LILLYWHITE, H.*; SAWYER, W. G.; HEATWOLE, H.; Univ. of Florida, Gainesville; Univ. of Florida, Gainesville; North Carolina State Univ., Raleigh: A Functional Interpretation of Pulmonary Structure in Marine Snakes

Snakes exhibit exceptional variation in pulmonary structure, related in part to demands of the gravitational environment. Terrestrial and especially arboreal species have short vascular segments, which may extend <10% of body length to minimize gravitational intravascular pressures when the animal is upright. The remainder of the lung is a simple, elongate, saccular structure that does not function directly in gas exchange. In contrast, the vascularized parenchyma of aquatic species constitutes a much larger proportion of the total lung structure, with saccular terminations either absent or restricted to a relatively short posterior segment. The gravitational pressure problem is eliminated in water because the hydrostatic pressure gradient of the medium approximately equals that in the pulmonary vascular system (which may also collapse for some length due to external pressures if a snake is at depth). Empirical data and mathematical model demonstrate a hypothesized advantage of the elongated vascular parenchyma in the aquatic species: it maximizes the air volume that can be stored in the lung and remain in contact with exchange surfaces without becoming positively buoyant. In deeper diving species, the posterior saccular lung is thick, muscular and mucogenic. Moreover, in the vascular lung, gas exchange surfaces are well developed in peripheral radial compartments but not in the central lung lumen. We hypothesize that collapse of more peripheral gas exchange units and storage of lung gas in the saccular segment both act to minimize nitrogen uptake at depth. Thus, oxygen uptake from the lung might be intermittent and dependent on active muscular control to move gas from the saccular segment or lumen to peripheral exchange surfaces.