Abstract
The present study was to investigate the effects of Methylococcus capsulatus protein (MCP) replacing fish meal on the growth performance, immunity, and intestinal health of Chinese soft-shelled turtle (Pelodiscus sinensis). Four isonitrogenous (47.00% crude protein) and isolipidic (6.00% crude lipid) diets were formulated for an 8-week feeding trial. The fish meal group (FM) contained 55% fish meal. MCP was used to replace 10%, 15%, and 20% of the fish meal in the FM group to create MCP10, MCP15, and MCP20 treatment groups. No significant differences were observed in the weight gain, feed conversion efficiency, physical indicators, or muscle amino acid composition among all diet treatments (P > 0.05). The MCP15 group showed the best growth performance. There were no significant differences in serum biochemistry among the groups (P > 0.05), except for aspartate aminotransferase, which exhibited a significant linear increase trend with the increase of MCP replacement level (P = 0.049). In the MCP20 group, the complement C3 content, immunoglobulin M content, and lysozyme activity in serum were significantly lower than those in the FM, MCP10, and MCP15 groups (P < 0.05), showing a quadratic relationship characterized by an initial increase followed by a decrease as the MCP level in the feed increased (P < 0.05). The intestinal morphology in all treatment groups appeared normal and the expression levels of intestinal inflammatory cytokines showed no significant differences (P > 0.05). The community structure of the intestinal microbiota, as well as the phylum and genus composition underwent significant alterations in response to the MCP supplementation. The recommended practice for Chinese softshell turtle diet involves substituting 15% MCP for fish meal, which enhances growth, boosts immune response, and does not adversely affect serum biochemistry, amino acid composition, or intestinal health.
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