Synchrony on the reef: how environmental factors shape coral spawning patterns in corals in the Maldives.

Elucidating our knowledge on the reproductive phenology of scleractinian corals and the environmental drivers of reproductive synchronicity is pivotal for assessing gene flow between populations and the potential for ecosystem recovery. The timing of gamete release in sessile broadcast spawning corals is key to successful reproduction; and is dependent on a complex interaction between an organism's genes and external environmental factors. In this study we assessed the effect of various environmental factors on the spawning timing and synchronicity of corals in the Maldives. A total of 3,026 colonies from 24 species of were recorded spawning between October 2021 and May 2024: 1,709 from 20 species in North Male Atoll and 1,317 from 18 species in Baa Atoll. Generalised linear models (GLMs) were used to estimate the effect of average daily wind speed (mph), total daily precipitation (mm), tide depth (m), and mean sea surface temperature (SST) over a 30-day period prior to spawning, on the proportion of colonies to spawn per day and the spawning day deviation to full moon. Models were run for all corals, and three species in which more than 30 days of spawning were observed enabling robust statistical models to be tested: , , and , to determine the presence of species-specific relationships. Based on additional GLMs, we found that a change in SST does not determine the likelihood of spawning to occur in a given month, but does significantly predict the number of colonies to spawn per month. We also found that the relationship between SST and spawning as a predictor of probability or synchronicity on a monthly temporal scale can be species specific. We found a significant, positive correlation between daily precipitation levels and the proportion of colonies to spawn per day, however, there were some variations between species. Additionally, a higher proportion of colonies spawned closer to the full moon. Spawning events of corals closer to the full moon are significantly correlated with lower tide depths across both atolls. This knowledge will be beneficial for the management of reef systems in the Maldives following a global bleaching event, due to increased reliance on targeted conservation measures to retain diversity and re-populate degraded reefs, such as in-situ larval settlement. While our analyses of environmental factors goes someway in explaining variability in spawning patterns within the genus in the Maldives, we must also conclude there are other factors which remain unexplored, or there is a wide range of ecologically appropriate conditions for spawning. However, our results highlight the importance of considering environmental conditions, and species-specific relationships, when predicting spawning, due to the temporal and spatial deviations in timing and synchronicity observed within and between species.