Corals are autogenic ecosystem engineers; the complex structures they build support a vast diversity of marine life and provide key services such as fisheries, coastal protection, tourism and recreational activities (Grafeld et al., 2017; Graham and Nash, 2013; Woodhead et al., 2019). Globally, coral reefs are threatened by anthropogenic stressors such as ocean acidification, pollution and climatic changes (Emslie et al., 2024; Frölicher et al., 2018; Hoegh-Guldberg, 1999; Hughes et al., 2017, 2003; Kleypas et al., 1999; Spalding and Brown, 2015). Ocean warming events are a particularly prevalent threat to coral reefs, as elevated sea surface temperatures (SST) can disrupt the reproductive cycles and spawning synchrony of corals (Paxton et al., 2016; Shlesinger and Loya, 2019) and can cause bleaching-induced mortality on large scales (Baker et al., 2008; Bruno and Selig, 2007; Hughes et al., 2018; Perry and Morgan, 2017a). Whilst coral bleaching can reduce a colony’s immediate growth rates, reproductive output, and larval supply, studies have shown coral reefs have the capacity to recover from mass bleaching events (Cantin and Lough, 2014; Donati et al., 2025; Gilmour et al., 2013; Gold and Palumbi, 2018; Gouezo et al., 2019; Graham et al., 2024; Pisapia et al., 2016). Reef recovery and repopulation is driven by the growth and reproduction of surviving colonies and the successful settlement of recruits (Adjeroud et al., 2017; Gilmour et al., 2013; Johnston et al., 2020). Hence, understanding the reproductive patterns of coral at the colony level is key for assessing how communities restructure following disturbances and for evaluating impacts on ecosystem function.

Reef-building corals reproduce by brooding or, for most species, via broadcast spawning events in which gametes are released into the water column for external fertilisation at the surface (Harrison, 2011). The success of broadcast reproductive events is dependent on synchronous release of gametes between colonies to enable cross-fertilisation (Levitan, 2004; Oliver and Babcock, 1992; Willis et al., 1985). The timing and extent of synchronous broadcast spawning events can vary greatly within and between taxa and biogeographic regions (Baird et al., 2022, 2009; Gouezo et al., 2020; Harrison, 2011; Harrison and Wallace, 1990; Mangubhai and Harrison, 2008; Moritz et al., 2025; Sheridan et al., 2025). An expansion of research on coral spawning in recent years has revealed reproductive patterns ranging from the highly synchronised mass spawning of over 100 different coral species as documented on the Great Barrier Reef (Babcock et al., 1986; Harrison et al., 1984; Oliver et al., 1988; Willis et al., 1985), to temporal reproductive isolation of 24 coral species studied in the Gulf of Eliat (Shlesinger et al., 1998; Shlesinger and Loya, 1985). Between these two extremes are records of multi-specific spawning (Monfared et al., 2023), asynchronous spawning (Gouezo et al., 2020), and extended breeding seasons (Mangubhai and Harrison, 2008) in other regions (summarised in Harrison, 2011). It has been hypothesised that there is a breakdown of spawning synchrony at low latitudes due to weaker fluctuations in environmental conditions that enable protracted breeding seasons (Oliver et al., 1988), as supported by studies on Kenyan reefs (Mangubhai and Harrison, 2008). However, synchronous spawning has been recorded in other equatorial regions (Gilmour et al., 2016; Gouezo et al., 2020; Monfared et al., 2023; Sheridan et al., 2025; Sola et al., 2016; Wijayanti et al., 2019).

The timing of sexual reproduction of reef-building corals has been linked to various environmental factors such as sea temperature (Keith et al., 2016; Lin and Nozawa, 2023; Nozawa, 2012; Monfared et al., 2023; Sheridan et al., 2025), solar insolation (Penland et al., 2004; van Woesik et al., 2006), regional wind fields (van Woesik, 2010), moon phase, sunrise (Bronstein and Loya, 2011; Moritz et al., 2025), and sunset (Baird et al., 2022; Lin and Nozawa, 2017). Whilst knowledge of coral reproduction has expanded in recent years, fundamental background information on the timing of spawning events is limited in some regions. Elucidating our understanding of spawning patterns across vast geographic scales is key for assessing the reproductive phylogeny of scleractinian corals and hence the impacts of climatic changes and the potential for recovery of coral reef ecosystems.

The Maldivian archipelago is considered the world’s seventh largest reef system (MEE, 2015). With 80% of all land area lying less than 1 metre above sea level, the islands that comprise the Maldives rely heavily on the 2,041 distinct coral reefs identified for protection, and provision for the population (Dhunya et al., 2017; Dryden et al., 2020; Naseer and Hatcher, 2004). In the Maldives, observations of asynchronous coral spawning in diverse assemblages over multiple lunar phases have been reported (Harrison and Hakeem, 2007). Monfared et al. (2023) noted extended breeding seasons of 22 species of Acropora over an 8 month period, detailing peak multi-specific spawning events coinciding with the transitional monsoon period, and asynchronous spawning between Acropora species and between colonies of the same species over multiple lunar phases. Further, Sheridan et al. (2025) assessed the degree of spawning synchrony of Acropora and found that larger, multi-specific spawning events occur more frequently than single species spawning events. These studies reported exact timings of Acropora spawning and correlated earlier spawning events relative to the full moon with lower tide depths, wind speeds, daily precipitation and higher sea surface temperatures.

However, to date, knowledge of spawning patterns and exact spawning times of non-Acropora corals is comparatively limited. Increasingly, restoration initiatives are utilising sexual reproduction for reef replenishment, including larval propagation, assisted fertilisation and selective breeding (Dela Cruz and Harrison, 2020; Harrison, 2024a, 2024b; Humanes et al., 2021; Koch et al., 2022; Pollock et al., 2017; Randall et al., 2020). Such techniques rely on accurately predicting when corals will spawn; advancing knowledge on the months of spawning and exact time of spawning for each region is pivotal for success and optimization of these initiatives. With both asexual and sexual restoration methods practised widely across the Maldives, understanding reproductive patterns of corals at the assemblage level is key for managing reef recovery via restorative intervention and for predicting the extent of community shifts following bleaching events. This study provides key baseline data for restoration initiatives utilising sexual reproduction. Here, we document the exact timing and sexual reproductive strategies for 375 individual colonies of 10 different genera of corals between February 2024 and April 2025. Data collection occurred before, during, and following the fourth global bleaching event confirmed by the International Coral Reef Initiative (International Coral Reef Initiative, 2024). This study describes two peak seasons for multispecific spawning events in Baa Atoll [as first identified for Acropora by Monfared et al. (2023)], as well as taxon-specific differences in reproductive synchronicity, noting instances of annual and bi-annual spawning, split spawning of individual colonies of various genera over multiple days in a lunar month and over multiple lunar months in a year. To date, this study is the first to report exact spawning times of non-Acropora genera in the Maldives.

A total of 449 observations of spawning from 375 individual colonies of 10 different genera (Acropora (N = 162), Astrea (N = 9), Astreopora (N = 16), Cyphastrea (N = 21), Dipsastrea (N = 20), Echinopora (N = 1), Galaxea (N = 60), Goniastrea (N = 134), Montipora (N = 25) and Porites (N = 2)) were recorded at Landaa Giraavaru, Baa Atoll, between February 2024 and April 2025 (Table 1). The majority of studied coral species (99.3% of colonies observed) released gametes in multi-specific spawning events during two peak seasons; in March and April (N = 326) and between October and December (N = 120) (Figure 2).

All colonies recorded spawning released gametes approximately one to three hours after sunset (Figure 3) within a few days of the full moon (Figure 4). Depending on the taxa, spawning occurred as a single event or extended spawning patterns (via asynchronous spawning amongst colonies of a species and/or split spawning of individual colonies) was observed.

Neither SST (p = 0.200) nor wind speed (p = 0.071) were statistically significant predictors of the probability of spawning occurrence in the binomial GLM (Table 2). However, solar insolation was a significant predictor (p = 0.047), with a positive association (β = 2.079 ± 1.046), indicating that higher solar insolation increased the likelihood of spawning occurring in a given month.

In the beta regression analysis, all three environmental variables (average SST, wind speed, and solar insolation) were found to significantly influence the proportion of colonies spawning per month (Figure 5). Average wind speed had the strongest effect (β = –0.709 ± 0.219, p = 0.001), with a negative association, suggesting that higher wind speeds were associated with a lower proportion of colonies spawning (Table 3). SST also showed a significant positive effect (β = 0.701 ± 0.304, p = 0.021), indicating increased spawning with higher SST. Solar insolation was similarly positively associated with the proportion of spawning events (β = 0.585 ± 0.250, p = 0.019).

The results of this study expand on previous observations of Acropora coral spawning in the Maldives (Monfared et al., 2023; Sheridan et al., 2025) and provide new information about the exact time of spawning onset and months of spawning for 13 species of 12 genera of non-Acropora corals in this region. We support the identification of two peak spawning seasons in the Maldives by Monfared et al. (2023) in March and April and between October and December, in which large, multispecific spawning events occur. We note variations in spawning patterns within and between taxa, documenting the occurrence of extended breeding via asynchronous spawning of colonies within a species and split-spawning of individual colonies across lunar months. We also note the occurrence of spawning as an annual or bi-annual event for individual colonies, demonstrating that the number of annual reproductive cycles and degree of spawning synchrony varies among taxa in this region. Our data suggests varied and complex patterns of reproduction and provides crucial foundational observations of coral spawning in the Maldives, a data-deficient region where little is known about the reproduction of non-Acropora corals.

Varied spawning patterns were observed between taxa in this study. Goniastrea retiformis colonies spawned in only one peak season per year. Temporal segregation between March/April and November spawning cohorts, coupled with a single gametogenic cycle annually may reduce gamete encounter rates and the chance of successful fertilisation (Shlesinger and Loya, 2019). Conspecific coral populations that spawn in different seasons may become reproductively isolated and experience limited gene flow, which over time can lead to genetic divergence (Rosser, 2015; Rosser et al., 2020). While our single-year dataset limits long-term inferences, these findings warrant further investigation into the genetic structure of G. retiformis populations in the Maldives. Region-wide data collected over extended time periods is needed to assess the degree of temporal segregation and potential for genetic divergence in this population.

Contrastingly, spawning of individual colonies over bi-annual spawning seasons was noted in Galaxea fascicularis and some colonies of Acropora humilis, A.plantaginea and A.secale. Split-spawning, where coral colonies release gametes over consecutive lunar months, was commonly noted in colonies G.fascicularis, for some species of Acropora, and for Cyphastrea micropthalma. This reproductive strategy has been widely documented across multiple taxa and regions (Baird et al., 2022, 2009; Bastidas et al., 2005; Bauman et al., 2011; Foster et al., 2018; Hock et al., 2019; Kongjandtre et al., 2010; Moritz et al., 2025; Willis et al., 1985). Split-spawning may lead to multiple smaller or staggered spawning events, reducing reproductive synchrony and gamete encounter rates, and increasing the vulnerability of gametes released to predation (Oliver and Babcock, 1992). However, whilst large spawning events maximise gamete encounter rates, at high gamete densities polyspermy can reduce fertilisation success (Levitan, 2004); as such, split-spawning may increase local fertilisation success by preventing gamete saturation. Further, by releasing gametes over multiple days following the full moon and in consecutive lunar months, the risk of total reproductive failure in a single event is reduced (Babcock et al., 1986; Harrison et al., 1984). Split-spawning can enable corals to realign reproductive timing with favourable environmental conditions, particularly when lunar cycles may cause spawning to occur outside optimal windows (Foster et al., 2018; Hock et al., 2019). Split-spawning is often associated with suboptimal environmental conditions in the lead-up to the full moon, or when thermal or photic thresholds are not met synchronously across coral populations (Baird et al., 2009; Harrison et al., 1984; Keith et al., 2016; Nozawa et al., 2015; Sweeney et al., 2011). Foster et al. (2018) noted that years with slower seasonal rises in SST showed increased incidences of split spawning, suggesting strong environmental cues may drive more synchronised spawning in a single lunar cycle. As such, corals in tropical regions where seasonal changes are less distinct may show less synchronised spawning. Split-spawning has been documented on the Great Barrier Reef (GBR) and in Western Australia most commonly when the full moon falls in the first half of the month (Baird et al., 2011; Gilmour et al., 2009; Willis et al., 1985), but can also occur when the full moon falls in the second half of the month (Babcock et al., 1986; Foster et al., 2015). Our study shows that split-spawning occurs in G.fascicularis and Acropora when the full moon falls in both the first and second half of the month, supporting observations of no consistent relationship between split-spawning and full moon timing. Long-term studies have shown that split-spawning is not an annual feature but rather occurs periodically (Foster et al., 2018), but recent research on daytime spawning corals has documented individual colonies of Porites rus spawning over five consecutive months each year (Moritz et al., 2025); as such, more data collected over extended time periods would be needed to identify the extent, environmental drivers, and impact on successful fertilisation of split-spawning in this region.

It has been hypothesised that equatorial reefs exhibit extended breeding seasons due to weaker environmental seasonality enabling multiple spawning events in a year (Oliver et al., 1988). Whilst the results of some studies support this hypothesis (Mangubhai and Harrison, 2008), others at similar latitudes have countered it (Baird et al., 2002; Chelliah et al., 2015; Guest et al., 2005b; Monfared et al., 2023; Gouezo et al., 2020; Novriansyah et al., 2023; Penland et al., 2004; Sola et al., 2016; Wijayanti et al., 2019). Bouwmeester et al. (2021) used data from 90 sites across the Indo-Pacific and found no correlation between latitude and reproductive synchrony, indicating that other factors drive reproductive synchronicity. In the Maldives, Monfared et al. (2023) noted that Acropora corals spawn over an extended period of eight months of the year. Further analysis of the degree of spawning synchrony found that Acropora corals in the Maldives spawn most frequently and in greater numbers during multispecific spawning events than they do in single species events, with 95.1% of colonies in Baa Atoll and 96.7% of colonies in North Male Atoll spawning during multispecific events (Sheridan et al., 2025). Our study concurs with these findings and reveals that previously unstudied genera at the same site in Baa Atoll also follow similar patterns, indicating similar environmental drivers of spawning.

The timing of peak spawning seasons in the Maldives coincide with the transition between the southwest and northeast monsoon, characterised by a reversal in wind direction and current (Aleem, 2013; Litster, 2016; Schott et al., 2009; Su et al., 2021). Similar patterns have been noted on other equatorial reefs (Wijayanti et al., 2019) and a global analysis spanning seven regions by van Woesik (2010) revealed that regional periods of low wind speeds were often tightly coupled with mass spawning events. Transitional periods of reduced wind intensity offer evolutionary advantages for coral reproduction by increasing successful fertilisation (Babcock et al., 1994) and enhancing larval retention, ultimately supporting local recruitment and population resilience (Elmhirst et al., 2009). However, low current (induced or not by wind) may limit the dispersal of larvae to other reef systems, reducing larval exchange between reef systems and genetic mixing, impacting metapopulation resilience (Cowen et al., 2000; Jones et al., 2009). As an archipelago comprising approximately 1,192 islands spread across 26 geographic atolls (Maldives National Statistical Bureau, 2020), more research from different regions is needed to assess the extent of larval dispersal and genetic connectivity in the Maldives.

In this study, while wind speed was not a significant predictor of the occurrence of spawning in a given month, it had the strongest influence on the proportion of colonies spawning each month. The largest multispecific spawning events observed occurred in March, April and November (annually); periods that displayed abrupt drops in average wind speed. Notably, these months coincided with the transitional periods between monsoon seasons in the Maldives. The Hulhangu (Southwest) monsoon occurs from May to September, while the Iruvai (Northeast) monsoon occurs from December to February (Aleem, 2013). These results are consistent with equatorial studies conducted in Palau (Penland et al., 2004), the Karimunjawa Archipelago in Indonesia (Wijayanti et al., 2019), Singapore (Guest et al., 2005a) and in Baa and North Malè Atolls in the Maldives (Monfared et al., 2023; Sheridan et al., 2025), all of which documented large multispecific coral spawning during the inter-monsoonal periods. These transition phases mark distinct and pronounced environmental shifts that may act as major spawning cues for gamete release in regions where wind conditions are rarely calm (van Woesik, 2010), and where changes in SST and solar insolation are less pronounced than in reefs at higher latitudes.

The duration of the gamete release is understood to be correlated with sustained periods of low wind speeds (van Woesik, 2010). A study on Kenyan reefs at a similar latitude to this study (4°S) found no evidence of a main spawning period to be during the inter-monsoon season; instead, a protracted breeding season of approximately 8 months was observed (Mangubhai and Harrison, 2008). This may be linked to Kenya’s less sharply defined seasonal wind transitions, with spawning occurring predominantly during the Northeast monsoon (December to March), when conditions are relatively calm and stable. Whilst our study concurs with findings of two peak spawning seasons in the Maldives coinciding with monsoonal transitions, we consider that extensive surveys of spawning in Acropora in this region revealed asynchronous spawning occurring outside of these peaks during the Northeast monsoon (Monfared et al., 2023; Sheridan et al., 2025). These patterns suggest that where there is no abrupt drop in wind speed, the persistence of calm conditions over a sustained period of time may too serve as a cue for coral spawning.

Monsoonal transitions are typically associated with rapid increases in sea surface temperature (SST), which have been identified as a key driver of coral spawning across various temporal and vast geographic scales (Keith et al., 2016; Novriansyah et al., 2023). SST has been shown to influence both gametogenic development and the timing of gamete release, with several studies correlating earlier or more synchronous spawning events with higher SSTs (Howells et al., 2014; Lin and Nozawa, 2023; Moritz et al., 2025; Nozawa, 2012; Monfared et al., 2023; Sun et al., 2024). Moritz et al. (2025) noted that at both large scales and at the individual colony level, increases in temperature induced earlier spawning. The role of SST as a spawning cue appears to vary by region and species. As such, peak spawning months could be inferred from rapid rises in SST, but not months where few colonies spawned. Sheridan et al. (2025) noted that while change in SST was a significant predictor of the number of colonies to spawn within a month, it did not significantly predict the month in which spawning occurred. Our study suggests that SST influences the intensity of spawning rather than the timing, as a higher proportion of colonies spawning in a given month were significantly correlated with higher average monthly SSTs. Sheridan et al. (2025) also found that Acropora species differed in their response to SST changes indicating that thermal cues alone do not universally regulate spawning patterns. In Palau, Gouezo et al. (2020) recorded spawning over an extended period of nine months with no consistent correlation between SST and the timing of reproductive events, suggesting that temperature alone may not serve as a universal proximate cue. These studies underscore the complexity of coral reproductive cues and highlight the need for further research to understand the interplay of various environmental factors influencing coral spawning.

Solar insolation cycles play an important role in entraining the endogenous reproductive rhythms of corals on broad seasonal scales (Mangubhai and Harrison, 2008; Gouezo et al., 2020; van Woesik et al., 2006). Studies have shown that in some equatorial regions the rate of change of solar insolation is a reliable predictor of coral spawning, with gamete release often occurring after the peak of solar insolation maxima, during the subsequent fall (Mangubhai and Harrison, 2008; Gouezo et al., 2020; van Woesik et al., 2006; Penland et al., 2004). Our study found that solar insolation significantly influenced both the likelihood of spawning occurrence and the proportion of colonies spawning in a given month. This dual influence underscores its importance as a key environmental cue. Studies have demonstrated that a combination of increased solar irradiance and high SST can result in oxidative stress to coral tissues (Yakovleva et al., 2009; Hoegh-Guldberg and Jones, 1999). As such, timing spawning to occur after peak solar insolation may serve as an evolutionary advantage to improve larval viability and reduce post-spawning stress on parent colonies. These studies highlight the complex interplay of multiple environmental factors influencing scleractinian coral spawning.

This study provides new insights into the timing, synchrony, and species-specific patterns of coral spawning in the Maldives, offering the first detailed records for several non-Acropora genera in this region. We found that larger spawning events in a given month were significantly correlated with lower wind speeds, higher sea surface temperatures, and increased solar insolation, highlighting the influence of environmental conditions on both the occurrence and extent of coral reproduction. Because more frequent and severe bleaching events can damage the reproductive fitness of corals, studying the successful reproduction and recruitment of surviving colonies is central to maintaining coral populations and restoring reef structure and function. Understanding when and how corals reproduce is critical for evaluating the potential of coral populations to recover following disturbance. With restoration initiatives in the Maldives increasingly utilising sexual reproduction (e.g. larval propagation and assisted fertilisation), these findings serve as an important baseline for reef managers, researchers, and restoration practitioners. However, as this study is based on a single year of observations from one atoll, caution must be taken in extrapolating trends across space and time. Long-term, region-wide monitoring is needed to identify the environmental drivers of coral spawning and assess how spawning patterns may shift under future climate scenarios.