Behavioral and Physiological Requirements for Artificial Shelters in Juvenile Sea Cucumbers Apostichopus japonicus

Shelters can enhance the growth efficiency of sea cucumbers, while the preference of sea cucumbers for shelters varies among individuals. Therefore, this study investigated the behavioral and physiological requirements of the sea cucumber <i>Apostichopus japonicus</i> for artificial shelters. In this experiment, we considered sea cucumbers that spent more than 80% of their time (2880 s) inside the shelter as the sheltered sea cucumbers and those that spent less than 20% of their time (720 s) inside the shelter as the non-sheltered sea cucumbers. We found that mouth tentacle grasping times in the sheltered group were significantly lower than in the non-sheltered group, while foraging selections of both groups were not significantly different. This indicates that feeding is the behavioral requirement for the sheltered group instead of foraging. The height of the intestinal crease was significantly shorter in the sheltered group than in the non-sheltered group. Further, the defecation rate and 5-HT content in the intestinal tract of the non-sheltered group were significantly lower than those of the sheltered group. This indicates that the sheltered group has a greater demand for food digestion than the non-sheltered group. Compared with the non-sheltered group, the sheltered group showed higher relative abundances of Gammaproteobacteria and Bacteroidia in the gut microbiota. The thermal tolerance was significantly worse in the sheltered group. Furthermore, there was no significant difference in movement distance after mechanical disturbance between the two groups. Cortisol content showed no significant difference either. These indicate that the sheltered sea cucumbers do not require shelters for stress relief in the absence of external handling stress. This study clarified the behavioral and physiological requirements of sea cucumbers on shelters and enriched our understanding of the shelter dependence of sea cucumbers.