THE ROLE OF OROBRANCHIAL O2 CHEMORECEPTORS ON THE CONTROL OF AQUATIC SURFACE RESPIRATION IN TAMBAQUI, Colossoma macropomum.

The present study examined the role of orobranchial O2 chemoreceptors on cardiorespiratory responses, aquatic surface respiration (ASR), and development of inferior lip swelling in the tambaqui, Colossoma macropomum, during long-term exposure to hypoxia. Intact fish (control) and two groups of denervated fish (bilateral denervation of the branches of cranial nerves V or V+VII to the orobranchial cavity) were exposed to severe hypoxia (PwO2 = 10 mmHg) for 360 min. Respiratory frequency (fR), ventilation amplitude (VAMP) and heart rate (fH) were recorded simultaneously with the occurrence of ASR. To verify the effect of hypoxia on the development of the inferior lip swelling, the dimensions of the lips were measured before and after the experiments. Control fish increased fR, VAMP and total ventilation (V & TOT), and demonstrated a significant bradycardia during the first 60 min of exposure to hypoxia. Heart rate returned to normoxic levels during the last hour hypoxia. The increases in fR, VAMP and V & TOT were not abolished by denervation in both groups of fish (V or V+VII)