Abstract
Gelatinous zooplankton exhibit a wide range of propulsive swimming modes. One of the most energetically efficient is the rowing behaviour exhibited by many species of schyphomedusae, which employ vortex interactions to achieve this result. Ctenophores (comb jellies) typically use a slow swimming, cilia-based mode of propulsion. However, species within the genus Ocyropsis have developed an additional propulsive strategy of rowing the lobes, which are normally used for feeding, in order to rapidly escape from predators. In this study, we used high-speed digital particle image velocimetry to examine the kinematics and fluid dynamics of this rarely studied propulsive mechanism. This mechanism allows Ocyropsis to achieve size-adjusted speeds that are nearly double those of other large gelatinous swimmers. The investigation of the fluid dynamic basis of this escape mode reveals novel vortex interactions that have not previously been described for other biological propulsion systems. The arrangement of vortices during escape swimming produces a similar configuration and impact as that of the well-studied ‘vortex rebound’ phenomenon which occurs when a vortex ring approaches a solid wall. These results extend our understanding of how animals use vortex–vortex interactions and provide important insights that can inform the bioinspired engineering of propulsion systems.
Highlights
Planktonic ctenophores typically use cilia, organized into ctene rows, for propulsion
2 hydrodynamic interactions occurring during lobe flapping by Ocyropsis spp. have not been well studied, the similarities of gelatinous body form and broad body contractions shared with oblate scyphomedusae suggest analogous hydrodynamic patterns underlying propulsion by both groups
The maximum swimming speeds normalized by body size illustrate the extraordinary capabilities of this group of oceanic ctenophores when compared with other large gelatinous zooplankton
Summary
Planktonic ctenophores typically use cilia, organized into ctene rows, for propulsion. 2 hydrodynamic interactions occurring during lobe flapping by Ocyropsis spp. have not been well studied, the similarities of gelatinous body form and broad body contractions shared with oblate scyphomedusae suggest analogous hydrodynamic patterns underlying propulsion by both groups. In this case, rowing propulsion by oblate scyphomedusae might serve as a useful model for understanding hydrodynamic processes powering escape swimming by Ocyropsis spp
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