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

Abstract

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 Ca2+ 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.