BRAINERD, E.L.; LANDBERG, T.*: Ventilatory Mechanics of an Elongate Aquatic Salamander: Amphiuma tridactylum
We investigated the mechanism of lung ventilation in an aquatic salamander, Amphiuma tridactylum. Breathing episodes, which occur at intervals ranging from less than 1 to 50 min, are characterized by a single large exhalation followed immediately by a variable number of buccal pumps which refill the lungs. Unlike the majority of salamanders which use a two-stroke buccal pump, Amphiuma uses a four-stroke buccal pump. Four-stroke breathing is also found in another aquatic salmander (Cryptobranchus), in an aquatic frog (Xenopus), and in basal ray-finned fishes (eg. Amia and Lepisosteus), and probably evolved independently in all of these taxa. In Amphiuma, the combination of relatively large tidal volumes and a four-stroke buccal pump results in highly effective exchange of pulmonary gases with the environment. Electromyography of the m. transversus abdominis (TA), abdominal pressure measurements and blow-hole pneumotachography indicate that both passive and active mechanisms contribute to exhalation. Exhalation generally begins in the absence of abdominal muscle activity. This “passive” phase of exhalation may be driven by tissue elasticity, hydrostatic pressure, and/or smooth muscle contraction in the lungs. In most breaths, the passive phase is followed by an active phase in which an increase in pleuroperitoneal pressure, powered by activity in the TA, joins the passive exhalation mechanisms. The use of hypaxial mucles to power active exhalation has now been found in five salamander families (Sirenidae, Cryptobranchidae, Proteidae, Ambystomatidae, and Amphiumidae), providing evidence that aspiration breathing in amniotes may have evolved in two stages: first active exhalation only and subsequently the addition of active inhalation.