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Zooplankton: Not-so-passive motion in turbulence

Physicists show that despite their limited swimming abilities, zooplankton called calanoid copepods display active, energetic behaviour in turbulent flows

Springer

Imagine a species that is only one millimetre long and has only a limited swimming ability. Yet, its mobility is sufficient for moving, feeding and reproducing in freshwater and seawater. That's exactly what a type of zooplankton of the crustaceans family - namely the calanoid copepods - does. In a study published in EPJ E, physicists shed new light on how these zooplankton steer large-scale collective motion under strong turbulence. To do so, the authors study the zooplankton's small-scale motion mechanisms when subjected to background flow motion. These findings are the work of François-Gaël Michalec from the Institute of Environmental Engineering, ETH Zurich, Switzerland, and European colleagues. Ecological applications in the field of zooplankton behaviour ecology include, for example, modelling the feeding efficiency of their predator, fish larvae.

Michalec and colleagues previously showed the ability of certain calanoid copepods to adjust their swimming efforts according to the background water flow. In this study, the idea was first to quantify to which extent copepods' self-induced motion - which consists of a succession of intermittent periods of slow swimming with strong relocation jumps - results in alternately low and high swimming speeds in calm water. To do so, the authors reconstructed the trajectories of a large number of copepods swimming freely by video tracking their motion in 3D.

Then, the authors focused on elucidating the relative contribution of relocation jumps and turbulence, which is also intermittent, to the substantial fluctuations in speed that are observed in copepods swimming under background flow motion. By using an original statistical analysis method, they precisely identified the contribution of both small and large time scales to these animals' speed fluctuations.

They found that at short time scales, due to the copepods' frequent relocation jumps, the intermittent nature of their self-induced motion amplifies the intermittent properties of the underlying flow.

Reference: Characterization of intermittency in zooplankton behaviour in turbulence. F-G. Michalec, F.G. Schmitt, S. Souissi, and Markus Holzner (2015), Eur. Phys. J. E 38: 108, DOI 10.1140/epje/i2015-15108-2

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