Flying underwater in the optimal Strouhal number range

The Antarctic shelled pteropod (Limacina helicina antarctica, or “sea butterflies”) swims with a pair of parapodia (or “wings”) via a unique flapping propulsion mechanism that incorporates similar techniques as observed in small flying insects. It is highly unusual to observe a “flying” aquatic organism, especially in the intermediate Re regime. The pteropod achieves this locomotive behavior by hyper-pitching its body to effectively generate upward thrust during both the power and recovery strokes. For the specimens observed in this study, the shell size range is 1.8 – 4 mm, the swimming velocity is 14 – 30 mm/s, and the shell pitches forward-and-backward at 1.9 – 3 Hz. The non-dimensional variables characterizing the motion of swimming pteropods are flapping, translating, and pitching Re (i.e. Re_f, Re_U, and Re_Ω). The flapping and translating Re may be combined to form a Strouhal number based on the wing stroke amplitude, A. The results reveal that L. helicina antarctica swims in the optimal power efficiency range of St_A between 0.2 and 0.4, which is consistent with many taxa. L. helicina antarctica are threatened by ocean acidification, and the concern is that their shell degradation may adversely affect their ability to swim in the optimal St_A range.