This study examined the distribution and orientation of gill O2 chemoreceptors in Oreochromis niloticus and
their role in cardiorespiratory responses to graded hypoxia. Intact fish, and a group with the first gill arch excised
(operated), were submitted to graded hypoxia and their cardiorespiratory responses (oxygen uptake -
V_ O2, breathing frequency - fR, ventilatory stroke volume - VT, gill ventilation - V_ G, O2 extraction from the
ventilatory current - EO2, and heart rate - fH) were compared. Their responses to bolus injections of NaCN
into the bloodstream (internal) or ventilatory water stream (external) were also determined. The V_ O2 of operated
fish was significantly lower at the deepest levels of hypoxia. Neither reflex bradycardia nor ventilatory
responses were completely abolished by bilateral excision of the first gill arch. EO2 of the operated group was
consistently lower than the intact group. The responses to internal and external NaCN included transient
decreases in fH and increases in fR and Vamp (ventilation amplitude). These cardiorespiratory responses
were attenuated but not abolished in the operated group, indicating that chemoreceptors are not restricted
to the first gill arch, and are sensitive to oxygen levels in both blood and water.