Investigation of the Fermentation Process of Moringa oleifera Leaves and Its Effects on the Growth Performance, Antioxidant Capacity, and Intestinal Microbiome of Procambarus clarkii

<i>Moringa oleifera</i> is renowned for its high antioxidant activity. However, few studies have been conducted on its effects on aquatic animals. The aim of this experiment was to investigate the optimal fermentation process of <i>M. oleifera</i> leaves and to evaluate the effects of fermented <i>M. oleifera</i> leaves on crayfish (9.11 ± 0.3 g) in terms of growth performance, antioxidant capacity, and gut microbiological parameters. By optimizing the fermenting material/water ratio, fermentation time, temperature, and strain, the optimal fermentation conditions of a 10% water ratio + 48 h + 30 °C + inoculation with 2% <i>B. amyloliquefaciens</i> (10⁷ CFU mL^-1) were obtained. These conditions resulted in notable increases in the contents of the total protein, total phenols, flavonoids, and amino acids (<i>p</i> < 0.05) while also leading to a notable decrease in the content of tannins in contrast to those of unfermented <i>M. oleifera</i> leaves (<i>p</i> < 0.05). The fermented <i>M. oleifera</i> (FMO) leaves were incorporated at five concentrations, including 0% (control (CT)), 0.25% (0.25FMO), 0.5% (0.5FMO), 1% (1FMO), and 2% (2FMO). The results showed that the 1FMO group performed better in terms of the final body weight (FBW), weight gain rate (WGR), and specific weight gain rate (SGR) compared with the CT group (<i>p</i> < 0.05). In addition, amylase and lipase activities were significantly higher in the 1FMO and 2FMO groups compared with the other groups (<i>p</i> < 0.05). The fermented <i>M. oleifera</i> leaves significantly increased the catalase (CAT) activity in the crayfish (<i>p</i> < 0.05). The superoxide dismutase (SOD) activity was significantly increased in the 0.25FMO, 1FMO, and 2FMO groups, and the malondialdehyde (MDA) content was significantly decreased while the glutathione peroxidase (GSH-Px) content was significantly increased in the 0.5FMO, 1FMO, and 2FMO groups (<i>p</i> < 0.05). Furthermore, the 1FMO group was observed to significantly increase the abundance of Firmicutes while simultaneously reducing the abundance of <i>Aeromonas</i> (<i>p</i> < 0.05) and adjusting the structure of the intestinal microbiome. In conclusion, this study established the optimal fermentation conditions for <i>M. oleifera</i> and obtained a product with high nutrient and low tannin contents. Furthermore, the incorporation of 1% FMO was demonstrated to facilitate growth, enhance the antioxidant capacity, and optimize the gut microbiology in crayfish.