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The Journal of Experimental Biology 205, 91-100 (2002)
© 2002 The Company of Biologists Limited

Nitrogen metabolism and excretion in the mangrove killifish Rivulus marmoratus II. Significant ammonia volatilization in a teleost during air-exposure

N. T. Frick and P. A. Wright^*

Department of Zoology, University of Guelph, Guelph, Ontario, Canada N1G 2W1

*Author for correspondence (e-mail: patwrigh{at}uoguelph.ca)

Accepted 22 October 2001

The mangrove killifish Rivulus marmoratus can tolerate prolonged periods of air-exposure (>1 month). During these periods of emersion, we hypothesized that R. marmoratus would convert potentially toxic ammonia into urea and free amino acids (FAAs). In air-exposed fish, both ammonia (J_Amm) and urea (J_Urea) excretion continued at approximately 57 % and 39 %, respectively, of submerged rates. Remarkably, approximately 42 % of the total ammonia excreted during air-exposure was through NH₃ volatilization. Ammonia did not accumulate in whole-body tissues of air-exposed fish, but levels of both urea and some FAAs (primarily alanine and glutamine) were up to twofold higher after 10 days. The activities of the ornithine–urea cycle enzymes carbamoyl phosphate synthetase III and ornithine transcarbamylase increased (by approximately 30 % and 36 %, respectively) in whole-body tissues of air-exposed fish, while levels of arginase remained unchanged. The activities of enzymes involved in amino acid and oxidative metabolism were not significantly different between control and air-exposed fish. Partitioning of the anterior and posterior ends of immersed fish revealed that just over half (57 %) of the total nitrogen (ammonia+urea) was excreted through the anterior end of the fish, presumably via the branchial tissues, while emersed fish increased excretion via the posterior end (kidney+skin). R. marmoratus do not undergo a shift towards ureotelism during air-exposure. Rather, we propose that R. marmoratus are able to survive on land for extended periods without significant ammonia accumulation because they continuously release ammonia, partially by NH₃ volatilization.

Key words: urea, air-tolerant teleost, amino acid, glutamine, glutamate, alanine, urea cycle enzyme, gaseous NH₃ excretion, mangrove killifish, Rivulus marmoratus.

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