Effects of circadian rhythm on behavior and physiology of the sea cucumber Apostichopus japonicus

The present study investigated the behavioral and physiological responses to the circadian rhythm in the sea cucumber Apostichopus japonicus. We found that righting behavior of sea cucumbers was significantly faster at night than that in daytime (P < 0.001). We thus suggest aqua-farmers carry out seeding at night in stock enhancement. The number of tentacle swings was significantly higher at night than that in daytime (P = 0.005). Thus, we suggest aqua-farmers provide diets before the peak of sea cucumber feeding at night. There was no significant difference in foraging behavior and defecation behavior during the day and at night. This indicates that not all behaviors have different characteristics in circadian rhythm. In addition, we found that cortisol concentration was significantly higher at night than that in daytime (P = 0.021). This suggests that sea cu-cumbers are probably more prone to be stressed at night. However, there was no significant difference in 5-HT and melatonin during the day and at night, suggesting that 5-HT and melatonin are probably not affected by circadian rhythm. The present study clarifies the behavioral and physiological responses to circadian rhythm and provides valuable information into sea cucumber aquaculture.