THE MUDSKIPPERS PERIOPHTHALMODON SCHLOSSERI AND BOLEOPHTHALMUS-BODDAERTI CAN TOLERATE ENVIRONMENTAL NH3 CONCENTRATIONS OF 446 AND 36 MU-M, RESPECTIVELY

The aim of this study was to elucidate if the mudskipper Periophthalmo don schlosseri , in relation to its capability to survive on land, has acquired a greater capacity to detoxify ammonia than more aquatic spe cies. 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 concentrati ons (LC50) of unionized ammonia (NH3) for P. schlosseri were 643, 556 and 536 mu M, respectively. The corresponding LC50 values for B. bodda erti were 77.1, 64.0, and 60.2 mu M. The relatively high tolerance of P. schlosseri to ammonia could be partially due to the presence of hig h activities of glutamine synthetase (GS) and glutamate dehydrogenase (GDH, aminating) in its brain. When P. schlosseri and B. boddaerti wer e exposed to their sublethal NH3 concentrations of 446 and 36 mu M, re spectively, both mudskippers detoxified ammonia by converting it to fr ee amino acids (FAA). This led to increases in concentrations of total FAA (TFAA) in the brain, liver and muscle. Increases in TFAA concentr ations in the brain were mainly due to increases in glutamine concentr ations. The activities of GS and GDH in the brain of both mudskippers increased significantly after they were exposed to their respective su blethal 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 eff ect on any of the parameters studied when P. schlosseri was exposed to 36 mu M of environmental NH3. These results suggest that P. schlosser i might be able to maintain a low steady state level of internal ammon ia during ammonia loading at a concentration which is lethal to other fishes.