Oxygen sensitive afferent information arising from the first gill arch of yellowfin tuna

Single nerve fiber discharge was recorded from O2 sensitive receptors in the first gill arch of the yellowfin tuna, Thunnus albacares, in vitro. These receptors were innervated by the vagus nerve and increased their discharge in response to decreasing perfusion rate, decreasing perfusion PO2 and, in most fibers, to decreasing external PO2. Fibers responding to environmental hypoxia exhibited an exponential increase in discharge to decreasing external PO2 with a sensitivity similar to that exhibited by cat carotid body chemoreceptors. Indirect evidence suggests that these receptors are located near the gill vasculature and are more sensitive to changes in arterial PO2 than water PO2. Their response characteristics and hypoxic sensitivity strongly implicate them as the afferent limb in the cardiac responses and perhaps also the ventilatory responses exhibited by tuna to environmental hypoxia.