Pulmonary Structure and Function in Marine Snakes.

The lung of marine snakes varies in structure and function related to diversity of phylogeny, behavior, and environment. All species are secondarily marine and retain dependence on aerial breathing, although some also exchange gases across the skin. Generally, there is an elongated functional right lung, and the left lung is vestigial or absent. Respiratory gases are exchanged in the 'vascular lung' segment, which may include the trachea, whereas the more distal 'saccular lung' has a poorly vascularized muscular wall, terminates blindly, and facilitates storage of oxygen. In terrestrial scansorial species of snakes, the vascularized segment of the lung is relatively short to prevent gravity-induced edema when the body is vertical or upright. In contrast, the vascular lung in marine snakes is relatively long. In the file snake Acrochordus granulatus, an exceptionally elongated vascular lung is shown to maximize oxygen storage when the snake is submerged at neutral buoyancy in shallow-water habitats. In deeper diving sea snakes, the entire lung is shorter, and the saccular segment functions to store oxygen. Movement of air within the lung is possible by means of body movements, but such behavior in naturally diving snakes is not well understood. Marine snakes avoid decompression sickness (the bends) by 'metering' the transfer of nitrogen from lung to seawater by varying pulmonary bypass of blood flow via intraventricular shunts and simultaneously varying blood flow to the skin. These complex cardiovascular actions are also influenced by the need of metabolizing tissues for oxygen uptake from lung stores or ambient water.