Bending dynamics of cilia from low to intermediate Reynolds numbers

Cilia are hair-like flexible appendages used in biological systems to generate flow, typically at low Reynolds numbers, with individual cilia lengths on the order of microns. However, ctenophores (comb jellies) use longer cilia (~1mm), fused together in paddle-like ctenes, to generate flow at intermediate Reynolds numbers. We examine the bending dynamics of ctenes and compare them to low-Re cilia to find distinct differences. We used high-speed video recordings of ctenes' movements for several beating cycles to calculate the curvature along the length of each tracked ctene. From this information, we determined bending locations and corresponding angles across multiple scales and species. We compare the patterns we observe to previously published waveforms for cilia used by low-Re microorganisms and discuss potential hydrodynamic causes of observed differences. We explore the overall role of flexibility—known to increase efficiency and performance across a variety of organisms and propulsive strategies—for biological propulsors, and how it may scale from low to intermediate Re. Our results, which include kinematic tracking at high spatiotemporal resolution, also provide a foundation for future efforts toward detailed numerical modeling of flexible propulsors.