THE POTENTIAL ROLE OF HYPERCARBIA IN THE TRANSITION FROM WATER-BREATHING TO AIR-BREATHING IN VERTEBRATES.

Abstract

In the transition from waterto air-breathing among the vertebrates, it is commonly theorized that aquatic hypoxia was the selective factor directing fishes toward the development and/or enhancement of air-breathing capabilities, probably during the Devonian. At this stage, the gills could still serve as a site for CO2 elimination, because of the high diffusivity of CO2 in water, thus averting respiratory acidosis. As transitional forms left the water, the gills were lost, and CO2 elimination was taken over by the skin, thus avoiding a respiratory acidosis while the [HCO3-] evolutionarily increased to buffer the increase in Pco2 associated with air-breathing. Finally, as [HCO3-] approached the high levels typical of present endotherms, the skin was no longer needed as a CO2 exchanger, the PCO2 of the body fluids could be maintained at high levels, and acute control of acid-base regulation, via the ventilatory control of CO2 elimination, was taken over by the lungs (Rahn, 1966; Rahn and Howell, 1976). This permitted the comification of the integument and consequent reduction of water loss, thereby permitting exploitation of inland environments. Studies of extant animals appear to support this theory, as fishes are characterized by a low PCO2 and [HCO3-] and achieve acid-base balance largely by regulating their [HCO3-I (Cameron, 1978; Heisler, 1980, 1982b),whileendothermshaveahighPC02and[HCO3-i and use ventilation to control pH. Amphibians are frequently noted to have intermediate levels of PCO2 and [HCO3-], but much of such data is on anurans, and they are a rather specialized group that is questionable for use as an argument in formulating evolutionary theory (Gans, 1970a, 1970b).