Ventilatory mechanics and the effects of water depth on breathing pattern in the aquatic caecilian Typhlonectes natans

The breathing pattern in the aquatic caecilian Typhlonectes natans was inve stigated by recording airflow via a pneumotachograph under unrestrained nor mal physiological conditions. Ventilatory mechanics were assessed using air flow and pressure measurements from the buccal cavity and trachea, The brea thing pattern consisted of an expiratory phase followed by a series of 10-1 5 small buccal pumps to inflate the lung, succeeded by a long non-ventilato ry period. T, natans separate the expiratory and inspiratory gases in the b uccal cavity and take several inspiratory pumps, distinguishing their breat hing pattern from that of sarcopterygians. Hydrostatic pressure assisted ex halation. The tracheal pressure was greater than the water pressure at that depth, suggesting that pleuroperitoneal pressure as well as axial or pulmo nary smooth muscles may have contributed to the process of exhalation. The frequency of lung ventilation was 6.33 +/- 0,84 breaths h(-1), and ventilat ion occurred via the nares, Compared with other amphibians, this low ventil atory frequency suggests that T. natans may have acquired very efficient pu lmonary respiration as an adaptation for survival in their seasonally fluct uating natural habitat, Their respiratory pathway is quite unique, with the trachea separated into anterior, central and posterior regions. The anteri or region serves as an air channel, the central region is attached to the t racheal lung, and the posterior region consists of a bifurcated air channel leading to the left and right posterior lungs, The lungs are narrow, elong ated, profusely vascularized and compartmentalized. The posterior lungs ext end to approximately two-thirds of the body length. On the basis of their b reathing pattern, it appears that caecilians are phylogenetically derived f rom two-stroke breathers.