Quality evaluation of indoor- and outdoor-cultured sea cucumber (Apostichopus japonicus) seedlings: Insight from survival and immune performance in response to combined stress of hyperthermia and hyposalinity

The mass mortalities of sea cucumber Apostichpous japonicus have prevailed in northern China, mainly attributing to the emergence of extreme environmental conditions, that is hyperthermia and hyposalinity. The high-quality sea cucumber seedlings appear to possess more robust resistance to adverse conditions. There are usually indoor- and outdoor-cultured seedlings in industrial production of sea cucumbers. Although the outdoor-cultured sea cucumbers are practically considered to be more strong and robust, the effective evaluation approach to distinguish these seedlings has been scarce. The current study compared survival and immune performances of indoor- and outdoor-cultured A. japonicus under combined exogenous stressors, that is hyperthermia and hyposalinity. Results based on secondary stress induction revealed that the activities of immune enzymes and levels of catecholamines in body wall of outdoor-cultured seedlings were prominently higher than those of indoor-cultured seedlings recovered for 0-72 hr following sublethal stress (30 degrees C and 25 psu of salinity). The opposite case occurred on immune enzymes in coelomic fluid of the two sources of seedlings except for myeloperoxidase. Importantly, the outdoor-cultured seedlings, which were recovered for 72 hr after sublethal stress, exhibited a 93% of cumulative survival rate following 7 days of recovery after lethal stress (33 degrees C and 20 psu of salinity), 27% higher than the indoor-cultured seedlings. Collectively, the outdoor-cultured A. japonicus seedlings showed more superior quality than the hatchery-produced seedlings in terms of survival and immune performance. These findings provide practically useful information towards quality distinction of the indoor- and outdoor-cultured sea cucumbers, which could benefit the aquaculture industry to obtain high-quality seedlings