Responses of hemolymph osmolality and tissue water of Penaeus chinensis Osbeck juveniles subjected to sudden change in salinity

Hemolymph osmolality and tissue water of laboratory-reared Penaeus chinensis Osbeck juveniles (0.83 to 1.86 g) were investigated, after they had been transferred individually from 10, 20, 30 and 40 ppt to 10, 20, 30 and 40 ppt for 0.25, 0.5, 1, 2, 5 and 10 d, respectively. Hemolymph osmolality and tissue water of shrimp were stablilized within 5 d after they had been subjected to a sudden change in salinity from each salinity level. Hemolymph osmolality had a positively linear relationship with medium osmolality. Tissue water decreased with increased medium osmolality, and decreased with increased hemolymph osmolality. The mean (SD) isosmotic point was 703 (8) mOsm kg^–1 which is equivalent to 24.2 (1.0) ppt. P. chinensis juveniles exhibited hyperosmotic regulation in salinities below isosmotic value, and hypoosmotic regulation in those above. The shrimp originally adapted to high salinity levels (30 and 40 ppt) showed less fluctuation of tissue water than those adapted to low salinity levels (10 and 20 ppt) when they were subjected to a sudden change in salinity.