Using a Shell as a Wing: Fluid Dynamics and Kinematics of Atlantid Heteropod Swimming

Atlantid heteropods are a type of holoplanktonic marine snail with a single swimming fin and a coiled aragonite shell. Swimming is important for heteropod predation and diel vertical migration, but their mode of locomotion has not been well described or quantitatively studied. We used a high-speed stereophotogrammetry system to measure the 3D swimming kinematics of Atlanta selvagensis, a warm-water species collected off the coast of Bermuda. With a body length of 2.2 mm, an average beat frequency of 9 Hz, and mean swimming speed of 27 mm/s, the atlantid heteropod inhabits an intermediate Reynolds number regime (Re=50). We find that previous work incorrectly described atlantid heteropods as sculling through the water using only their swimming fin. Instead, the results show that this species uniquely synchronizes motion of its highly flexible swimming fin and of its rigid, flattened shell, both of which are used as wings, to propel itself through the water. Both the fin and shell flap through stroke angles of approximately 180º at high angles of attack and clap together at the end of each half-stroke. Time resolved 2D flow measurements with a micro-PIV system reveal the complex vortices generated by this motion and by the flow interaction between the two wings.