The ammonia excretion rate was used as an index of the physiological state of Penaeus monodon poltlarvae (PL) by monitoring excretion rates after handling and transfer to physiologically different media. The ammonia excretion rate of postlarval P. monodon increased from a mean of 0.543 Âµg to 0.815 Âµg total NH<sub>3</sub>-N per mg dry wt per hour after transfer from 32 to 20 ppt sea water. PL transferred to 40 ppt showed a significant decrease in excretion rate to 0.423 Âµg total NH<sub>3</sub>-N per mg dry wt per hour. These changes in excretion rate after transfer to different salinities were only temporary since acclimatized PL showed only minor deviations in excretion rate from the control. PL transferred from normal (8.2) to low (7.2) pH had elevated ammonia excretion rates, from 0.591 Âµg to 0.733 Âµg total NH<sub>3</sub>-N per mg dry wt per hour, while those transferred to high pH (9.2) had excretion rates that dropped drastically to 0.259 Âµg. About 40% of the PL in high pH were moribund after the 150 min exposure. The ammonia excretion rate of PL after a 1 hour feeding was twice that of unfed PL during the first 15 min after transfer. This difference narrowed with time and was no longer significant at 75 min after transfer which also the duration that represented the postprandial surge in ammonia excretion after the 1 hour feeding
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