The ampullariid Pomacea urceus uses its ctenidium and lung in respiration. The snails reach the surface with their siphon and ventilate their lung by withdrawing their head-foot in a pumping action. The lung gas also serves to change the buoyancy of the snails. After a ventilation a snail does not necessarily return to the same buoyancy level. They may achieve overall specific gravities of <1 to > 1 after a ventilation. They may remain submerged (s.g. > 1) or float (s.g.
Bailey-Matthews National Shell Museum
Burky, Kathleen A. and Burky, Albert J., "Buoyancy Changes as Related to Respiratory Behavior in an Amphibious Snail, Pomacea Urceus (Müller), from Venezuela" (1977). Biology Faculty Publications. 320.