The principal reasons for doubting the existence of specific CO2 chemoreceptors were (i) the observations that hyperventilatory responses to hypercapnia could be prevented or attenuated by concurrent hyperoxia an ventilation volume in rainbow trout (Oncorhynchus mykiss) regardless of blood acid–base status (Randall, 1982), (iii) the apparent absence of any physiological benefit of hyperventilation during hypercapnia given its limited impact on blood acid–base status (i.e. hyperventilation cannot prevent respiratory acidosis during exposure to elevated CO2), and (iv) the lack of a hyperventilatory response in goldfish (Carassius auratus) exposed to hypercapnia (PCO2 = 24 mmHg) (Dejours, 1973).