A COMPARATIVE STUDY ON THE ENVIRONMENTAL AMMONIA TOLERANCE OF TWO MUDSKIPPERS

Abstract

Periophthalmodon schlosseri had a much higher tolerance to NH4Cl than Boleophthalmus boddaerti. The 24 h, 48 hand 96 h median lethal concentrations (LC50 ) of NH4Cl for P. schlosseri were 140 mM, 125 mM and 115 mM, respectively, whereas those for B. boddaerti were 17.5 mM, 14 mM and 13.8 mM, respectively. When P. schlosseri and B. boddaerti were exposed to their respective sublethal NH4Cl concentration at 100 mM or 8 mM over a time course from 3 h to 144 h ( 6 days), there were increases in the NH4+ levels in the plasma, brain, liver and muscle of both fishes, presumably mainly due to ammonia entry down its concentration gradient. The level of NH4+ accumulated in the plasma of ammonia exposed B. boddaerti was the lowest (at about 2 µmol/ml) among the tissues studied, while that in the brain was the highest, reaching 19 µmol/g wet weight. During the initial periods of ammonia exposure, the liver of P. schlosseri accumulated more NH4+ than the brain, but after 48 h, the level in the brain was higher than that in the liver. Both mudskippers detoxified ammonia by converting it to FAA, leading to increases in total free amino acid (TFAA) concentrations in the brain, liver and muscle. Increases in TFAA concentrations were mainly contributed by glutamine, glutamate, alanine and glycine. High glutamine synthetase (GS) activities were detected in the brains of both mudskippers, but GS activity was undetectable in the livers. The activity of GS and that of glutamate dehydrogenase (GDH) in the aminating direction from the brain of ammonia exposed P. schlosseri and B. boddaerti increased significantly as compared to the respective control value. GDH activity in the aminating direction from the brain of P. schlosseri was approximately 3 times higher than that in B. boddaerti. Urea production was not utilized as a means for environmental ammonia detoxification in both mudskippers. There was no significant change in the NH4+, urea and TFAA in the tissues of P. schlosseri exposed to 8 mM NH4Cl for 6 days as compared to those of the 6 d starvation control. Thus, different from B. boddaerti, when exposed to 8 mM NH4Cl, P. schlosseri did not keep the level of NH4+ in its tissues low by converting it to FAA. Since the blood pH of P. schlosseri and B. boddaerti were comparable, the partial pressure gradient of NH3 for its diffusion should be similar for both mud skippers. Results obtained indicate that P. schlosseri might have a more efficient system to excrete the ammonia that entered back to the ambient medium. In comparison to B. boddaerti, C. micropeltes and O. mossambicus, the Na+,K+-ATPase activity in the gill of P. schlosseri was higher by 3-12. times. It was found that NH4 could stimulate branchial ATPase activity in the presence of Na+, and the NH4+, Na+-ATPase activity was ouabain sensitive. It is likely that P. schlosseri, with a high level of NH4+,Na+-ATPase activity, could actively transport the NH4+ from the circulation and remove it via the apical Na+ /NH4+ exchange mechanism even during ammonia loading.