Evolution of Oxygen Secretion in Fishes and the Emergence of a Complex Physiological System

We have reconstructed the events that led to the evolution of a key physiological innovation underpinning the large adaptive radiation of fishes, namely their unique ability to secrete molecular oxygen (O₂). We show that O₂ secretion into the swimbladder evolved some 100 million years after another O₂-secreting system in the eye. We unravel the likely sequence in which the functional components of both systems evolved. These components include ocular and swimbladder countercurrent exchangers, the Bohr and Root effects, the buffering power and surface histidine content of hemoglobins, and red blood cell Na⁺/H⁺ exchange activity. Our synthesis reveals the dynamics of gains and losses of these multiple traits over time, accounting for part of the huge diversity of form and function in living fishes.