Fertilization and Sperm Chemotaxis in Ascidians

Prior to fertilization, spermatozoa of many animals and plants show chemotactic behavior toward eggs. Chemotactic behavior was first described in ferns (1) , and their attractant was identified as the bimalate ion (2 ). In animals, sperm chemotaxis to the egg was first observed in the hydrozoan Spirocodon saltatrix (3 ) and is now widely recognized in all species from cnidarians to human (for reviews, see refs. 4 –6 ).