Dietary Supplementation with <i>Rhodotorula mucilaginosa</i> Enhances Resistance to <i>Aeromonas veronii</i> Infection in Red Claw Crayfish (<i>Cherax quadricarinatus</i>)

The objective of this study was to evaluate the effects of dietary supplementation with different levels of <i>Rhodotorula mucilaginosa</i> (0.0 g/kg, 0.1 g/kg, 1.0 g/kg, and 10.0 g/kg) on resistance to <i>Aeromonas veronii</i> infection in red claw crayfish (<i>Cherax quadricarinatus</i>) (initial body weight of 0.13 ± 0.06 g). The investigation combined a 56-day feeding trial with a subsequent 7-day infection challenge to assess cumulative mortality, immune and antioxidant enzyme activities, and the relative expression of immune-related genes. During the <i>A. veronii</i> infection test, the cumulative mortalities for the 0.1 g/kg, 1.0 g/kg, and 10.0 g/kg groups were 44.44%, 38.89%, and 38.89%, respectively, all significantly lower (<i>p</i> < 0.05) than that of the control group (58.33%). Compared with the control group, after infection with <i>A. veronii</i>, the activities of acid phosphatase, alkaline phosphatase, catalase, and superoxide dismutase in the hepatopancreas and alkaline phosphatase, lysozyme in the hemolymph of red claw crayfish in the 1.0 g/kg group significantly increased (<i>p</i> < 0.05). The activities of aspartate aminotransferase and alanine aminotransferase in the hemolymph of red claw crayfish in the 1.0 g/kg group significantly decreased (<i>p</i> < 0.05). The relative expression levels of serine protease inhibitor, crustacean hyperglycemic hormone, anti-lipopolysaccharide factor, and superoxide dismutase genes in the hepatopancreas of red claw crayfish in the 1.0 g/kg group were significantly upregulated (<i>p</i> < 0.05). In conclusion, <i>R. mucilaginosa</i> could significantly improve the antibacterial ability of red claw crayfish against <i>A. veronii</i>. In this experimental context, the ideal addition level of <i>R. mucilaginosa</i> is determined to be 1.0 g/kg.