19Spermatozoa are attracted to their conspecific female gamete by diffusive molecules 20 released from the egg investments, a process called chemotaxis. The decapeptide 21 speract induces metabolic and permeability changes in Strongylocentrotus purpuratus 22 sea urchin sperm. In spite of decades since speract purification from S. purpuratus egg 23 investments, sperm chemotaxis has not been demonstrated in this species. By studying 24 how the stimulus function, which spermatozoa experience during the accumulation of 25 bound chemoattractants throughout their trajectory, influences both their motility 26 response and their internal Ca 2+ oscillations, we were able to show, for the first time, 27 that S. purpuratus spermatozoa exhibit chemotaxis under sufficiently steep speract 28concentration gradients. We demonstrate that this process arises through frequency 29 entrainment of the coupled metabolic oscillators. 30