Effects of abrupt decline in salinity on growth,immunity and histological structure of respiratory trees in juvenile sea cucumber Apostichopus japonicus

The juvenile sea cucumber Apostichopus japonicus with body weight of 2.42 g and 3.5 g held in sea water with a salinity of 33 was directly stocked into tanks of each 50 cm×40 cm×30 cm with a salinity of 33,30,26 and 22,ad libitum fed and reared routinely at water temperature of 10.0-13.0 ℃ for 60 days to elucidate the effects of abrupt decline in salinity on growth,survival,immunity and histological structure of respiratory trees in the sea cucumber juveniles.It was found that 2.23%-11.11% of the juveniles exposed to abrupt decline in salinities showed evisceration in the experimental groups,significantly higher in the group with a salinity of 30 ppt than in the group with a salinity of 33(P0.05).However,the juveniles had decrease in weight gain by 5.0% and 62.5% under a salinity of 30 and 26,indicating that the lower limit of salinity is 26 for the growth in the sea cucumber juveniles.The activity of serum lysozyme increased with increase in salinity while the serum superoxide distmutase(SOD) activity decreased with increase in salinity.No acid phosphatase(ACP) activity was determined in the juveniles exposed to abrupt decline in salinity.The respiratory tree consists of coelothelial cells layer,muscular coat layer,and endothelial cell layer,indistinct haemocoel and wide center antrum.At a salinity of 29,there were no coelothelial cells,many vesicles in the endothelial cells,and narrow center antrum.At a salinity of 24,and 18,coelothelial cells were found to increase,haemocoel to thicken and endothelial cells to extend into the center antra.