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

Acute ( 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.