Tube feet regulate aggregation behavior of the sea cucumber Apostichopus japonicus

Revealing the mechanism of aggregation behavior is of great importance in purposefully guiding and manipulating aggregation behavior. For the first time, the present study clarified that tube feet regulate aggregation behavior and investigated the underlying molecular mechanism in sea cucumber (Apostichopus japonicus Selenka), which belongs to the phylum Echinodermata, class Holothuroidea, order Aspidochirotida, family Stichopodidae, and genus Apostichopus. Residence time was significantly higher in the region with 20 sea cucumbers than in the region with 5 sea cucumbers, indicating that sea cucumbers themselves do play a role in the aggregation process. In addition, the proportion of aggregation in the area with tube feet was significantly higher than that of those without tube feet. This indicates that tube feet can attract sea cucumbers to aggregate. Further, the proportion of sea cucumbers aggregated in the region with crushed tube feet was significantly higher than that in the uncrushed tube feet region. This further verifies the role of tube feet in aggregation behavior of sea cucumbers. We found that the proportion of sea cucumbers aggregated in region with intact tube feet was significantly higher than in region where tube feet were removed. This further suggests that tube feet, rather than other parts, attract sea cucumbers to aggregate. Interestingly, the present study found that sea cucumbers significantly aggregated on the rough stone with 20 sea cucumbers attached than on the acrylic plate with 20 sea cucumbers attached. Consistently, the aggregation behavior significantly enhanced after the injection of potassium ion solution. This suggests that potassium ions probably affect the aggregation behavior of sea cucumbers. We also found that the proportion of aggregation was significantly lower in sea cucumbers injected with HC-030031 (25 μL) than those injected with DMSO (25 μL), while the aggregation proportion of sea cucumbers injected with 5-HT (25 μL) was significantly higher than that injected with normal saline (25 μL). These results suggest that the aggregation behavior may be manipulated by adhesion proteins released by the tube feet, and are probably achieved through TRPA1 pathway perception and 5-HT conduction.