Vertebrates reduce arterial blood pH (pHa) when body temperature increases. In water breathers, this response occurs primarily by reducing plasma HCO3 − levels with small changes in the partial pressure of CO2 (PCO2). In contrast, air breathers mediate the decrease in pHa by increasing arterial PCO2 (PaCO2) at constant plasma HCO3 − by reducing lung ventilation relative to metabolic CO2 production. Much less is known about bimodal breathers, which utilize both water and air. Here, we characterized the influence of temperature on arterial acid–base balance and intracellular pH (pHi) in the bimodal-breathing swamp eel, Monopterus albus. This teleost uses the buccopharyngeal cavity for gas exchange and has very reduced gills. When exposed to ecologically relevant temperatures (20, 25, 30 and 35°C) for 24 and 48 h, pHa decreased by −0.025 pH units (U) °C−1 in association with an increase in PaCO2, but without changes in plasma [HCO3−]. pHi was also reduced with increased temperature. The slope of pHi of liver and muscle was −0.014 and −0.019 U °C−1, while the heart muscle showed a smaller reduction (−0.008 U °C−1). When exposed to hypercapnia (7 or 14 mmHg) at either 25 or 35°C, M. albus elevated plasma [HCO3 −] and therefore seemed to defend the new pHa set-point, demonstrating an adjusted control of acid–base balance with temperature. Overall, the effects of temperature on acid–base balance in M. albus resemble those in airbreathing amniotes, and we discuss the possibility that this pattern of acid–base balance results from a progressive transition in CO2
excretion from water to air as temperature rises.