A completely randomized experimental design carried out to investigate the effects of different levels of Pediococcus acidilactici (PA) including 0 (basal diet as a control diet), 1 × 106, 2 × 106, 4 × 106, and 8 × 106 colony-forming unit (CFU) per gram of the diet for 60 days on the mucosal immunity responses, growth, and reproductive performance, in zebrafish, Danio rerio (with mean weigh ± SE: 120 ± 10 mg). The obtained results revealed that the best growth and reproduction indices were related to the concentration of 4 × 106 CFU PA g−1 diet (P < 0.05). The maximum activities of mucosal immune responses including total protein, alternative complement system, IgM, and lysozyme were observed in the fish fed with 4 × 106 CFU PA g−1 diet (P < 0.05). Furthermore, the maximum alkaline phosphatase activity of skin mucus was recorded in the fish fed with 8 × 106 CFU PA g−1 diet (P < 0.05). Fish fed with 4 × 106 CFU PA g−1 diet had the highest villus length and width of the intestine (P < 0.05). Supplementing the diet with 4 × 106 CFU PA g−1 diet more significantly enhanced Cyp19a gene expression in comparison with this in other groups. Hence, PA with a concentration of 4 × 106 CFU g−1 diet can be considered as a proper level of probiotic for improving the health, growth, and reproductive performance of the D. rerio.

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