Physiology and evolution of oxygen secreting mechanism in the fisheye.

Most teleost fishes possess a unique system for tissue oxygen supply, where oxygen is delivered to the retina at partial pressures that exceed one atmosphere, providing a steep gradient for oxygen diffusion through their thick avascular retinas. This exceptional physiological system works through the elaborate interplay between highly pH-sensitive hemoglobins, acid-producing metabolic pathways, and a retinal vasculature with specialized structural and functional properties. This graphical review summarizes recent advances in understanding the mechanisms underlying retinal oxygen secretion and their impact on visual processing. Further, it discusses how the evolution of this complex physiological system provided the essential physiological exaptations for the adaptive improvements of vision in early teleost evolution. Finally, it summarizes knowledge gaps and directions for future research on this unique system for tissue oxygen supply.