Vigorous self-locomotion drives efficient mate finding in planktonic copepods swimming in turbulence

Calanoid copepods are the most abundant metazoans in the ocean. An open question is how these tiny organisms find mates when ambient flow motion challenges their limited swimming abilities and places physical constrains on their navigation. We track simultaneously and in 3D the motion of flow tracers and copepods in turbulence and we quantify the contribution of turbulence and organism motility to encounters to understand their ability to reproduce in such challenging conditions. We show that the encounter rate of inert carcasses in turbulence is lower than that of living copepods swimming in calm water, indicating that being passively transported by turbulence provides less mating opportunities than self-locomotion in calm hydrodynamic conditions. By swimming very vigorously in turbulence, copepods achieve an encounter rate that is not only larger than that resulting from advection alone, but also larger than that resulting from their less vigorous motion in calm water where swimming strategies and olfactory orientation are possible. Copepods further convert frequent encounters at short separations to actual contact events via directed motion toward nearby conspecifics. Inertial effects do not result in preferential concentration, reducing the geometric collision kernel to the clearance rate, which we model accurately by considering the separate contributions of organism motility and turbulent advection. The model shows that the mechanism underpinning higher encounter rates in copepods swimming in turbulence is the independent and vigorous self-locomotion of individual organisms. We suggest that this behavior provides evolutionary advantages because it sustains efficient mate finding in spite of the fundamental constraints imposed by turbulence advection on plankton motion. This advantage may account for the ability of planktonic calanoid copepods to thrive in turbulent environments such as estuaries and the pelagic ocean. The model is semi-empirical but requires only knowledge of the probability density function of the velocity in calm water, which is readily accessible experimentally, and therefore we expect it to remain valid for organisms displaying a wide range of motility patterns, allowing accurate predictions when studying mating, predation, and resource exploitation in the plankton.