Dietary Lactobacillus plantarum Supplementation Improves Growth and Modulates Hepatic and Immune-Related Responses in Tongue Sole (Cynoglossus semilaevis)

Lactic acid bacteria (LAB) are increasingly used as functional feed additives in aquaculture. This study evaluated the effects of dietary Lactobacillus plantarum supplementation on growth performance, hepatic biochemical status, lipid-related indices, antioxidant status, and immune-related responses in Chinese tongue sole (Cynoglossus semilaevis). A total of 1620 juveniles (initial body weight 8.11 ± 0.23 g) were randomly assigned to three dietary treatments for 10 weeks: a control diet (CON) and diets supplemented with 500 mg/kg (LAB1) or 1000 mg/kg (LAB2) LAB (three replicate tanks per treatment). Compared with CON, LAB supplementation improved growth performance, with LAB2 showing significantly higher final body weight, weight gain rate, and specific growth rate. LAB altered hepatic function-related indices and reduced hepatic lipid peroxidation, as indicated by lower malondialdehyde (MDA). Hepatic lipid-related indices were also modulated, with LAB2 showing reduced total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C), triacylglycerol (TG), and total bile acid (TBA), together with increased high-density lipoprotein cholesterol (HDL-C). Serum immune indices showed non-linear responses, with C3, C4, and IgM increasing in LAB1 but decreasing in LAB2, whereas lysozyme showed an overall decreasing trend. qPCR analysis showed that LAB supplementation upregulated hepatic IGF-I and TGF-β1 expression, downregulated IL-8, TNF-α, and G6PD expression, and reduced FAS expression at the higher dose. Overall, dietary L. plantarum at 500–1000 mg/kg improved growth performance and was associated with changes in hepatic lipid-related indices, oxidative status, and immune-related responses in tongue sole. These results support further evaluation of L. plantarum as a functional feed additive in this species.