Schizochytrium Supplementation in Compound Feed: Effects on Growth, Metamorphosis, Intermediate Metabolism, and Intestinal Health of Bullfrogs (Lithobates catesbeianus)

Abstract

Schizochytrium is often added to feed to enhance the growth and health of farmed animals, yet research on its effects on amphibians remains relatively scarce. Here, this study investigated the effects of dietary Schizochytrium meal on growth, metamorphosis, intermediate metabolism, and intestinal health of bullfrogs. Six compound feeds (S0–S5) containing different gradients of Schizochytrium meal (0.00, 2.00, 5.00, 10.00, 15.00, and 20.00 g/kg diets) were formulated. After 90 days, the S4 group (15.00 g/kg) exhibited significantly superior growth performance, with the weight gain rate (WGR) increasing by up to 23.78% compared to the control (S0). Metamorphosis rate (MR) peaked at 23.33% in the S4 group. The enzyme activities of digestion (amylase (AMS), lipase (LPS), protease), brush border membrane (Na⁺, K⁺-ATPase, alkaline phosphatase (AKP), γ-glutamyl transferase (γ-GT), creatine kinase (CK), and antioxidation (superoxide dismutase (SOD), catalase (CAT)), as well as microvilli length and mucosal epithelial cell height in the intestine were the highest in the S4 group. Intestinal microbial diversity (Ace index) significantly increased by 41.28% in the S4 group, which also promoted beneficial bacteria. Key genes related to the GH-IGF-1 axis, metabolism, and intestinal barrier function were significantly upregulated with increasing Schizochytrium levels up to 15.00 g/kg, whereas pro-inflammatory genes showed an opposite trend. Overall, dietary supplementation with Schizochytrium meal at 15.00 g/kg promotes growth, metamorphosis, and intestinal health in bullfrog tadpoles by modulating the GH-IGF-1 axis, enhancing digestion and absorption, and improving intestinal integrity. Optimal Schizochytrium meal levels were identified as 13.27 g/kg.