Ontogenesis of oxygen chemoreception in aquatic vertebrates

In aquatic vertebrates, peripheral O2 chemoreceptors initiate compensatory physiological and behavioural responses to hypoxia, beginning at very early stages of development, to maintain sufficient gas exchange across the skin or gills. This review highlights the morphological and physiological studies, particularly those of zebrafish, that have contributed to the current understanding of the development of O2 chemoreception and the response to hypoxic challenges in embryonic and larval stages of fish and amphibians. The gills appear to be the primary site of O2 chemoreception in developing aquatic vertebrates and initiate ventilatory changes, and adult-like O2-sensitive neuroepithelial cells (NECs) are found in the gills in larval stages of zebrafish and Xenopus laevis. However, evidence from zebrafish studies indicates that extrabranchial O2 chemoreceptors appear before gill NECs and regulate responses to hypoxia that develop earlier. The developmental and evolutionary significance of the internal migration of O2-chemoreceptive sites with changes in respiratory organs is also discussed.