By Orlando Timmerman
Coral: Habitual stargazers
The natural world moves to a cosmic rhythm. From the surging tides that follow the moon’s pull to the chorus of songbirds hailing the sunrise, life on Earth is intricately tied to planetary motion. Corals, it turns out, are no exception. A team of scientists has recently deepened our understanding of how corals time their spawning, investigating the roles of both moonlight and darkness1. Since coral spawning is essential for reef recovery after disturbances and has potential applications in larval propagation and transplantation, these insights are crucial to ongoing efforts to conserve our valuable reef ecosystems2.
A mythical submerged snowstorm
Coral spawning – the process in which corals release billions of sperm and egg cells, or gametes – sounds almost mythical. Once a year, in an event often likened to an underwater snowstorm, each species of coral colonies coordinate their spawning to within the period of a few nights, typically around the full moon. This synchrony maximises the chances of successful fertilisation between individuals of the same species. But since it’s a little too wet at the seafloor to keep a calendar, how do they manage this precise timing?
A spawning coral on the Great Barrier Reef. Image credit: Gabriel Guzman via Australian Geographic.
How do corals keep time?
Previous research has shown that variations in moonlight play a key role, triggering genetic and physiological processes that lead to gamete maturation and release3. However, given the amount of variation in other conditions in the messy, complex environment of the shallow oceans, the full story is far more complex. In fact, corals are picky and species-specific in their spawning cues. For example, a particular species of Acropora – the finger-like branching coral which is arguably the poster child of Indo-Pacific reefs – is one example of a coral which bucks the trend, with different colonies choosing different months to spawn, leading to a ‘split spawning’ event4. In an attempt to untangle the messy web of environmental influences, a research team set out to investigate whether this species’ spawning was a response to moonlight itself or to the period of post-sunset darkness.
To investigate this, fragments of coral were carefully removed from their home colonies and whisked across the water to environment-controlled tanks, where researchers monitored their health before relocating them to an experimental site near the Palau International Coral Reef Center. This allowed the Newcastle-based scientists better access to the coral fragments throughout the study.
To test the role of light exposure, researchers developed a simple but elegant solution: custom-made pyramidal hoods. After a period of adjustment to their new home, the corals were tucked in at sunset. Each night, for a period before and after the full moon, some of the coral fragments received a completely opaque hood – akin to a full eye-mask – which blocked out moonlight entirely. Some were kept under wraps the entire night while some were allowed to see the night sky for a period. Additional corals were given transparent covers, and a final group was left uncovered. This careful design ensured that any observed differences in spawning could be attributed to light levels rather than simply the presence of a covering.
To monitor spawning, researchers placed collection cups upside down over each fragment. Every morning, when the hoods were removed for the day, these cups were checked for signs of spawning: tiny gametes, no larger than grains of table salt. Events were classified as ‘major’ or ‘minor’ based on the number of gametes released.
A complex timepiece
The study showed that the corals spawned in two rounds: one set in March, one set in April. In contrast to previous studies, the corals were able to spawn synchronously in the absence of moonlight5. In addition, the corals kept in darkness for at least two consecutive nights advanced their spawning compared to the control samples. This indicates that periods of post-sunset darkness can act as a trigger for spawning in certain Acropora species rather than moonlight, suggesting that this mechanism is more widespread than previously thought.
Covered corals advanced their spawning times relative to those left out in the open1.
A changing ocean, a changing clock?
Despite advances in understanding how lunar phases and levels of darkness influence coral spawning, the precise mechanisms remain elusive. Clearly, light is just one piece of a complex puzzle with interactions between other factors including sea temperature complicating the picture6. Humans contribute to the complexity, with artificial light potential impacting coral spawning7; and as ocean conditions shift under climate change, we will see whether corals can adapt their timing to match new environmental cues. Future research will be essential to determine how resilient these lunar timekeepers are – and whether they can adapt to a changing world before time runs out.
1 [Main article] de la Torre Cerro, R. et al. (2025) ‘Evaluating the role of moonlight-darkness dynamics as proximate spawning cues in an Acropora coral’, Coral Reefs. Available at: https://doi.org/10.1007/s00338-025-02618-9.
2 Project Coral (2025) Horniman Museum and Gardens. Available at: https://www.horniman.ac.uk/project/project-coral/ (Accessed: 18 February 2025)
3 Kaiser, T.S. and Neumann, J. (2021) ‘Circalunar clocks—Old experiments for a new era’, BioEssays, 43(8), p. 2100074. Available at: https://doi.org/10.1002/bies.202100074.
4 Gouezo, M. et al. (2020) ‘Multispecific coral spawning events and extended breeding periods on an equatorial reef’, Coral Reefs, 39(4), pp. 1107–1123. Available at: https://doi.org/10.1007/s00338-020-01941-7.
5 Kaniewska, P. et al. (2015) ‘Signaling cascades and the importance of moonlight in coral broadcast mass spawning’, eLife, 4. Available at: https://doi.org/10.7554/elife.09991.
6 Lin, C.-H. and Nozawa, Y. (2017) ‘Variability of spawning time (lunar day) in Acropora versus merulinid corals: a 7-yr record of in situ coral spawning in Taiwan’, Coral Reefs, 36(4), pp. 1269–1278. Available at: https://doi.org/10.1007/s00338-017-1622-5.
7 Davies, T.W. et al. (2023) ‘Global disruption of coral broadcast spawning associated with artificial light at night’, Nature Communications, 14(1), p. 2511. Available at: https://doi.org/10.1038/s41467-023-38070-y.
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