| Summary: | The goal of this study was to assess osmotic stress in Litopenaeus vannamei postlarvae (PL12) in three different progressive osmotic stress levels ranging from sea water (35‰) to low salinity (5‰). The survival of PL12 reduced as medium salinity decreased. PL12 acclimated to a gradual acclimation of 5‰ showed significantly higher survival (with a survival percentage (SP) of 90%) than those acclimated to a gradual acclimation of 10‰ (SP = 80%) and 15‰ (SP = 58%). At the end of the acclimation period (at 5‰), PL12 acclimated to a gradual acclimation of 15‰ (Trial 3) exhibited significantly greater hemolymph osmolality (696 mOsm/kg H2O) than those gradually acclimated to a gradual acclimation of 10‰ (546 mOsm/kg H2O, Trial 2) and 5‰ (450 mOsm/kg H2O, Trial 1). The levels of Na+, K+ and Cl- in the hemolymph of PL12 decreased with decreasing salinity in all three trials. Na+-K+-ATPase levels of PL12 increase as salinity decreases. Total hemocyte count levels decreased when the salinity decreased from 35‰ to 5‰. The longer acclimation time may account for the greater survival rate, as it likely ensures ion balance between the shrimp hemolymph and the surrounding environment thus minimizing osmoregulatory stress.
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