Practice question

Sea urchin reproduction involves broadcast spawning where eggs and sperm meet in seawater at low concentrations, requiring efficient guidance mechanisms. Eggs achieve this via diffusible small peptides that establish steep chemical gradients attracting sperm by chemotaxis. Peptides bind flagellar receptors triggering guanylate cyclase, cGMP rise, potassium efflux, intracellular alkalization, and CatSper-mediated calcium pulses causing transient flagellar asymmetry and turning up-gradient. Repeated sampling creates biased random walk toward egg source. Electrical signals, temperature gradients, or phototaxis play negligible roles in this marine invertebrate, where chemotaxis optimizes fertilization efficiency and species-specificity via peptide-receptor matching.