Practice question

Broadcast spawning sea urchins release gametes freely into seawater where sperm must locate relatively immobile eggs efficiently to ensure reproductive success. Egg jelly coat surrounding oocyte releases species-specific peptides that diffuse forming stable chemical gradient extending millimeters. Spermatozoa sense gradient concentration via receptor guanylate cyclase and CatSper Ca2+ channels, adjusting flagellar waveform asymmetry to bias swimming up gradient, classic chemotaxis analogous to bacterial navigation. Electrical signals, temperature gradients or light cues are not directional guides for marine gametes. This peptide-based chemoattraction substantially increases fertilization probability. Resact and speract peptides exemplify chemical attractants guiding sperm propulsion in open ocean.