Abstract Friedrich

Sperm cells are guided to the egg by chemical cues in a process termed chemotaxis. Chemotactic navigation requires a tight feedback loop of sensing and motility. The sperm flagellum combines sensing and motility in one: Chemoattractant receptors on the flagellar surface signal to an intraflagellar signaling cascade that controls the shape of the flagellar beat.
We put forward a theory of how sampling a concentration gradient along helical paths allows sperm of marine species to steer up-gradient. This navigation principle is confirmed by recent high-speed tracking of sea urchin sperm in three space dimensions. A simple steering feedback explains how helical swimming paths bend to align with a concentration gradient. This deterministic navigation strategy is fundamentally different from those employed by most bacteria (biased random walk) or immune cells (spatial comparison), and can be shown to be particularly well suited for fast swimmers operating at the limits of chemical detection.