Dogmas and controversies in the handling of nitrogenous wastes: Ureotely and ammonia tolerance in early life stages of the gulf toadfish,Opsanus beta

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Abstract
SUMMARY The marine gulf toadfish (Opsanus beta) is an unusual teleost fish as it is able to switch between ammoniotelism and ureotelism in response to a variety of laboratory conditions. The present study integrates field work conducted in Biscayne and Florida Bays, USA with laboratory studies to examine ureotelism during the early life history stages of O. beta. Adult toadfish voluntarily nested in artificial shelters placed amongst seagrass beds and were found to be predominantly ureotelic under natural conditions as the internal shelter water had mean urea and ammonia concentrations (N=51) of 14.2±1.6 μmol N l–1 and 8.9±0.9 μmol N l–1, respectively. Toadfish successfully spawned in shelters, providing eggs, larvae and juvenile toadfish for laboratory study. In the lab, juvenile toadfish were also ureotelic and urea was excreted in pulsatile events that accounted for 62.0±5.9% of total urea-N excreted. Excretion rates of urea-N and ammonia-N were 1.018±0.084 μmol N h–1 g–1 and 0.235±0.095 μmol N h–1 g–1, respectively. Field-collected eggs, larvae and juveniles expressed significant levels of the ornithine–urea cycle enzymes carbamoyl-phosphate synthetase III, ornithine transcarbamylase and arginase and the accessory enzyme glutamine synthetase, all of which increased in activity as toadfish developed through early life stages. In juveniles, the ammonia 96-h LC50 value was 875 μmol N l–1 and there was a 3-fold increase in ornithine transcarbamylase activity in the 1000 μmol N l–1 NH4Cl treatment. The results are discussed in the context of the causal factor(s) for ureotelism in toadfish. Furthermore, the results of this study suggest it is unlikely that the adaptive significance of ureotelism in toadfish is a means to prevent fouling nests with ammonia and in turn poisoning offspring; however, additional study is warranted.