The response of the copepod Acartia tonsa to the hydrodynamic cues of small-scale, dissipative eddies in turbulence

This study quantifies the behavioral response of a marine copepod (Acartia tonsa) to individual, small-scale, dissipative vortices that are ubiquitous in turbulence. Vortex structures were created in the laboratory using a physical model of a Burgers vortex with characteristics corresponding to typical dissipative vortices that copepods are likely to encounter in the turbulent cascade. To examine the directional response of copepods, vortices were generated with the vortex axis aligned in either horizontal or vertical directions. Tomographic particle image velocimetry was used to measure the volumetric velocity field of the vortex. Three-dimensional copepod trajectories were digitally reconstructed and overlaid on the vortex flow field to quantify A. tonsa’s swimming kinematics relative to the velocity field and to provide insight to the copepod behavioral response to hydrodynamic cues. The data show significant changes in swimming kinematics and an increase in relative swimming velocity and hop frequency with increasing vortex strength. Furthermore, in moderate-to-strong vortices, A. tonsa moved at elevated speed in the same direction as the swirling flow and followed spiral trajectories around the vortex, which would retain the copepod within the feature and increase encounter rates with other similarly behaving Acartia. While changes in swimming kinematics depended on vortex intensity, orientation of the vortex axis showed minimal significant effect. Hop and escape jump densities were largest in the vortex core, which is spatially coincident with the peak in vorticity suggesting that vorticity is the hydrodynamic cue that evokes these behaviors.