and Lenfant, 1968; Sanchez et al.,

and Lenfant, 1968; Sanchez et al., 2001). Studies involving manipulations of aquatic O2 content by far exceed studies involving manipulations of aerial O2 content. However, experimental manipulations of aerial O2 content have the potential to offer insight into the presence and role of ABO-O2 chemoreceptors. Air-breathing fish generally respond to aerial hypoxia by reducing apnoea (i.e. breath-hold) duration, whereas hyperoxia often lengthens it (Shelton et al., 1986). However, conclusions arising from experiments involving aerial O2 manipulations are limited in that the activity of chemoreceptors within the ABO cannot be discriminated from those located remotely, in the efferent vasculature (Graham, 1997). Simulated breathing by injection of either hypoxic or hyperoxic gas into the ABO produces the same results as changes in aerial gas composition (e.g. Johansen and Lenfant, 1968). However, gas injection also increases the volume of the ABO, and such experiments may therefore be confounded by activation of ABO mechanoreceptors that transmit information about the rate or extent of organ wall deformation (Milsom, 1990; Pack et al., 1990). Evidence for ABO-O2 chemoreceptors was found for the swamp eel, Monopterus albus, by Graham et al. (Graham et al., 1995). M. albus was observed to expel severely hypoxic or anoxic breaths within a few seconds of inspiration. The rapidity