Surfing along concentration filaments: sperm chemotaxis in physiological shear flows

TL;DR

This paper investigates how shear flows influence sperm chemotaxis by creating concentration filaments, revealing an optimal flow strength that enhances sperm-egg encounters through a balance of molecular spreading and filament distortion.

Contribution

The study provides a theoretical framework explaining the existence of an optimal flow strength for chemotaxis in shear flows, supported by simulations and experimental comparisons.

Findings

Identification of an optimal flow strength for chemotaxis effectiveness.

Demonstration of sperm 'surfing' along concentration filaments.

Agreement between theory, simulations, and experiments.

Abstract

Many motile biological cells navigate along concentration gradients of signaling molecules: This chemotaxis guides for instance sperm cells from marine invertebrates, which have to find egg cells in the ocean. While chemotaxis has been intensively studied for idealized conditions of rotationally symmetric gradients in still water, natural gradients are usually distorted, e.g., by turbulent flows in the ocean. Recent experiments and direct numerical simulations with sperm cells and bacteria surprisingly suggest the existence of an optimal flow strength at which chemotaxis is more effective than for still water. We use sperm chemotaxis in simple shear flow as a prototypical example to understand the origin of such an optimal flow strength theoretically: We quantify how flow accelerates spreading of signaling molecules released by the egg, but distorts the resulting concentration field into long and thin filaments. The competition between these two effects sets an optimal flow strength that maximizes sperm-egg encounter. We characterize how sperm cells `surf' along concentration filaments, typical for scalar turbulence, revealing a general navigation paradigm in the presence of flow. We compare both simulation and theory with previous experimental results and find good agreement.