Swimming strategy of settling elongated micro-swimmers by reinforcement learning

Particular types of plankton in aquatic ecosystems can coordinate their motion depending on the local flow environment to reach regions conducive to their growth or reproduction.Investigating their swimming strategies with regard to the local environment is important to obtain in-depth understanding of their behavior in the aquatic environment.In the present research,to examine an impact of the shape and gravity on a swimming strategy,plankton is considered as settling swimming particles of ellipsoidal shape.The Q-learning approach is adopted to obtain swimming strategies for smart particles with a goal of efficiently moving upwards in a two-dimensional steady flow.Strategies obtained from reinforcement learning are compared to those of naive gyrotactic particles that are modeled considering the behavior of realistic plankton.It is found that the elongation of particles improves the performance of upward swimming by facilitating particles’resistance to the perturbation of vortex.In the case when the settling velocity is included,the strategy obtained by reinforcement learning has similar performance to that of the naive gyrotactic one,and they both align swimmers in upward direction.The similarity between the strategy obtained from machine learning and the biological gyrotactic strategy indicates the relationship between the aspherical shape and settling effect of realistic plankton and their gyrotactic feature.

A review on gyrotactic swimmers in turbulent flows

Many marine plankton species are motile and perform daily vertical migrations,traveling across water columns over distances of tens of meters.It is intriguing that these tiny and slow swimmers can travel in a certain direction within a turbulent environment.One way to do that is by exploiting gravitaxis,which is a form of taxis characterised by the directional movement of an organism in response to gravity.Many plankton species are able to generate a gravitational torque(e.g.,due to a nonuniform mass distribution)that reorients them upwards.However,the swimming direction is disturbed by the shearing motions and the velocity fluctuations that characterise oceanic turbulence:these can generate a viscous torque that may destabilize the swimmer.The directed locomotion resulting from the combination of gravitational and viscous torques in a flow is termed gyrotaxis,which is known to lead to a non-uniform spatial accumulation of swimmers in patches or layers.These phenomena depend strongly on the non-linear dynamics that originate from the fluid motions,and the study of gyrotactic swimmers in complex flows is attracting growing attention.Numerical simulations of the Navier-Stokes equations coupled with suitable models of gyrotactic swimmers have proven their capability to provide valuable insight into the dynamical and statistical properties of self-propelled organisms.In this paper,we review recent studies and key findings on gyrotactic swimmers in turbulent flows.First,we introduce the most recent results concerning the orientation and vertical migration of gyrotactic swimmers in isotropic turbulence.Second,we discuss the findings on the accumulation of the swimmers.Last,we review recent progresses concerning the behaviour of gyrotactic swimmers in free-surface turbulence.