Locomotion at intermediate Reynolds numbers: particle shadow velocimetry measurements of ctenophore swimming across species

Ctenophores (comb jellies) are the largest animals that employ cilia for locomotion. They use millimeter-scale appendages (ctenes) composed of bundled cilia, which create flow by beating metachronally in rows. Past work has provided considerable understanding of ciliary flows at the micron scale (low Reynolds numbers), but less is known about how cilia operate at intermediate Reynolds numbers. We use ctenophores as a model to investigate these intermediate-scale ciliary flows. Conventional particle image velocimetry (PIV) methods are impractical at scales around a few millimeters, as the light sheet becomes nontrivially thick compared to the field of view; additionally, flows near surfaces can be difficult to resolve due to reflections. Particle shadow velocimetry (PSV), conversely, uses a narrow focal depth in lieu of a laser sheet and can measure smaller fields of view containing complex moving boundaries. We use PSV to investigate flows generated by several species of ctenophores which have different swimming and feeding strategies. We observe differences in ctene beating frequency and consequently oscillatory Reynolds number (ranging from 15 to 75) across species. We also analyze differences and similarities between the planar velocity fields measured for each species