Proper development of the O2 sensing system is essential for survival, particularly in amphibious fishes that may encounter a wide range of environmental O2 levels. In this thesis, I investigated how environmental conditions during early life stages modulate the development of O2-sensitive chemoreceptors (neuroepithelial cells; NECs) and the scope for NEC flexibility in the amphibious mangrove rivulus (Kryptolebias marmoratus). NEC development in K. marmoratus was largely unaffected by environmental O2 availability during embryonic stages. However, exposure to fluctuating water-air conditions during larval and juvenile development altered the constitutive skin NEC phenotype, as well as the scope for NEC flexibility in the gills and skin of adults. Collectively, my findings suggest that the larval and/or juvenile stages may represent a critical period during which the development of the O2 sensing system and its capacity for phenotypic flexibility can be shaped by environmental conditions.