Periodic respiration of gill-breathing fishes

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Abstract
Rhythmic and arrhythmic patterns of breathing are common among gill-breathing fishes. Irregular, short bouts of apnea occur in most fishes during feeding, while long apneic periods are routine for many open-water fishes such as scombrids, which ram ventilate during cruise swimming. During ram ventilation, the work of gill ventilation is transferred from the respiratory to the swimming musculature, with energy savings due to reductions in drag and inertial losses. Noncontinuous swimmers, such as some benthic and midwater marine and freshwater species, seldom cease rhythmic respiratory movements or resort to ram ventilation. When quiescent, they may adopt patterns of secondary cycling, alternating between respiratory pauses and short periods of rhythmic branchial pumping. Types and locations of chemo- and mechano-receptors that trigger changes in respiratory patterns of fish are being identified, as are the reflex pathways linking them to brainstem respiratory centers. A new mechanoreceptor is described that overlies the adductor mandibulae jaw muscles and may be of use in the modulation of cyclic respiratory movements. Respiratory switching control between rhythmic and ram gill ventilation is discussed.

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