RESUMO
Three fish species of the family Batrachoididae, the gulf toadfish (Opsanus beta), the oyster toadfish (Opsanus tau), and the plainfin midshipman (Porichthys notatus) demonstrated exceptionally high tolerances to elevated water ammonia with 96-h LC50 values of 9.75, 19.72 and 6 mM total ammonia, respectively. Using pH values we calculated the corresponding unionized ammonia (NH(3)) values to be 519, 691 and 101 µM, respectively. These values are well above typical values for most teleost fishes, but close to those of ureotelic fish examined to date. Following sublethal high ammonia exposure (HAE) blood and tissue (brain, liver and muscle) sampling confirmed that internal ammonia levels rose substantially in all three species, suggesting that they were not simply avoiding toxicity by impermeance to ammonia. The three species of batrachoidids can be characterized in the following manner with respect to the inabilities to synthesize and excrete urea, based on these studies and prior research: O. beta (fully ureotelic)>O. tau (moderately ureotelic)>P. notatus (ammoniotelic). While some of the high ammonia tolerance for O. beta and O. tau can be explained by their ability to detoxify it to urea, other mechanisms must be at play for P. notatus. Further experiments determined that all three species possess rather high activities of glutamine synthetase (GSase) in brain especially (60-180 U g(-1)), that glutamine accumulates in many tissues, and that LC50 values are correlated positively with brain GSase activity. Taken together, our results suggest that alternative/additional mechanisms for ammonia detoxification via urea synthesis must be considered to explain the exceptionally high ammonia tolerance of this group.
