Swimming Kinematics and Hydrodynamics of a Subtropical Sea Angel

Sea angels (gymnosomatous pteropods) are small zooplanktonic shell-less marine snails inhabiting the meso- and epipelagic zones. They swim in an intermediate Reynolds number regime using highly flexible, wing-like parapodia in order to capture prey, avoid predators, and perform diel vertical migration. However, the kinematics and fluid dynamics of gymnosome swimming are not well understood, particularly for species residing in low-viscosity, subtropical waters. Here we use high-speed stereophotogrammetry and dual brightfield particle image velocimetry (PIV) systems to investigate the swimming of the rare subtropical species Pneumoderma atlantica, captured off the coast of Bermuda. In particular, we quantify wing kinematics for hovering and slow upwards swimming and compare our results with morphologically similar temperate and polar species, which can be up to twice as large and swim in water up to twice as viscous. Like tiny insects flying in a similar regime, the chordwise Reynolds number appears to be inversely related to the wing angle of attack and stroke plane. Thus both the small, warm-water and the large polar gymnosomes seem to use their wings more like paddles to generate upward forces while the temperate species seems to use its parapodia more like wings to generate lift. Further, we provide the first flow measurements of a swimming gymnosome, these at somewhat higher swimming speeds, which show that gymnosomes employ an unsteady flow interaction between the wings and body (similar to the clap-and-fling mechanism) twice during each stroke cycle which likely generates additional lift. These findings provide insight into how similar locomotion modes may be adapted to different viscosities and into the widespread use of lift-generating, clap-and-fling-like mechanisms among marine snails.