Diel planulation by the brooding coral Favia fragum (Esper, 1797)

Numerous studies addressing annual timing of planulation in scleractinians document a tightly synchronized process that coincides with lunar periodicity. In contrast, diel variation has received little attention, yet it may have important implications for post-metamorphic success. In summer 2006 and 2007, nightly release of planulae by Favia fragum was examined in Bermuda. Planulation occurred throughout the night with a significant peak between 04:00 hrs and 06:00 hrs. The effects of larval release time on settlement behavior, metamorphic success, in situ growth and mortality were also analyzed between larvae released from 22:00 hrs to 00:00 hrs (post-sunset) and 04:00 hrs to 06:00 hrs (pre-dawn) in July of 2007. The percentage of planulae observed testing the substrate for settlement at 4-hr intervals over 48 hrs post-release was significantly higher for those released pre-dawn than those released post-sunset. Additionally, percent metamorphic success after 48 hrs of exposure to a settlement tile was significantly higher for planulae released pre-dawn. Growth rates for pre-dawn planulae over 40 days post-release were initially significantly higher than post-sunset released planulae; however, this difference did not persist over time. Planulation near dawn and the resulting benefits are hypothesized to be a consequence of immediate exposure to light, which may provide increased energy from photoautotrophic symbionts, and also serves as a settlement cue inducing rapid settlement thereby aiding in predator avoidance and natal retention.