The mudskippers Periophthalmodon schlosseri and Boleophthalmus boddaerti can tolerate environmental NH₃ concentrations of 446 and 36 μM, respectively

The aim of this study was to elucidate if the mudskipper Periophthalmodon schlosseri, in relation to its capability to survive on land, has acquired a greater capacity to detoxify ammonia than more aquatic species. The tolerance of P. schlosseri to environmental ammonia was much higher than that of another mudskipper, Boleophthalmus boddaerti, and those of other fishes. The 24, 48, and 96 h median lethal concentrations (LC₅₀) of unionized ammonia (NH₃) for P. schlosseri were 643, 556 and 536 μM, respectively. The corresponding LC₅₀ values for B. boddaerti were 77.1, 64.0, and 60.2 μM. The relatively high tolerance of P. schlosseri to ammonia could be partially due to the presence of high activities of glutamine synthetase (GS) and glutamate dehydrogenase (GDH, aminating) in its brain. When P. schlosseri and B. boddaerti were exposed to their sublethal NH₃ concentrations of 446 and 36 μM, respectively, both mudskippers detoxified ammonia by converting it to free amino acids (FAA). This led to increases in concentrations of total FAA (TFAA) in the brain, liver and muscle. Increases in TFAA concentrations in the brain were mainly due to increases in glutamine concentrations. The activities of GS and GDH in the brain of both mudskippers increased significantly after they were exposed to their respective sublethal concentrations of NH₃. Urea production and excretion were not utilized as a means for environmental ammonia detoxification in these mudskippers. The most intriguing results obtained were the lack of effect on any of the parameters studied when P. schlosseri was exposed to 36 μM of environmental NH₃. These results suggest that P. schlosseri might be able to maintain a low steady state level of internal ammonia during ammonia loading at a concentration which is lethal to other fishes.