Comparative study of ureogenesis in freshwater, air‐breathing teleosts

Abstract

Abstract The ureogenic potential of five species of freshwater, air‐breathing teleosts was studied. The activity of all five enzymes of the ornithine‐urea (c‐u) cycle could be assayed in four out of the five species. The enzymatic activities were generally high. Physiological levels of ammonia and urea and of enzymes of the o‐u cycle have been considered to be associated with the potential for terrestrial life in different species. The species having an efficient o‐u cycle had low levels of ammonia and high levels of urea in various tissues. Ammonia was the major excretory product in the aquatic medium in all five air‐breathing species. However, the rate of excretion of urea was greater than that of exclusively freshwater teleosts reported earlier. There was apparently no diurnal variation in the rates of excretion of ammonia. The rate of excretion of urea was significantly higher during the night than during the day in only two of the species examined, namely, Heteropneustes fossilis and Anabas testudineus . These observations strengthen our earlier suggestion that active ureogenesis through a functional o‐u cycle does exist in some freshwater, air‐breathing teleosts.