Effects of high density on behavior, physiology and disease resistance of the sea cucumber Apostichopus japonicus under stress conditions

High stocking density is widely adopted due to the efficient water utilization in sea cucumber aquaculture. However, bacterial infection challenge and acute temperature decrease are common and inevitable problems in the aquaculture of sea cucumbers. This study investigated the impacts of high density on survival, behavioral and physiological response of Apostichopus japonicus under stress conditions (bacterial infection challenge and an acute temperature decrease). In experiment II, our results showed that morbidity significantly increased in A. japonicus and that feeding behavior significantly reduced in asymptomatic individuals at high density after bacterial challenge. However, foraging behavior was not significantly affected in asymptomatic individuals. These results suggest that high density probably affects the survival rate of sea cucumbers after the SUS challenge and inhibits feeding behavior of asymptomatic individuals. Moreover, in experiment III, righting and feeding behaviors were significantly inhibited in A. japonicus at high density after acute temperature decrease. However, crawling behavior and feeding preferences were not significantly affected in A. japonicus. These results indicated that high density increased the susceptibility to acute temperature decrease stress in A. japonicus. In addition, in experiment I, we further found that superoxide dismutase (SOD) activity and the level of pyruvate kinase (PK) significantly increased in A. japonicus at high density. Increased SOD and PK are probably the important reasons that cause a decreased stress resistance, when sea cucumbers are exposed to bacterial infection or acute temperature decrease. In conclusion, our study clarified the influences of increased stocking density on survival, behavioral and physiological responses of A. japonicus under stress conditions. We further revealed the possible reasons why high density affects the survival, behavior, and physiology in A. japonicus under stress.