Preferential intracellular pH regulation is a common trait amongst fishes exposed to high environmental CO2

Acute (< 96 h) exposure to elevated environmental CO2 (hypercarbia) induces a pH disturbance in fishes that is often compensated by concurrent recovery of intra- and extracellular pH (pHi and pHe, respectively; coupled pH regulation). However, coupled pH regulation may be limited at PCO2 tensions far below levels that some fishes naturally encounter. Previously, four hypercarbia tolerant fishes had been shown to completely and rapidly regulate heart, brain, liver, and white muscle pHi during acute exposure to>4 kPa PCO2 (preferential pHi regulation) before pHe compensation was observed. Here we test the hypothesis that preferential pHi regulation is a wide spread strategy of acid-base regulation among fish by measuring pHi regulation in 10 different fish species that are broadly phylogenetically separated, spanning 6 orders, 8 families and 10 genera. Contrary to previous views, we show that preferential pHi regulation is the most common strategy for acid-base regulation within these fishes during exposure to severe acute hypercarbia and that this strategy is associated with increased hypercarbia tolerance. This suggests preferential pHi regulation may confer tolerance to the respiratory acidosis associated with hypercarbia and we propose that it is an exaptation that facilitated key evolutionary transitions in vertebrate evolution, such as the evolution of air breathing.