Dietary Bacillus improves behavior, intestinal health, and growth of juvenile sea cucumbers Apostichopus japonicus at low temperature

The traditional overwintering process of sea cucumbers (Apostichopus japonicus) requires burning a large amount of coal to raise the water temperature. It is useful but costly and not environmentally friendly. Bacillus is proposed as a cheap and green alternative. Therefore, this study intended to achieve cleaner production of A. japonicus by adding Bacillus to their diet at low temperature (7 °C) to achieve the production efficiency of sea cucumbers cultured by heating water to 11 °C. Here, we found that number of crawl steps, relative food intake, relative fecal outputs, amylase and proteinase activities, body weight and weight gain rate significantly reduced, and intestinal morphology and intestinal microbiota were also worse in sea cucumbers at low temperature (7 °C), compared with the sea cucumbers cultured at 11 °C. This suggests that low temperature negatively affect the behaviors, intestinal health, and growth of A. japonicus. However, the adverse effects on the behavioral capacities (such as number of crawl steps, relative food intake and relative fecal outputs), intestinal health (such as digestive enzyme activities and intestinal morphology), and growth (such as body weight and weight gain rate) of sea cucumbers under low temperature conditions were compensated after adding dietary Bacillus (Bacillus methylotrophicus and Bacillus amyloliquefaciens) at 10⁷ CFU/g. These traits reached the level of those in sea cucumbers cultured at 11 °C with no significant difference, and were significantly different from those cultured at 7 °C without adding Bacillus. Importantly, the intestinal microbiota structure of sea cucumbers was greatly improved after the addition of dietary Bacillus, reducing the proportion of Proteobacteria and the consequent probability of diseases. In conclusion, the results suggest that dietary supplementation with Bacillus can reverse the adverse effects caused by 4 °C of temperature difference (between 7 °C and 11 °C) on sea cucumbers. Therefore, we recommend that aquaculture farmers use a combination of burning limited coal to increase water temperature to 7 °C and feeding Bacillus to improve the production efficiency of A. japonicus in winter.