The aim of this study was to investigate the effects of light-dark (LD) cycles and lunar phases on ocular melatonin rhythms in the seagrass rabbitfish, Siganus canaliculatus, a lunar-synchronized spawner. Under a natural 24-hr LD (12.00:12.00) cycle, ocular melatonin levels were low during daylight hours. The levels significantly elevated to peak during the mid-dark phase at 24.00 hr and then declined sharply in the early morning around 06.00 hr. These rhythms disappeared under either constant light (LL) or constant dark (DD) conditions. Melatonin levels remained low in LL compared with those in DD condition. These results suggest that ocular melatonin rhythms in the seagrass rabbitfish are suppressed in the presence of light. When fish were exposed to natural moon phases, ocular melatonin concentrations were higher around the new moon than both the first quarter and full moon phases. Exposure to experimental new moon conditions caused a significant increase in melatonin levels while those of the fish exposed to experimental full moon conditions were decreased. These results suggest that the seagrass rabbitfish perceives moonlight through the eye and that moonlight directly causes melatonin suppression.