Underwater Flight of the Pteropod

The swimming characteristics achieved by flapping wings, translating motion, and shell pitching are studied from observations of shelled Antarctic pteropods (aquatic snails nicknamed `sea butterfies'). A portable tomographic particle image velocimetry (tomo-PIV) system was used to study these pteropods (Limacina helicina antarctica) swim with a pair of parapodia (or “wings”) via a unique flapping propulsion mechanism that incorporates similar techniques as observed in small flying insects. 

The swimming velocity is typically 14 – 30 mm/s for pteropod size ranging 1.5 – 5 mm, and the pteropod shell pitches forward-and-backward at 1.9 – 3 Hz. The pitching motion of the shell effectively positions the parapodia such that they flap downwards during both power and recovery strokes. The tomo-PIV measurements reveal the influence of the vortex structure created and shed from the parapodia on the generated lift forces. 

The non-dimensional variables characterizing the motion of swimming pteropods are flapping, translating, and pitching Reynolds numbers (i.e. Ref, ReU, and ReΩ). The observed specimens swim within the same optimal Strouhal number range as observed for a broad range of species in air and water. The complex combination of body kinematics, flow structures, and resulting force balance may be significantly altered by thinning of the pteropod shell, thus making pteropods an indicator of the detrimental effects of ocean acidification.

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