Evaluating the role of moonlight-darkness dynamics as proximate spawning cues in an Acropora coral

For sessile broadcast spawning marine invertebrates, such as corals, successful sexual reproduction depends on conspecifics spawning synchronously. The precise monthly, lunar, and diel timing and the extent of synchrony, i.e., proportion of population reproducing at the same time, are likely to play a key role in coral population recovery, persistence, and adaptation. Despite its importance, the mechanisms by which different environmental factors trigger corals to spawn on specific dates within the lunar cycle remain poorly understood. Periods of darkness post-sunset around full moon of the spawning month have been shown to induce spawning in merulinid corals, whereas for Acropora, moonlight is considered the main determinant driver of night of spawning. Here, we conducted two manipulative field experiments around full moon in Palau using the common table coral Acropora aff. hyacinthus to disentangle the role of moonlight and darkness post-sunset as proximate cues. Coral fragments were assigned to three treatments providing different post-sunset darkness conditions, versus control and procedural control fragments exposed to natural conditions. In contrast to previous studies on Acropora, we found that Acropora aff. hyacinthus can spawn synchronously in the absence of moonlight during the nights leading to spawning. Corals exposed to darkness post-sunset for at least two to three consecutive nights advanced their spawning compared to controls. This finding indicates that periods of darkness post-sunset can act as an inducer for spawning in Acropora as well as in merulinid corals, suggesting that this mechanism may be more widespread than previously thought.