Freshwater snail feeding: lubrication-based particle collection on the water surface.

The means by which aquatic animals such as freshwater snails collect food particles distributed on the water surface are of great interest for understanding life at the air-water interface. The apple snail stabilizes itself just below the air-water interface and manipulates its foot such that it forms a cone-shaped funnel into which an inhalant current is generated, thereby drawing food particles into the funnel to be ingested. We measured the velocity of this feeding current and tracked the trajectories of food particles around and on the snail. Our experiments indicated that the particles were collected via the free surface flow generated by the snail's undulating foot. The findings were interpreted using a simple model based on lubrication theory, which considered several plausible mechanisms depending on the relative importance of hydrostatic pressure, capillary action and rhythmic surface undulation.