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 intracellular and extracellular pH (pH_i and pH_e, respectively; coupled pH regulation). However, coupled pH regulation may be limited at CO₂ partial pressure (P_CO ) 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 pH_i during acute exposure to >4 kPa P_CO  (preferential pH_i regulation) before pH_e compensation was observed. Here, we test the hypothesis that preferential pH_i regulation is a widespread strategy of acid-base regulation among fish by measuring pH_i regulation in 10 different fish species that are broadly phylogenetically separated, spanning six orders, eight families and 10 genera. Contrary to previous views, we show that preferential pH_i 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 that preferential pH_i 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.