4.4 Article

Active gyrotactic stability of microswimmers using hydromechanical signals

Journal

PHYSICAL REVIEW FLUIDS
Volume 7, Issue 1, Pages -

Publisher

AMER PHYSICAL SOC
DOI: 10.1103/PhysRevFluids.7.014311

Keywords

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Funding

  1. Vetenskapsradet [2018-03974_VR]
  2. Knut and Alice Wallenberg Foundation [2019.0079]
  3. National Natural Science Foundation of China [91752205]
  4. Institute for Guo Qiang of Tsinghua University [2019GQG1012]
  5. joint China-Sweden mobility programme (NSFC-STINT) [11911530141, CH2018-7737]
  6. Swedish Research Council [2018-03974] Funding Source: Swedish Research Council

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Many plankton species migrate vertically in the ocean using a gyrotactic mechanism. This study proposes an active mechanism for gyrotactic stability that allows alignment with, or against, gravity. The mechanism is minimally affected by turbulence and is more effective than passive mechanisms.
Many plankton species undergo daily vertical migration to large depths in the turbulent ocean. To do this efficiently, the plankton can use a gyrotactic mechanism, aligning them with gravity to swim downwards or against gravity to swim upwards. Many species show passive mechanisms for gyrotactic stability. For example, bottom-heavy plankton tend to align upwards. This is efficient for upward migration in quiescent flows, but it is often sensitive to turbulence which upsets the alignment. Here we suggest a simple, robust active mechanism for gyrotactic stability, which is only lightly affected by turbulence and allows alignment both along and against gravity. We use a model for a plankton that swims with a constant speed and can actively steer in response to hydrodynamic signals encountered in simulations of a turbulent flow. Using reinforcement learning, we identify the optimal steering strategy. By using its setae to sense its settling velocity transversal to its swimming direction, the swimmer can deduce information about the direction of gravity, allowing it to actively align upwards. The mechanism leads to a rate of upward migration in a turbulent flow that is of the same order as in quiescent flows, unless the turbulence is very vigorous. In contrast, passive swimmers with typical parameters of copepods show much smaller upward velocity in turbulence. Settling may even cause them to migrate downwards in vigorous turbulence.

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