The amphibious fish Kryptolebias marmoratus uses different strategies to maintain oxygen delivery during aquatic hypoxia and air exposure.

Despite the abundance of oxygen in atmospheric air relative to water, the initial loss of respiratory surface area and accumulation of carbon dioxide in the blood of amphibious fishes during emersion may result in hypoxemia. Given that the ability to respond to low oxygen conditions predates the vertebrate invasion of land, we hypothesized that amphibious fishes maintain O2 uptake and transport while emersed by mounting a co-opted hypoxia response. We acclimated the amphibious fish Kryptolebias marmoratus, which are able to remain active for weeks in both air and water, for 7 days to normoxic brackish water (15‰, ~21kPa O2; control), aquatic hypoxia (~3.6kPa), normoxic air (~21 kPa) or aerial hypoxia (~13.6kPa). Angiogenesis in the skin and bucco-opercular chamber was pronounced in air- versus water-acclimated fish, but not in response to hypoxia. Aquatic hypoxia increased the O2-carrying capacity of blood via a large (40%) increase in red blood cell density and a small increase in the affinity of hemoglobin for O2 (P50 decreased 11%). In contrast, air exposure increased the hemoglobin O2 affinity (decreased P50) by 25% without affecting the number of red blood cells. Acclimation to aerial hypoxia both increased the O2-carrying capacity and decreased the hemoglobin O2 affinity. These results suggest that O2 transport is regulated both by O2 availability and also, independently, by air exposure. The ability of the hematological system to respond to air exposure independent of O2 availability may allow extant amphibious fishes, and may also have allowed primitive tetrapods to cope with the complex challenges of aerial respiration during the invasion of land.