The ventilation mechanism of the Pacific hagfish Eptatretus stoutii

We made anatomical and physiological observations of the breathing mechanisms in Pacific hagfish Eptatretus stoutii, with measurements of nostril flow and pressure, mouth and pharyngo-cutaneous duct (PCD) pressure and velum and heart impedance and observations of dye flow patterns. Resting animals frequently exhibit spontaneous apnea. During normal breathing, water flow is continuous at a high rate (~125 ml kg⁻¹ min⁻¹ at 12°C) powered by a two-phase unidirectional pumping system with a fast suction pump (the velum, ~22 min⁻¹) for inhalation through the single nostril and a much slower force pump (gill pouches and PCD ~4.4 min⁻¹) for exhalation. The mouth joins the pharynx posterior to the velum and plays no role in ventilation at rest or during swimming. Increases in flow up to >400 ml kg⁻¹ min⁻¹ can be achieved by increases in both velum frequency and stroke volume and the ventilatory index (product of frequency x nostril pressure amplitude) provides a useful proxy for ventilatory flow rate. Two types of coughing (flow reversals) are described. During spontaneous swimming, ventilatory pressure and flow pulsatility becomes synchronised with rhythmic body undulations.