ACTIVE BRANCHIAL AND RAM GILL VENTILATION IN FISHES

Abstract

1. Characteristics of cyclic breathing movements have been examined in a number of fish species at rest and during swimming acceleration to velocities above those sufficient to induce transfers from active to passive gill ventilation (ram gill ventilation). They were trained to swim in one of several types of swimming tunnels after electrode implants to permit recording of ECG's and ventilatory muscle EMG's.2. Transfer to the ram mode of gill breathing is marked by a drop-out of individual cyclic breathing movements as swimming increases from rest. When the swimming speed reaches about 65 cm· sec-1, most fish that use ram gill ventilation complete conversion to the ram mode (35 to 82 cm·sec-1, range of all fish tested). This is equivalent to an across-gill differential pressure of 2.1 cm H2O as measured with a combined impact-reverse pitot tube as an approximate model. Generally, a fish must be swimming to ram gill ventilate, but some exceptions are noted such as the shark-riding remoras.3. Control of the transfer between gill ventilation modes appears basically to be a reflex shut-down of rhythmic breathing initiated by mechanoreceptive detection of water flow-velocity and the detection of swimming movements. Reflex transitions between active and passive breathing seem to happen too rapidly for a sufficient change of respiratory gas concentrations to occur and allow triggering of chemoreceptors.4. No unusual events are detectable in the ECG's of swimming fish as they reach and surpass speeds sufficient to maintain the ram mode. Cardioacceleration varies as between sluggish and active swimmers, but seems to be independent of the mode of gill ventilation used.5. Generally, demersal fishes that depend mainly upon the opercular phase for active gill ventilation, strongly aided by a well-developed branchiostegal system, do not ram ventilate. Conversely, nearly all species in which the work share of active ventilation is about equally buccal and opercular, probably use ram gill ventilation when they reach the requisite swimming speed. These are mostly midwater to pelagic in habits so for them, transfer of the muscular work of gill ventilation from the branchial to the swimming musculature seems to serve a dual function—a reduction in the cost of breathing, and an improvement in swimming efficiency.