In mammals there are several structures that have been implicated as oxygen sensing structures; however, all of these structures are situated to detect aortic or internal hypoxia since mammals rarely run into environmental hypoxia.  These structures include the type I cells of the carotid body, the neuroepithelial bodies of the lungs as well as some central and peripheral neurons and vascular smooth muscle cells.
In fish, the neuroepithelial cells (NEC) have been implicated as the major oxygen sensing cells.  NEC have been found in all teleost fish studied to date, and are likely a highly conserved structure within many taxa of fish.  NEC are also found in all four gill arches within several different structures, such as along the filaments, at the ends of the gill rakers and throughout the lamellae.  Two separate neural pathways have been identified within the zebrafish gill arches both the motor and sensory nerve fibre pathways.  Since neuroepithelial cells are distributed throughout the gills, they are often ideally situated to detect both arterial as well as environmental oxygen.