Na/K-ATPase activity and osmo-ionic regulation in adult whiteleg shrimp Litopenaeus vannamei exposed to low salinities

Na/K-ATPase activity and osmo-ionic regulation were examined in Litopenaeus vannamei exposed to low salinities. The hemolymph osmolality of shrimp exposed to very low salinities (0.5 ppt and 1 ppt) decreased rapidly after 6 h from 800 mOsm to 540–560 mOsm; these individuals could not survive beyond 1 day. Under exposure to 7 ppt, osmolality levels decreased rapidly to 630 and 570 mOsm after 6 h and 1 day respectively, whereas in shrimp exposed to 18 ppt, levels declined slowly from 800 mOsm to 700 mOsm after 6 h and thereafter remained stable for 7 days until the end of the experiment. Hemolymph sodium ion levels showed a pattern of change similar to that of hemolymph osmolality, with initial levels being 380 mmol/L and decreasing to 180 mmol/L under very low salinity. Changes in calcium, potassium, and magnesium ion concentrations were also investigated. Na/K-ATPase activity was 3.98 μmol ADP/mg protein/hour in the gills of control individuals maintained at 28 ppt for the experimental duration. Levels increased in individuals exposed to 18 ppt, 7 ppt and 3 ppt for 7 days to 5.06, 6.51, and 5.81 μmol ADP/mg protein/hour, respectively. Based on the above, it is suggested that L. vannamei could not survive in low salinity (< 1 ppt) due to the loss of osmoregulatory capacity when the shrimps were transferred directly from high salinities to low salinities.