The development of the O-sensing system in an amphibious fish: consequences of variation in environmental O levels.

Proper development of the O-sensing system is essential for survival. Here, we characterized the development of the O-sensing system in the mangrove rivulus (Kryptolebias marmoratus), an amphibious fish that transitions between hypoxic aquatic environments and O-rich terrestrial environments. We found that NECs formed in the gills and skin of K. marmoratus during embryonic development and that both NEC populations are retained from the embryonic stage to adulthood. We also found that the hyperventilatory response to acute hypoxia was present in embryonic K. marmoratus, indicating that functional O-sensing pathways are formed during embryonic development. We then exposed embryos to aquatic normoxia, aquatic hyperoxia, aquatic hypoxia, or terrestrial conditions for the first 30 days of embryonic development and tested the hypothesis that environmental O availability during embryonic development modulates the development of the O-sensing system in amphibious fishes. Surprisingly, we found that O availability during embryonic development had little impact on the density and morphology of NECs in the gills and skin of K. marmoratus. Collectively, our results demonstrate that, unlike the only other species of fish in which NEC development has been studied to date (i.e., zebrafish), NEC development in K. marmoratus is largely unaffected by environmental O levels during the embryonic stage, indicating that there is interspecies variation in O-induced plasticity in the O-sensing system of fishes.