247 Fish Physiology and Biochemistry 16: 247-258, 1997. © 1997 Kluwer Academic Publishers. Printed in the Netherlands. Blood oxygen transport and hemoglobin function in three tropical fish species from northern Australian freshwater billabongs R.M.G. Wells 1 , J. Baldwin 2 , R.S. Seymour 3 and R.E. Weber 4 1 School of Biological Sciences, The University of Auckland, Private Bag 92019 Auckland, New Zealand; 2 Department of Ecology and Evolutionary Biology, Monash University, Clayton, Victoria 3168 Australia; 3 Department of Zoology, University of Adelaide, Adelaide, SA 5005, Australia; 4 Department of Zoophysiology, University of Aarhus, Aarhus C, DK-8000, Denmark Accepted: October 16, 1996 Keywords: hemoglobin, blood oxygen transport, tropical fish, osteoglossid, red cell organic phosphates Abstract The oxygen binding properties of whole blood and hemoglobin were studied in three phylogenetically dis- tant tropical teleosts inhabiting freshwater billabongs that exhibit marked seasonal oxygen stratification. The water breathing saratoga (Scleropages jardinii, Family Osteoglossidae), derived from an ancient divergence of the stem line leading to all other extant teleosts, has a low blood oxygen carrying capacity, Bohr and Root effect, and a high blood oxygen affinity. Erythrocytes contain a single major hemoglobin component, and equimolar concentrations of ATP and GTP. The tarpon (Megalops cyprinoides, Family Megalopidae), a facultative air-breather, has the highest blood oxygen carrying capacity and Bohr effect, and a low blood oxygen affinity and Root effect. Erythrocytes contain a single major hemoglobin and ATP as a putative regulator of hemoglobin oxygen affinity. Barramundi (Lates calcarifer, Family Centropomidae), is an obli- gate water- breather with intermediate blood oxygen binding properties, and the smallest Root effect. Eryth- rocytes contain at least 7 hemoglobins, and equimolar concentrations of ATP and GTP. Functional proper- ties of these three blood oxygen transport systems are considered in terms of the respiratory environment and demand for oxygen. Our interpretation supports the hypothesis that the process of speciation can lead to divergence in physiological mechanisms, irrespective of past or present selection pressures. Introduction Vertebrate hemoglobins display great flexibility in adapting to environmental conditions, metabolic demand, and modes of life. These adaptations, which involve modifications of both hemoglobin structure and the intraerythrocytic environment, raise questions about the levels of functional plas- ticity of hemoglobin-oxygen binding in different taxa. One way of addressing these questions is to make comparisons among phylogenetically di- verse animals occupying the same habitat but re- sponding to respiratory challenges through differ- ent behavioural, anatomical or physiological me- chanisms. We have taken this approach in examin- ing the oxygen binding properties of whole blood and hemoglobin from three fish species inhabiting freshwater billabongs on the monsoonal flood plains of northern Australia. The saratoga (Scleropages jardinii, Family Os- teoglossidae), a suspected facultative air breather (Merrick and Schmida 1984), belongs to a relict group of primary freshwater fishes derived from an ancient divergence of the stem line leading to all other extant teleosts (Nelson 1984). Little is known of hemoglobin regulation in this family, apart from Arapaimi gigas, being the only fish reported to uti- lize inositol polyphosphates (Isaacks et al. 1978a), 1 Correspondence to: R.M.G. Wells at the above address.