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A feeding experiment was conducted to investigate the effects of dietary administration of synbiotic with Bacillus lincheniformis WS-2 (CGMCC No. 12813) and alginate oligosaccharides (AOS) on the growth, innate immune response, and intestinal microbiota of the sea cucumber Apostichopus japonicus and its resistance to Vibrio infection. Sea cucumbers were given a control diet (non-supplemented), pro diet (basal diet plus 1 × 109 cfu (g diet)−1B. lincheniformis WS-2), syn diet (basal diet plus 1 × 109 cfu (g diet)−1B. lincheniformis WS-2 and 10 g (kg diet) −1 AOS) or pre diet (basal diet plus 10 g (kg diet) −1 AOS) over a period of 60 days, and the growth performance and various innate immune parameters of the animals were evaluated after 30 and 60 days of feeding. No significant difference in growth performance was observed between the group fed with the syn and the group fed with the pro diet, but both these groups exhibited significant (P < 0.05) enhancement in growth performance compared to the control group. At the same time, both syn and pro diets also resulted in the animals having significantly higher levels of amylase, protease and alginate lyase activities compared to the con diet. Individuals fed with the syn or pro diet showed enhanced levels of various immune enzyme activities, compared to those fed with the con diet. At the end of the growth period, the sea cucumbers were challenged with Vibrio splendidus via intraperitoneal injection. The survival rates of sea cucumbers fed with the syn, pro or pre diet were significantly improved compared to that of sea cucumbers fed with the con diet, with sea cucumbers fed with synbiotic having the highest survival. In addition, increased proportions of Bacillus and Lactococcus were found in the intestinal tract of sea cucumbers fed with the syn diet (9.5% and 7.3%) compared to those of sea cucumbers fed with the pro diet (6.1% and 4.6%), con diet (4.0% and 3.4%), or pre diet (5.2% and 6.8%) after 60 days of feeding. Furthermore, the proportion of Vibrio in the intestinal tracts of sea cucumbers fed with the pro diet (2%) or syn diet (3.1%) was lower than that of sea cucumbers fed with the con diet (5.5%) or pre diet (3.8%), although no significant difference was detected between the pro diet and syn diet groups (P > 0.05). Overall, the results suggested that dietary synbiotic consisting of Bacillus lincheniformis and alginate oligosaccharides (AOS) could have positive benefit for sea cucumber aquaculture.Sea cucumbers fed with synbiotic showed better growth performance than the control.Dietary synbiotic significantly improved disease resistance in sea cucumbers.Synbiotic improved the diversity of gut microbiota in sea cucumbers.