Gill Na⁺/H⁺ and Cl⁻/HCO₃⁻ Exchange Systems Evolved Before the Vertebrates Entered Fresh Water Available to Purchase

Recent studies have demonstrated that the transport mechanisms for Na⁺ and Cl⁻uptake across freshwater fish gills (i.e. Na⁺/H⁺, Na⁺/NH4⁺ and C1⁻/HCO₃⁻ exchange systems) also occur in the branchial epithelium of marine teleosts and elasmobranchs, despite the presumed NaCl load in these hypo-regulating fishes (Evans, 1973, 1975a, 1977, 1982; Evans, Kormanik & Krasny, 1979). It was originally proposed (Evans, 1975b) that this occurrence was a remnant of the freshwater ancestry of the marine fishes. However, this proposition is based upon the assumption that the primary function of the system is NaCl regulation by freshwater organisms, and that the carrier which favoured the excretion of unwanted acid, nitrogen and base arose secondarily, as a means to maintain approximate electroneutrality across the epithelial membrane. The alternative proposition, viz, that acid/base regulation and nitrogen excretion by marine ancestors of the vertebrates were the driving forces for the origin of these ionic exchange systems, is an intriguing one, especially since a corollary is that the branchial transport systems necessary for NaCl regulation in fresh water actually arose before they were ‘needed’. One way to test this proposition is by examining the role that external Na⁺ and Cl⁻ play in the excretion of H⁺, ammonia, and base by hagfishes, the only chordate group which has apparently never entered fresh water, and which represent the only modern relatives of the primitive chordates that predated the entry of the vertebrates into fresh water (Hardisty, 1979).