Effects of stocking density on behavior, physiology, and gene expression of the sea cucumber Apostichopus japonicus

The behavioral and physiological mechanisms induced by high density remain mostly unknown in sea cucumbers, although it has been widely adopted to increase the space usage efficiency in aquaculture. The present study focused on investigating how high density affects fitness-related behaviors, physiology, and gene expression in the sea cucumber Apostichopus japonicus. We found that high density significantly increased crawling behavior of sea cucumbers in short-term (one week), while significantly inhibited feeding behavior in long-term (seven weeks). Foraging behavior was not significantly affected in sea cucumbers at high density. Our study found that short-term high density did not have a significant impact on the cortisol and glucose concentration in A. japonicus. There was no significant difference in serotonin (5-HT) of sea cucumbers between high and low density. Gamma-aminobutyric acid (GABA) significantly increased in sea cucumbers at high density. Thus, GABA probably plays a crucial role in regulating the behavior of A. japonicus at high density. In addition, we conducted a transcriptome analysis on intestinal tract of sea cucumbers at two different stocking densities. There were 117 differentially expressed genes (DEGs) were discovered, including 72 up-regulated DEGs and 45 down-regulated DEGs. Gene Ontology (GO) functional annotation revealed that the DEGs were significantly enriched in the processes of oxidation-reduction, dioxygenase activity, and oxidoreductase activity. Additionally, the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis showed a significant enrichment of DEGs in glycerolipid metabolism and PPAR signaling pathways. We highlighted LPL encoding lipoprotein esterase, which probably plays an important role in lipid metabolism in A. japonicus under high density. In conclusion, the present study clarified the behavioral and physiological reactions and further revealed the related molecular mechanisms of A. japonicus in a high-density condition. This provides valuable information into the management of the high-density aquaculture of sea cucumbers. •High density increases crawling in short-term and inhibits feeding in long-term.•GABA involved in the physiological process in A. japonicus at high density.•Antioxidant defense and lipid anabolism are regulated in A. japonicus at high density.