NOTE Coral spawn timing is a direct response to solar light cycles and is not an entrained circadian response A. K. Brady Æ J. D. Hilton Æ P. D. Vize Received: 26 September 2008 / Accepted: 29 March 2009 / Published online: 12 April 2009 Ó Springer-Verlag 2009 Abstract Broadcast spawning corals release gametes into the oceans with extraordinarily accurate timing. While the date of spawning is set by the lunar cycle, the hour/minute of spawning is set by the solar cycle. In this report, we describe experiments that test whether the time of spawn- ing is regulated by an entrained biological clock or whether it is directly controlled by the solar cycle. Montastraea franksi samples were collected on the morning of the predicted spawning. Fragments from colonies were kept under three different lighting conditions and spawning monitored. The three conditions were sunset times of 0, 1 or 2 h earlier than normal. Fragments from the same colony spawned differently under these three conditions, with an early sunset causing a corresponding early shift in spawning. These results indicate that spawn timing is not controlled by a circadian rhythm and that it is directly controlled by local solar light cycle. Keywords Broadcast spawning Á Circadian rhythm Á Biological clock Á Mass spawning Introduction Broadcast spawning by corals is an amazingly precise temporal process. Every year, on just one or two evenings, many of the individuals of each broadcast spawning spe- cies release their gametes in a time window that is usually approximately 30 min long. With only a simple nerve net and no specialized light sensing organs to sense and interpret their local environment (Shelton 1975), these simple cnidarians somehow manage to tell the time of year, the time of the lunar month, and the time after sunset. Precise timing is critical in the ocean environment to achieve high fertilization rates. As corals generally cannot self-fertilize, gamete dilution and drift result in extremely low fertilization rates when corals spawn more than 2 h after their neighbors on the reef (Levitan et al. 2004). Each of the three different elements that control spawn timing season, moon phase and sunset time, must be read accu- rately to achieve the extraordinary accuracy of the broad- cast. Although it has not yet been demonstrated, it is assumed that these three independent inputs somehow converge to select the exact moment of spawning (e.g., Harrison et al. 1984; Babcock et al. 1986; Hunter 1988; van Veghel 1993; Hagman et al. 1998; Mendes and Woodley 2002; van Woesik et al. 2006). The month of spawning is set by local weather cycles. The key environmental variable correlated with the selec- tion of the month of spawning is solar insolation cycles (Penland et al. 2004; van Woesik et al. 2006), but exactly how solar radiation and the weather patterns they drive are sensed or responded to by corals remains unknown. The night of spawning is set by the lunar cycle. Lunar cycles can be used to accurately predict the broadcast spawning night in many locations (e.g., Willis et al. 1985) and altering lunar irradiance cycles can change planula release cycles in brooding corals (Jokiel et al. 1985). Low levels of light such as moonlight can be perceived by corals and can result in changes in gene transcription (Levy et al. 2007). However, exactly how lunar light entrains broadcast spawning also remains unknown. A third key environmental component of broadcast spawn timing is sunset time, which sets the Communicated by Environment Editor Prof. Rob van Woesik A. K. Brady Á J. D. Hilton Á P. D. Vize (&) Department of Biological Sciences, University of Calgary, 2500 University Drive NW, Calgary, Alberta T2N 1N4, Canada e-mail: pvize@ucalgary.ca 123 Coral Reefs (2009) 28:677–680 DOI 10.1007/s00338-009-0498-4