| Summary: | The retina of vertebrates has one of the highest O 2 demands and requires a constant O 2 supply. Four general types of retinal circulations have been described for O 2 delivery in extant vertebrates: 1) sole reliance on the choriocapillaris; 2) capillaries penetrating the retina; 3) capillarization of the inner side of the retina; and 4) O 2 secretion. Oxygen secretion is based on acid secretion into the blood resulting in a potent unloading of O 2 from so‐called Root effect hemoglobins (Hb), which is augmented by venous to arterial O 2 diffusion in a vascular rete lining the back side of the retina. This choroid rete mirabile (CRM) allows O 2 delivery at super‐atmospheric pressure in the avascular retina of some fishes that have the thickest retinae and some of the largest eyes (relative to body size) of all extant vertebrates. The suborder Notothenioidei, which consists of Antarctic and sub‐Antarctic fishes, provides a model system to unravel how the interactions between Hb function and retinal vascularity evolved to produce modern fish eyes. While several phyletically basal notothenioids have been shown to display similar ocular arrangements as in other Perciform species containing a CRM, more phyletically derived notothenioids have highly reduced CRM or have lost it completely. Of particular interest are the species of the family Channichthyidae, icefishes, that are unique among adult vertebrates in possessing pale white blood containing only few vestigial immature erythrocytes and no Hb. Without Hb, O 2 secretion is not functional, and icefishes have been demonstrated to possess heavily increased pre‐retinal capillarization most probably to maintain retinal oxygenation and preserve large eyes and thick retinae. However, whether available O 2 at different retinal levels actually follows the degree of vascularization has not been tested. Using thin O 2 sensors, we measured profiles of partial pressure of O 2 (PO 2 ) in the retina of six species of icefishes, two species of related, but red‐blooded ...
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