Lunar synchronization of testicular development and plasma steroid hormoneprofiles in the golden rabbitfish

Weekly changes in the gonadosomatic index (I-G) of male golden rabbitfish S iganus guttatus showed two peaks corresponding with the new moons in June a nd July. The testes developed synchronously toward the first lunar quarter in June and July. After the second spawning, the sperm duct and the seminif erous tubules were occupied by residual spermatozoa and spermatogonia, resp ectively. Weekly changes of plasma testosterone, 11-ketotestosterone and 17 ,20 beta-dihydroxy-4-pregnen-3-one coincided with those of testicular activ ity. They reached their peaks 1 week before spawning coinciding with the pe aks of I-G and testicular development. These cyclical changes in plasma ste roid hormones suggest that lunar periodicity is a major factor that synchro nizes testicular activity of S. guttatus. (C) 2000 The Fisheries Society of the British isles.