Effects of thermal and osmotic stress on growth, osmoregulation and Hsp70 in sea cucumber (Apostichopus japonicus Selenka)

This study deals with the effect of thermal and osmotic stress on growth, osmoregulation and Hsp70 level in the sea cucumber Apostichopus japonicus Selenka. Sea cucumbers were exposed to 30 °C (Hs treatment) or 20 °C (control) for 2 h, and then were challenged with osmotic shock, which was administered by transferring the sea cucumbers from natural seawater (about 32 ppt) to different salinities (20 ppt, 25 ppt, 30 ppt, and 40 ppt). After a 30-day experiment, the specific growth rate (SGR) and survival rate of sea cucumbers were measured. To elucidate the underlying physiological mechanisms, osmotic pressure of the coelomic fluid, Na+/K+-ATPase and Hsp70 level of the sea cucumbers from Hs treatment and control were measured at selected times (1 h, 2 h, 6 h, 24 h, and 48 h) after the osmotic shock. At the end of the 30-day experiment, SGR of Hs sea cucumber was higher than that of the control at salinity of 20 ppt. However, SGR of Hs sea cucumber was lower than that of the control at salinity of 30 ppt, 40 ppt and natural seawater. At salinity of 20 ppt, the survival rate of A. japonicus was only 20%. These results indicted that a salinity of 20 ppt exceeded the ecological tolerance for growth in juvenile A. japonicus. A prior heat shock could accelerate growth when the ambient salinity was below the ecological tolerance for growth (salinity of 20 ppt); on the other hand, it could retard growth when the ambient salinity was within the ecological tolerance for growth (salinity of 25 ppt, 30 ppt, 40 ppt and natural seawater). The osmotic pressure of the coelomic fluid changed rapidly and stabilized by 6 h after the osmotic shock. The temporal change pattern of Na+/K+-ATPase activity increased initially and reached the maximum value within 6 h. These results indicated that A. japonicus was an osmotic conformer, and the activity of Na+/K+-ATPase had an adaptive response to the change of ambient salinity. Both thermal and osmotic stresses could induce the up-regulation of Hsp70, which might partly explain the differences in growth of sea cucumbers at different salinities.