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First published online May 13, 2004
Journal of Experimental Biology 207, 2011-2020 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.00956

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

John F. Barimo^1,*, Shelby L. Steele², Patricia A. Wright² and Patrick J. Walsh¹

¹ Division of Marine Biology and Fisheries, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL 33149-1098, USA
² Department of Zoology, University of Guelph, Guelph, Ontario N1G 2W1, Canada

^* Author for correspondence (e-mail: jbarimo{at}rsmas.miami.edu)

Accepted 23 February 2004

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.

Key words: ureogenesis, ureotely, Opsanus beta, toadfish, ontogeny, ammonia toxicity, ornithine–urea cycle, ornithine transcarbamylase, Florida Bay, Biscayne Bay, Batrachoididae

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SPECIAL COLLECTION: DOGMAS AND CONTROVERSIES IN THE HANDLING OF NITROGENOUS WASTES

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