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The Journal of Experimental Biology 205, 79-89 (2002)
© 2002 The Company of Biologists Limited

Nitrogen metabolism and excretion in the mangrove killifish Rivulus marmoratus I. The influence of environmental salinity and external ammonia

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

At a field site in Belize, mangrove killifish Rivulus marmoratus inhabit hypersaline waters (up to 48 ) containing approximately 1 mmol l^–1 ammonia. We tested the hypotheses that R. marmoratus modify their nitrogen metabolism and excretion (i) by accumulating free amino acids (FAAs) and urea in the tissues during hyperosmotic stress and (ii) by shifting to ureotelism and accumulating FAAs during hyperammonia stress. Urea excretion (J_Urea) (but not ammonia excretion, J_Amm) displayed a diurnal pattern, with significantly less (75 %) urea excreted at night than during the day in both laboratory-reared clones and wild-caught killifish. When fish were exposed to hypersaline conditions (45  sea water), J_Urea was significantly reduced and tissue urea and FAA levels were elevated compared with those of control fish (15  sea water). When R. marmoratus were exposed to 0, 1, 2, 5 and 10 mmol l^–1 NH₄Cl (pH 8) for 48 h, no differences were found in J_Urea. Remarkably, prolonged exposure (10 days) to 5 mmol l^–1 NH₄Cl (pH 8) did not result in an elevation of tissue ammonia levels. In addition, tissue urea and total FAA levels did not differ between control and ammonia-exposed fish after 4 days. We propose that the euryhaline R. marmoratus retain urea and FAAs within their tissues in response to extreme osmotic stress. In contrast to many ammonia-tolerant fishes, R. marmoratus do not shift to ureotelism during prolonged hyperammonia stress, nor do they convert nitrogenous wastes into FAAs. The data suggest that killifish continue to eliminate ammonia despite an unfavourable blood-to-water gradient, thereby avoiding accumulation of ammonia.

Key words: urea, ammonia, excretion, amino acid, hyperosmotic stress, ureogenesis, ammonia detoxification, osmoregulation, ammonia exposure, mangrove killifish, Rivulus marmoratus.

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