Acute CO tolerance in fishes is associated with air breathing but not the Root effect, red cell βNHE, or habitat.

High CO (hypercapnia) can impose significant physiological challenges associated with acid-base regulation in fishes, impairing whole animal performance and survival. Unlike other environmental conditions such as temperature and O, the acute CO tolerance thresholds of fishes are not understood. While some fish species are highly tolerant, the extent of acute CO tolerance and the associated physiological and ecological traits remain largely unknown. To investigate this, we used a recently developed ramping assay, termed the Carbon Dioxide maximum (CD), that increases CO exposure until loss of equilibrium (LOE) is observed. We investigated if there was a relationship between CO tolerance and the Root effect, β-adrenergic sodium proton exchanger (βNHE), air-breathing, and fish habitat in 17 species. We hypothesized that CO tolerance would be higher in fishes that lack both a Root effect and βNHE, breathe air, and reside in tropical habitats. Our results showed that CD ranged from 2.7 to 26.7 kPa, while LOE was never reached in four species at the maximum PCO we could measure (26.7 kPa); CO tolerance was only associated with air-breathing, but not the presence of a Root effect or a red blood cell (RBC) βNHE, or fish habitat. This study demonstrates that the diverse group of fishes investigated here are incredibly tolerant of CO and that although this tolerance is associated with air-breathing, further investigations are required to understand the basis for CO tolerance.