Spiral-Sensing and Fold-Change Detection Direct Ascidian Sperm to the Egg

From microorganisms to marine sperm, successful navigation toward chemical cues relies on the integration of sensory inputs with locomotion. Ascidian sperms exhibit a robust chemotactic strategy by swimming along circular paths whose centres form large-scale spirals, guiding them toward attractant-releasing eggs. Although their intracellular Ca ²⁺ dynamics exhibit rhythmic bursts aligned with flagellar modulation, the underlying computational principles remain unclear. Here, we propose a minimal model that captures ascidian sperm chemotaxis through a combination of swimming trajectory control and an internal signalling model with fold-change detection (FCD). This model responds to relative rather than absolute changes in the stimulus concentration, achieving scale-invariant signal processing. Concurrently, the spiral swimming path enables spatial sampling of the concentration field, allowing the cell to infer gradient direction. Our findings demonstrate that the integration of a resilient signalling module and physically structured sampling trajectory enables effective chemotaxis in complex environments.