Is the Air-Breathing Organ a Significant Route for CO2 Excretion During Aquatic Hypercapnia in the Pirarucu, Arapaima Gigas?

The pirarucu is one of the very few obligate air-breathing fish, employing a gigantic, highly vascularized air-breathing organ (ABO). Traditionally, the ABO is thought to serve mainly for O2 uptake (ṀO2), with the gills providing the major route for excretion of CO2 (ṀCO2) and N-waste. However, under aquatic hypercapnia, a common occurrence in its natural environment, branchial ṀCO2 to the water may become impaired. Under these conditions, does the ABO become an important route of ṀCO2 excretion to the air? We have answered this question by measuring ṀCO2 and ṀO2 in both air and water phases, as well as the pattern of air-breathing, in pirarucu under aquatic normocapnia and hypercapnia (3