Portable tomographic PIV measurements of swimming shelled Antarctic pteropods
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A portable tomographic particle image velocimetry (tomographic PIV) system is described. The system was successfully deployed in Antarctica to study shelled Antarctic pteropods (Limacina helicina antarctica)—a delicate organism with an unusual propulsion mechanism. The experimental setup consists of a free-standing frame assembled with optical rails, thus avoiding the need for heavy and bulky equipment (e.g. an optical table). The cameras, lasers, optics, and tanks are all rigidly supported within the frame assembly. The results indicate that the pteropods flap their parapodia (or “wings”) downward during both power and recovery strokes, which is facilitated by the pitching of their shell. Shell pitching significantly alters the flapping trajectory, allowing the pteropod to move vertically and/or horizontally. The pronation and supination of the parapodia, together with the figure-eight motion during flapping, suggest similarities with insect flight. The volumetric velocity field surrounding the freely swimming pteropod reveals the generation of an attached vortex ring connecting the leading-edge vortex to the trailing-edge vortex during power stroke and a presence of a leading-edge vortex during recovery stroke. These vortex structures play a major role in accelerating the organism vertically and indicate that forces generated on the parapodia during flapping constitute both lift and drag. After completing each stroke, two vortex rings are shed into the wake of the pteropod. The complex combination of body kinematics (parapodia flapping, shell pitch, sawtooth trajectory), 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.
KeywordsParticle Image Velocimetry Vortex Ring Ocean Acidification Swimming Behavior Power Stroke
We thank the United States Antarctic Program for their support on RV Laurence M. Gould and at Palmer Station, Antarctica, that made this study successful. We gratefully acknowledge Dr. David Fields (Bigelow Laboratory for Ocean Sciences), Dr. Roi Holzman (Tel Aviv University), Dr. Rajat Mittal (Johns Hopkins University), Dr. Marc Weissburg (Georgia Institute of Technology), and Dr. Jun Zhang (New York University) for many fruitful discussions and their help in collecting pteropods in Antarctica. This work is supported by the US National Science Foundation (PLR-1246296).
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