The swimming characteristics achieved by flapping wings, translating motion, and shell pitching are studied from observations of shelled Antarctic pteropods (aquatic snails nicknamed “sea butterflies”). 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 geometric scaling of the wing span ($L$), wing chord ($c$), and minor shell diameter ($d$) with respect to the major shell diameter ($D$) reveal geometric similitude. Thus, the major shell diameter ($D$) is the only length scale required to describe the size of the pteropods. The motion of swimming pteropods is characterized using flapping, translational, and rotational Reynolds numbers (i.e., ${\text{Re}}_f, {\text{Re}}_U$, and ${\text{Re}}_{\mathrm{\Omega }}$). A critical value of the flapping Reynolds number, ${\text{Re}}_f=35$, is found for the onset of translating and pitching locomotion. Finally, the relationship is obtained for the Strouhal number (${\text{St}}_A=fA/U$) for the pteropods using the geometric scalings and the translational and flapping Reynolds numbers. The Strouhal number is found to be between 0.2 and 0.4, which indicates general agreement with other oscillating organisms moving with high propulsion efficiency.

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• Biomechanics
• Locomotion
• Swimming