Sand Substrate Thickness Regulates Growth Performance, Intestinal Antioxidant Defense, and Gut Microbiota in an Experimental Culture of Marsupenaeus japonicus

Kuruma shrimp (<i>Marsupenaeus japonicus</i>) exhibit natural sand-burrowing behavior, but the optimal sand substrate thickness for industrial farming remains unclear. This study evaluated the effects of different sand layer thicknesses on growth performance, intestinal health, oxidative status, and gut microbiota in <i>Marsupenaeus japonicus.</i> A 120-day controlled farming experiment was conducted using four sand substrate treatments: 0 cm (no sand), 5 cm, 10 cm, and 20 cm, with three replicate ponds per treatment. Growth indices, survival rate, intestinal histology, antioxidant enzyme activity, gene expression, and gut microbial composition were analyzed. Shrimp reared without sand showed markedly reduced growth and survival, increased intestinal damage, and higher oxidative stress. In contrast, shrimp cultured with a 10 cm sand layer exhibited improved growth and survival, lower intestinal oxidative damage and cell apoptosis, and healthier intestinal structure. This condition also supported a more diverse and stable intestinal microbial community and a lower abundance of opportunistic pathogenic bacteria compared with thinner or thicker sand layers. Overall, these results indicate that a sand substrate thickness of 10 cm provides the most favorable balance between growth, intestinal health, and microbial stability, offering practical guidance for optimizing kuruma shrimp aquaculture.