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

Weekly changes in the gonadosomatic index (I_G) of male golden rabbitfish Siganus guttatus showed two peaks corresponding with the new moons in June and July. The testes developed synchronously toward the first lunar quarter in June and July. After the second spawning, the sperm duct and the seminiferous tubules were occupied by residual spermatozoa and spermatogonia, respectively. Weekly changes of plasma testosterone, 11-ketotestosterone and 17,20β-dihydroxy-4-pregnen-3-one coincided with those of testicular activity. They reached their peaks 1 week before spawning coinciding with the peaks of I_G and testicular development. These cyclical changes in plasma steroid hormones suggest that lunar periodicity is a major factor that synchronizes testicular activity of S. guttatus .