🔹Title: How do copepods use hydrodynamic signals to 'see'?


🔹 Speaker: Tommaso Redaelli (I.R.P.H.E., Centrale Marseille, France)


🔹 Abstract: Copepods are one of the most abundant living organisms on Earth. As most plankton species, they have very limited eye, yet they manage to escape predators, attack preys and catch passive food particles. Experimental evidence show that these plankters exploit hydrodynamic signals to sense the environment. Inspired by these animals, this talk aims at two objectives: (1) modeling the hydrodynamic signals emitted by predators, preys and passive particles at low but non-negligible Reynolds number and (2) understanding how these signals can be processed to detect their source. First, we quantify the effects on motile organisms due to fluid inertia at finite Reynolds number using the method of matched asymptotic expansions (MAE). We show how the expression of the hydrodynamic force on the motile organism simplifies when fluid convective effects are small and how one can translate known inertial effects for nonmotile particles to motile ones. We then include fluid convective inertia in the calculations. We provide the analytical expressions for the hydrodynamic force and the flow field disturbance generated by an unsteady inertial active particle. Second, we develop a detection strategy based on hydrodynamic cues. Since planktonic organisms are advected by flow currents, the information  they can collect is at first order the flow gradient or its symmetric part. We describe a 'triangulation' strategy based on two measures of flow gradients. We show the good performance of this strategy for different Stokes flows and that it allows organisms to overcome a fundamental difficulty of tracking: the intrinsic symmetries of the flow.