Meeting Abstract
Sea butterflies, also called pteropods, are small, holoplanktonic marine snails with a shell composed of aragonite, a form of calcium carbonate that is sensitive to ocean acidification. Sea butterflies swim by flapping their highly flexible wing-like appendages. Pteropod swimming is not well studied but is important for diel vertical migration. Further, their negatively buoyant shells and pelagic lifestyle require an efficient use of energy. Previous studies show that the swimming hydrodynamics of Limacina helicina, a polar pteropod with a spiral shell, is similar to tiny insect flight aerodynamics and that unsteady lift generation techniques and forward-backward pitching are key features. However, swimming by diverse pteropod species with different shell shapes has not been examined. We present measurements of Cuvierina columnella, a warm water species with an elongated non-spiral shell collected off the coast of Bermuda. With a body length of 9 mm, wing length of 4.6 mm, mean chord length of 3.3 mm, wing beat frequency of 5 Hz, and mean swimming speed of 35 mm/s, these organisms swim at a body-based Reynolds number of approximately 300, a regime in which both inertial and viscous forces are important. Swimming kinematics acquired via a high speed stereophotogrammetry system reveals that the elongated shell correlates with reduced body pitching and that the wings bend approximately 180 degrees in each direction, overlapping at the end of each half-stroke. Time resolved two-dimensional flow measurements collected with a micro-PIV system show leading edge vortices present in both power and recovery strokes. Interactions between the overlapping wings and the shell also likely play a role in force generation.
