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Effects of Moonlight Exposure on Plasma Melatonin Rhythms in the Seagrass Rabbitfish, Siganus Canaliculatus

Tropical Biosphere Research Center, Sesoko Station, University of the Ryukyus, Motobu, Okinawa 905-0227, Japan; Marine Science Institute, University of Texas at Austin, 750 Channel View Drive, Port Aransas, Texas 78373;rahman{at}utmsi.utexas.edu

Byung-Ho Kim

Marine Research Institute, Cheju National University, Cheju-do 695-810, Korea

Akihiro Takemura

Tropical Biosphere Research Center, Sesoko Station, University of the Ryukyus, Motobu, Okinawa 905-0227, Japan

Chang-Bum Park

Young-Don Lee

Marine Research Institute, Cheju National University, Cheju-do 695-810, Korea

Influences of light-dark (LD) cycle and moonlight exposure on plasma melatonin rhythms in the seagrass rabbitfish, Siganus canaliculatus, a lunar synchronized spawner, were determined by time-resolved fluoroimmunoassay (TR-FIA). When the fish were exposed to a natural LD (12:12) cycle, plasma melatonin levels exhibited a clear daily rhythm, with higher levels at midnight and lower levels during the day. These rhythms were not evident under either constant light (LL) or constant dark (DD) conditions. Plasma melatonin levels under LL condition were low and high under DD condition. These results indicate that plasma melatonin rhythms are driven by LD cycle in this species. When the fish were exposed to the 4 lunar phases, plasma melatonin levels around the new moon were significantly higher than during the first quarter moon and the full moon. Exposure to experimentalnewmoonand full moonconditions caused significant increases and decreases of plasma melatonin levels, respectively. The synchronous rhythmicity of melatonin levels in the plasma support the hypothesis that the seagrass rabbitfish perceives moonlight intensity and responds with secretion of melatonin into the bloodstream.

Key Words: moonlight • melatonin • plasma • rabbitfish • lunar cycle

Journal of Biological Rhythms, Vol. 19, No. 4, 325-334 (2004)
DOI: 10.1177/0748730404266712


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