by regulating brain appetite-related genes and hormones.
- It improved intestinal health by enhancing gut morphology, enzyme activity, and antioxidant capacity.
- Earthworm powder shifted gut microbiota toward beneficial bacteria and increased short-chain fatty acids.

## Abstract

The substitution of fish meal (FM) with soybean meal (SBM) is a sustainable strategy for the aquaculture industry, but it often leads to reduced feed intake and poor growth in farmed fish. This study aimed to evaluate whether adding earthworm powder (EP) to the diet could mitigate these negative impacts in rice field eels (Monopterus albus). Our findings indicate that while high levels of SBM suppressed appetite and damaged the gut, the inclusion of 2.5% EP effectively reversed these effects. EP was found to stimulate eels’ feeding behavior by regulating brain signals and hormones, while simultaneously repairing intestinal tissue, strengthening the gut barrier, and balancing the community of gut bacteria. These results suggest that EP is a promising, natural feed additive that can support the use of SBM proteins in aquafeeds, thereby promoting the health and sustainable farming of Monopterus albus.

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 (Monopterus albus), 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 (agrp, npy) 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 zo-1, cla-12), and alleviated intestinal inflammation (downregulated tnf-α, il-1β). Furthermore, EP supplementation was associated with a shift in gut microbiota, including the suppression of the potential pathogen g_Clostridium_T and promotion of the beneficial bacterium g_Lactococcus_A, 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 Monopterus albus, offering a potential functional strategy for high-SBM aquafeeds.