Ocean warming is increasing the frequency, extent, and severity of tropical-coral bleaching and mortality. During 2014-2017, marine heatwaves caused the Third Global Coral Bleaching Event. We analyze data from 15,066 reef surveys globally during 2014-2017. Across all surveyed reefs, 80% and 35% experienced moderate or greater (affecting >10% of corals) bleaching and mortality, respectively. We assess the global extent of coral bleaching and mortality by applying bleaching response curves calibrated from surveyed reefs to predict bleaching globally, based on comprehensive remote-sensing of heat stress. These models predict that 51% and 15% of the world's coral reefs suffered moderate or greater bleaching and mortality, respectively, during one or multiple years, surpassing damage from any prior global coral bleaching event. Our findings demonstrate that the impacts of ocean warming on coral reefs are accelerating, with the near certainty that ongoing warming will cause large-scale, possibly irreversible, degradation of these essential ecosystems. With heat stress levels during this event surpassing those observed previously, the National Oceanic and Atmospheric Administration developed more extreme Bleaching Alert levels that are now being used during the ongoing Fourth Global Coral Bleaching Event.

Corals represent a complex assemblage consisting of a host cnidarian, symbiotic dinoflagellate microalgae, and associated microbiomes and viromes, collectively called the coral holobiont. Corals are foundational to tropical reefs, yet their global decline due to climate change and other stressors creates an uncertain future for this valuable ecosystem. Metabolomics is a powerful means to unravel biochemical interactions within the holobiont that underpin coral resilience and adaptation. However, the remote nature of reefs and the analytical demands of this technique often limit its application. Untargeted metabolomics presents analytical challenges that are amplified in complex samples like corals, such as identifying the biological source of metabolites. Here, we evaluate how different sample fixation methods and time delays before storage, unavoidable in field contexts, affect coral metabolome profiles. We further present a framework for mapping metabolites in holobiont samples to their coral host and algal symbiont origins and introduce a spectral library to improve and automate annotation of coral lipids. Additionally, we demonstrate how single samples can be used concurrently for metabolomics, DNA amplification, and proteomics. Together, our study provides a streamlined, field-adaptable workflow for coral metabolomics that enables larger-scale studies and broader adoption of metabolomics in coral reef research and conservation. ### Competing Interest Statement The authors have declared no competing interest. Michigan State University Climate Change Research Support Program U.S. National Science Foundation, https://ror.org/021nxhr62, NSF-ORCC-2307516 Defense Advanced Research Projects Agency, DARPA-HR001122C0134

Anthropogenic pollutants known as xenobiotics, such as pharmaceuticals, pesticides, or human metabolites, primarily originate on land and accumulate near populated coasts, where rainfall further increases their transport and harmful impacts on marine ecosystems. However, empirical evidence for such dynamics remains limited. Here, we investigate spatial and seasonal patterns of terrigenous xenobiotics along the land-sea continuum of Curaçao. Using solid-phase extraction and untargeted LC-HR-MS/MS metabolomics, we detected approximately 30,000 molecular features, including 1,394 xenobiotics that were most abundant in terrestrial waters, declined offshore, and correlated strongly with other terrestrial input indicators (fDOM). Results from three cross-seasonal campaigns (2021-2022) revealed that rainfall and increased water residence times were dominant drivers for xenobiotic abundances along the shore, with 3.5 times higher abundances during the 2022 wet season compared to drier periods. Additional factors were local landscape features, distance to fluvial runoff, hydrodynamic connectivity, and, for specific substance groups (e.g., human metabolites or personal care products), human activities in proximity to sampling locations. Our findings show that terrigenous xenobiotics spread island-wide, making them a broader concern beyond discharge sites. This first island-wide, seasonally resolved assessment provides a baseline for managing emerging contaminants and understanding their wider impact on Caribbean coastal ecosystems.

ABSTRACT Habitat configuration governs the movement of organisms across landscapes, thereby shaping both population structure and community assembly. While theoretical and empirical studies have assessed how habitat connectivity simultaneously influences intra‐ and interspecific diversity, direct comparisons across contrasting biogeographic regions remain limited. Here, we investigate patterns of genetic and species β‐diversity in tropical reef fishes across two ocean basins with distinct spatial configurations: the Caribbean Sea and the Western Indian Ocean. Using a comparative framework based on species occurrence data from five fish families and single nucleotide polymorphism (SNP) data from 19 species, we detected significant isolation by distance at both population and community levels in the Western Indian Ocean, but only at the community level in the Caribbean Sea. Additionally, genetic and species β‐diversity were positively correlated among species in the Western Indian Ocean, but not in the Caribbean Sea. Together, these results suggest that the shorter inter‐reef distances of the Caribbean Sea promote higher connectivity, leading to a decoupling of intra‐ and interspecific β‐diversity patterns.

In 2021, groups of the Indo-Pacific damselfish Neopomacentrus cyanomos were observed on reefs in Aruba, in the southern Caribbean. In 2016, a floating accommodation platform (flotel) used to support offshore petroleum extraction was towed from the southwestern Gulf of Mexico, where the damselfish was widespread and had been recorded on other petroleum platforms, first to Aruba and then to nearby Curacao. In 2025, this study aimed to investigate whether the flotel transported N. cyanomos to those two islands. Previously published underwater videos of the submerged flotel hull, recorded in Curacao in early 2017, were reviewed. In 2025, an information request was posted on social media for sightings of the damselfish in Curacao. The 2017 videos showed many N. cyanomos living on the underside of the flotel that permanently left Curacao in August 2017. In 2025, adult N. cyanomos were recorded living under a dock in Curacao, and in 2026, they were replaced by two small fish. These are the most recent sightings of that species in Curacao. The detection of N. cyanomos on the flotel in 2017 in Curacao, followed by the sightings on the reefs of both islands, five and eight years after the platform passed by Aruba and Curacao, provides evidence that the species was likely transported via the flotel, resulting in the establishment of a population, at least in Aruba.

ABSTRACT The pillar coral Dendrogyra cylindrus is a rare but iconic member of Caribbean reefs that has suffered range-wide losses. D. cylindrus is highly susceptible to stony coral tissue loss disease (SCTLD), and the outbreak has contributed to the functional extinction of Florida’s population of pillar corals. The coral microbiome can impact the health and disease resistance of coral colonies, yet little is known about what constitutes the core microbiome of D. cylindrus . This information is crucial for comparisons of healthy and diseased tissue in pathogen identification studies and can be applied to restoration efforts as a coral health metric. Therefore, we characterized the microbiomes of D. cylindrus colonies ahead of the SCTLD disease front in Belize and Curaçao. The most prevalent members of the D. cylindrus microbial community were bacteria for which taxonomy could not be assigned confidently beyond the level of domain as well as the putatively endosymbiotic genera Endozoicomonas , Ca . Amoebophilus, and Spiroplasma . The coral reefs of Belize and Curaçao represent distinct Caribbean marine ecoregions, and we documented regional differences in strains among predominant bacterial taxa. The understudied microbiome of D. cylindrus harbors unique bacterial lineages that are in danger of extinction along with its critically endangered coral host, and these bacterial lineages may be important bioindicators during restoration efforts. IMPORTANCE Tropical corals face global extinction if average temperatures rise by 2°C (3.6°F), which may occur as soon as 2050. Included in the loss of charismatic macrofauna like the majestic pillar coral is the loss of the biological and genetic diversity of its symbionts. Here we examined the bacterial and archaeal communities associated with Caribbean pillar corals and found that the microbiome was dominated by taxonomically unclassified and putatively endosymbiotic taxa. Endosymbiotic bacteria, which live inside the coral tissue, are likely to have evolved unique adaptations to become symbionts and may be important to the health and success of pillar corals in ecosystem restoration efforts.

Multicellularity evolved independently multiple times in eukaryotes 1–4 . Two distinct mechanisms underpin multicellularity 5 : clonality (serial cell division without sister-cell separation) and aggregation (whereby independent cells assemble into a multicellular entity). Clonal and aggregative multicellularity are traditionally considered to be mutually exclusive 1,6–8 , with rare exceptions 9 , and evolutionary hypotheses have addressed why multicellularity might diverge towards one or the other extreme 3,4 . Both animals and their sister group, the choanoflagellates, are currently known to acquire multicellularity only clonally 4,10,11 . Here we show that the choanoflagellate Choanoeca flexa 12 forms motile and contractile cell monolayers (sheets) through multiple mechanisms— C. flexa sheets can form purely clonally, purely aggregatively or through a combination of both processes. We characterize the life history of C. flexa in its natural environment—ephemeral splash pools on the island of Curaçao—and show that C. flexa undergoes reversible transitions between unicellularity and multicellularity during evaporation–refilling cycles. Different splash pools house genetically distinct strains of C. flexa and kin recognition constrains aggregation between them. We show that clonal-aggregative multicellularity is a versatile strategy for the robust establishment of multicellularity in this variable and fast-fluctuating environment. Our findings challenge former generalizations about choanoflagellates and expand the option space of choanozoan multicellularity.

Exometabolites released by benthic primary producers (BPP) are an integral part of the coral reef food web. Depending on their origin and composition these complex mixtures of dissolved organic compounds support a distinct microbial community. Which exudate components are preferentially used by microbes, how this preference differs between exudate types, and what molecular features drive the microbial community differentiation is still poorly understood. Here we use an untargeted metabolomics approach (liquid chromatography-tandem mass spectrometry (LC/MS-MS)) to assess the microbial uptake of exudates produced by BPP (mixed coral community, macroalgae, turf algae, and coral-macroalgae, coral-turf algae). We can show that that exudate compounds and especially those unique to a specific BPP or mixed community are the most favored substrate for microbes in the respective communities. Our data suggests that in each BPP treatment the unique combination of organic compounds is the main driver selecting for a specific microbial community composition rather than a specific single substance. This emphasizes the complexity of mechanisms and metabolisms that constitute and structure communities in ecosystems as intricate as coral reefs.

Coral reefs are critical ecosystems and biodiversity hotspots that provide ecological stability and essential services to coastal communities. The coral holobiont, a complex symbiotic system composed of the coral animal and a diverse array of associated microbes, plays a central role in coral health and resilience. While Symbiodiniaceae and bacterial symbionts have been extensively studied, much less is known about the diversity and function of microbial eukaryotes such as protists and fungi. These organisms are increasingly recognized as important, yet remain vastly underexplored. Here, we present the first global survey of the coral-associated eukaryome using an anti-metazoan 18S rRNA primer set to bypass host DNA amplification. Our dataset includes corals and related anthozoans from the Caribbean Sea, the Red Sea, and several locations in the Pacific Ocean, spanning a broad taxonomic and geographic range, and includes both healthy and diseased specimens. They reveal a eukaryome that is not only more diverse than that of global coastal waters but also surpasses the diversity of the well-studied coral bacterial microbiome. We recover diverse microbial eukaryotic communities, including Symbiodiniaceae, other known symbionts, potential pathogens, and previously uncharacterized lineages. These results reveal consistent patterns across coral groups and geographic regions. This study provides the most comprehensive taxonomic overview of coral-associated microbial eukaryotes to date, offering new insights into their roles within the holobiont. Our findings highlight the ecological significance of microbial eukaryotes and underscore the importance of incorporating them into broader coral reef research and conservation strategies. ### Competing Interest Statement The authors have declared no competing interest. Canadian Institutes of Health Research, MOP-42517 European Commission, FP7-PEOPLE-2012IOF - 331450 CAARL Tula Foundation, https://ror.org/0029jxk29 University of Miami, https://ror.org/02dgjyy92

Diel rhythms align physiological processes with light/dark cycles, driving predictable oscillations in gene expression and protein activity through tightly controlled transcriptional-translational feedback loops. This study presents in situ transcriptomic analyses of the stony coral Pseudodiploria strigosa and its photosymbionts, Breviolum sp., at key daily time points. P. strigosa shows precise transcriptional control: dawn triggers a molecular reset marked by RNA metabolism and protein turnover; midday emphasizes anabolic and phosphate-regulated pathways; dusk reflects transitional lipid and amino acid metabolism; and midnight reveals stress responses, mRNA catabolism, and mitochondrial organization. Photosymbionts display subtler diel patterns, with photoprotection at dawn, metabolite transport and nitrogen cycling through midday/dusk, and cell cycle and ion homeostasis at night. Microbial communities show time-dependent restructuring of co-occurrence networks, driving diel-related functional consequences like changes in microbial metabolism. These findings present a system-level molecular framework of diel regulation across the coral-photosymbiont-microbe holobiont, revealing time-specific transcriptional control of coordinated function and homeostasis.

Coral reef ecosystems are increasingly exposed to a wide variety of anthropogenic pollutants. This study evaluated the spatial and seasonal distribution of land-derived substances on fringing reefs of the Caribbean island of Curaçao during two wet- and two dry seasons (2021-2023), to determine whether spatial differences in δ15N values can serve as a proxy for other forms of pollution, including nutrient availability and potentially toxic elements. The spatial distribution of δ15N signatures of macroalgal tissues along the island's coast was compared to tissue concentrations of 32 nutrients and non-essential elements. Tissue concentrations of many micronutrients (Cu, Fe, Mn, Se, Si, Zn) and non-essential elements (Al, Pb, Ti) correlated positively with δ15N. Elemental concentrations and δ15N values showed a strong positive correlation with the amount of coastal alteration. Higher rainfall was associated with elevated δ15N values, and this association intensified with increased coastal alterations, while δ15N values decreased with depth. Elemental pollution was measured during one wet season, but since it correlated positively with δ15N, it is likely to follow similar temporal and spatial patterns as δ15N values. Certain non-essential elements were found in high concentrations at only a few sites near industrial, heavily developed areas, and a landfill. Despite such local deviations for specific elements, our findings support the use of isotopic signatures in benthic organisms as a proxy for anthropogenic inputs on coral reefs and show that precipitation increases the effect of land-use changes on pollution run-off to nearby coral reefs.

Coral reefs have experienced extensive coral loss and shifts in community composition worldwide. Despite this, some coral species appear naturally more resistant, such as Madracis mirabilis (herein Madracis ). Madracis has emerged as the dominant hard coral in Curaçao, comprising 26% of coral cover in reefs that declined by 78% between 1973 and 2015. Although life history traits and competitive mechanisms contribute to Madracis ’ success, these factors alone may not fully explain it, as other species with similar traits have not shown comparable success. Here, we investigated the potential role of microbial communities in the success of Madracis on Curaçao reefs by leveraging a low-bias bacterial and viral enrichment method for metagenomic sequencing of coral samples, resulting in 77 unique bacterial metagenome-assembled genomes and 2,820 viral genomic sequences. Our analyses showed that Madracis -associated bacterial and viral communities are 1.24-fold and 1.61-fold richer than the communities of five sympatric coral species combined. The Madracis -associated bacterial community was dominated by Ruegeria and Sphingomonas , genera that have previously been associated with coral health, defense against pathogens, and bioremediation. The viral community exhibited a 50% enrichment of proviruses relative to the viral communities of other corals. These proviruses have the genomic capacity to laterally transfer genes involved in antibiotic resistance, central metabolism, and oxidative stress responses, potentially enhancing the adaptive capacity of the Madracis microbiome and contributing to Madracis ’ success on Curaçao’s reefs. IMPORTANCE Understanding why some coral species persist and thrive while most are in fast decline is critical. Madracis mirabilis is increasingly dominant on degraded reefs in Curaçao, yet the role of microbial communities in its success remains underexplored. This study highlights the potential role of Madracis- associated bacterial and viral communities in supporting coral resilience and competitive success. By identifying key microbial partners and viral genes that may enhance host stress tolerance and defense against pathogens, we broaden the understanding of how the coral holobiont contributes to species persistence under environmental stress. These insights are valuable for predicting reef community shifts in a changing climate and open avenues for microbiome-informed strategies to support coral conservation and restoration.

Coral reefs have experienced widespread and accelerated decline, driven by a combination of global and local anthropogenic stressors. To contextualize these changes, we compared the composition of coral reef communities on Curaçao between 1973 and 2023 with that of corals preserved in fossil reefs from the Last Interglacial period (128–116 ka). These fossil reefs, exposed along the island’s leeward coast, provide a multi-millennial baseline of ecological variability. Here we show that the ecological transformation observed on modern reefs over the past five decades is unmatched when compared to the relatively stable community structure maintained for more than 12,000 years during the Last Interglacial. We propose that the global, rapid, and well-documented collapse of tropical coral reef ecosystems since the mid-20th century represents a stratigraphically relevant signal of anthropogenic change. We surmise that a well-characterized reef site—such as Curaçao—could, in principle, serve as the Global Boundary Stratotype Section and Point (GSSP) marking the onset of the Anthropocene.

Corals (Cnidaria; Anthozoa) play critical roles as habitat-forming species with a wide range, from warm shallow-water tropical coral reefs to cold-water ecosystems. They also represent a complex ecosystem as intricate holobionts made up of microbes from all domains of the Tree of Life that can play significant roles in host health and fitness. The corallicolids are a clade of apicomplexans that infect a wide variety of anthozoans worldwide and can influence the thermal tolerance of habitat-forming corals. Despite their potentially important impacts on reef ecosystems, much of the basic biology and ecology of corallicolids remains unclear. Apicomplexans often have a closed life cycle, with minimal environmental exposure and sometimes multiple hosts. Corallicolids have only been documented in anthozoan hosts, with no known secondary/reservoir hosts or vectors. Here, we show that abundant corallicolid sequences are recovered from bearded fireworms (Hermodice carunculata) in tropical reef habitats off Curaçao and that they are distinct from corallicolids infecting the corals on which the fireworms were feeding at the time of their collection. These data are consistent with a fireworm-specific corallicolid infection, not merely a byproduct of the worms feeding on infected corals. Furthermore, we suggest that H. carunculata is potentially a vector moving corallicolids among coral hosts through its feces. These findings not only expand our understanding of the ecological interactions within coral reef ecosystems but also highlight the potential role of host-associated parasites in shaping the resilience of reef habitats.

Corals (Cnidaria; Anthozoa) play critical roles as habitat-forming species with a wide range, from warm shallow-water tropical coral reefs to cold-water ecosystems [1-3]. They also represent a complex ecosystem as intricate holobionts made up of microbes from all domains of the Tree of Life, where many play significant roles in host health and fitness [4]. The corallicolids are a clade of apicomplexans that infect a wide variety of anthozoans across the world, and have been shown to influence the thermal tolerance of habitat-forming corals [1, 5]. Despite their potentially important impacts on reef ecosystems, much of the basic biology and ecology of corallicolids remains unclear. Apicomplexans often have a closed life cycle, with minimal environmental exposure, and sometimes multiple hosts. Corallicolids have only been documented in anthozoan hosts, with no known secondary/reservoir hosts or vectors [6]. Here, we show that abundant corallicolid sequences are recovered from bearded fireworms ( Hermodice carunculata ) in tropical reef habitats off Curacao, and that they are distinct from corallicolids infecting the corals on which the fireworms were feeding at the time of their collection. The data are consistent with an active infection of fireworms, as opposed to corallicolids being a byproduct of feeding on infected corals, and we propose that H. carunculata is potentially a vector moving corallicolids between coral hosts through its faeces. These findings not only expand our understanding of the ecological interactions within coral reef ecosystems but also highlight the potential role of host-associated parasites in shaping the resilience of reef habitats. ### Competing Interest Statement The authors have declared no competing interest.

Nutrient pollution has been a major contributor to coral decline throughout the Caribbean. Coral physiological responses to excess nutrients vary with nutrient forms (e.g. nitrate or ammonia), concentrations and nitrogen-to-phosphate (N : P) ratios. However, how these responses differ across nutrient contexts remains understudied. We show that Orbicella annularis photosymbiont densities respond differently to excess nitrogen in phosphorus-limited versus nitrogen-limited environments. Along Curaçao's leeward reef, excess nitrogen significantly decreased (p < 0.05) photosymbiont density under phosphorus-limited conditions (N : P > 16) with low phosphorus (mean = 0.07 µM ± 0.06). In contrast, data from Barbados indicate a significant increase (p < 0.01) in photosymbiont density under nitrogen-limited conditions (N : P < 16). These findings highlight how nutrient contexts shape coral responses to nitrogen inputs, emphasizing the need to consider nutrient dynamics in coral conservation strategies.

ABSTRACT Fishing pressure is the primary threat to coastal elasmobranch populations, and understanding its impact requires long‐term regional data—often lacking in complex, small‐scale fishery settings. This is the case for Curaçao, a southern Dutch Caribbean island with an unmonitored artisanal fishery where, according to anecdotes, elasmobranchs have severely declined but continue to be landed. In such data‐limited regions, fishers' local ecological knowledge (FLEK) is a valuable tool for reconstructing historical baselines. Using FLEK from 21 surveys, we quantified historical and current elasmobranch diversity around Curaçao. Participatory mapping identified spatial distributional changes of 14 elasmobranch species, comparing the time of the surveys with fishers' career beginnings. Temporal trends were analysed alongside shifts in fishing efforts, socioeconomic contexts and perceptions of fishery management. Between 1957 and 2009, we identified 36 spatial hotspots of elasmobranch richness, which declined to 14 hotspots from 2010 to 2022, with a 4.3‐fold greater likelihood of hotspots occurring in the past. Species richness in these areas significantly decreased from 7.44 ± 1.00 (mean ± s.e.) to 3.00 ± 1.18 species, while the number of fishers increased from 2.86 ± 0.23 fishers to 5.14 ± 0.49 per hotspot. Although not targeted, incidental elasmobranch catches are commonly retained. Most fishers expressed a desire for increased inclusion in fishery management but viewed elasmobranch‐specific measures as unnecessary, perceiving local populations as healthy. We thus provide critical spatial baseline data for evidence‐based conservation of elasmobranchs around Curaçao while emphasising the benefits and importance of engaging small‐scale fishers in managing elasmobranch populations.

Coral reefs have undergone extensive coral loss and shifts in community composition worldwide. Despite this, some coral species appear naturally more resistant, such as Madracis mirabilis (herein Madracis). Madracis has emerged as the dominant hard coral in Curaçao, comprising 26% of coral cover in reefs that declined by 78% between 1973 and 2015. Although life history traits and competitive mechanisms contribute to Madracis's success, these factors alone may not fully explain it, as other species with similar traits have not shown comparable success. Here, we investigated the potential role of microbial communities in the success of Madracis on Curaçao reefs by leveraging a low-bias bacterial and viral enrichment method for metagenomic sequencing of coral samples, resulting in 77 unique bacterial metagenome-assembled genomes and 2,820 viral genomic sequences. Our analyses showed that Madracis-associated bacterial and viral communities are 12% and 20% richer than the communities of five sympatric coral species combined. The Madracis-associated bacterial community was dominated by Ruegeria and Sphingomonas, genera that have previously been associated with coral health, defense against pathogens, and bioremediation. These communities also displayed higher functional redundancy, which is often associated with ecological resilience. The viral community exhibited a 50% enrichment of proviruses relative to other corals. These proviruses had the genomic capacity to laterally transfer genes involved in antibiotic resistance, central metabolism, and oxidative stress responses, potentially enhancing the adaptive capacity of the Madracis microbiome and contributing to Madracis's success on Curaçao's reefs. IMPORTANCE:Understanding why some coral species persist and thrive while most are in fast decline is critical. Madracis mirabilis is increasingly dominant on degraded reefs in Curaçao, yet the role of microbial communities in its success remains underexplored. This study highlights the potential role of Madracis-associated bacterial and viral communities in supporting coral resilience and competitive success. By identifying key microbial partners and viral genes that may enhance host stress tolerance and defense against pathogens, we broaden the understanding of how the coral holobiont contributes to species persistence under environmental stress. These insights are valuable for predicting key microbial community players in reef interactions and may inform microbiome-based strategies to support coral conservation and restoration.

The thermal structure of tropical reef systems is shaped by air-sea interactions, turbulent mixing, and subsurface-driven processes, yet their complex dynamics and interactions are not well understood. This study uses in situ observations and global model outputs to investigate the modulation of subsurface ocean properties by wind-driven Ekman transport, turbulent overturning, and semidiurnal temperature fluctuations, along a 70 km-long reef island coastline. Easterly trade winds prevailed for 80% of the year, during which coastal downwelling was favorable along the majority of the leeward coastline, with significant sub-island scale variability. In the surface Ekman layer, coastal downwelling and surface turbulent mixing modulated subsurface warming and mixed layer deepening. During periods of weaker winds, near-surface waters were less turbulent and buoyancy fluxes allowed for restratification. At all times, turbulence and mixing were intensified below the Ekman layer, and isopycnal depths were episodically modulated at semidiurnal frequency. On the reef, temperatures responded to Ekman transport and also varied at sub-inertial time-scales, specifically at semidiurnal frequencies. On the 60 m-deep reefs, semidiurnal temperature fluctuations drove cooling by up to 4°C. Wind and internally driven subsurface turbulence further stimulated vertical fluxes of heat and mass, relevant to local biophysical responses. This work reinforces the need to analyze the dynamic processes that regulate the subsurface biophysical structure in tropical island ecosystems.

Coral species abundance and biodiversity estimates are typically based on colony macromorphology. However, such measurements often underestimate the true diversity within coral communities because morphology does not necessarily reflect behavioral or genetic divergence. We previously reported on the unusual spawning behavior of the brain coral Diploria labyrinthiformis (Linnaeus, 1758) in Curaçao, Southern Caribbean, where this species spawns in both spring and autumn. Here, using data collected from 2013 to 2021, we show that in Curaçao, D. labyrinthiformis comprises two behaviorally and genetically distinct lineages, with 93% of colonies spawning exclusively in one season or the other. The two lineages could not be distinguished based on obvious macromorphological differences or depth but represented clearly distinct genetic clusters ( F ST  = 0.098) based on genome-wide sequencing. We tested for prezygotic and postzygotic gametic barriers between them by fertilising eggs released in spring 2019 with sperm collected and cryopreserved in autumn 2018. Fertilisation in this unidirectional cross was successful and the resulting larvae developed normally, thus eliminating complete gametic incompatibility or early life postzygotic barriers as explanations for their divergence. Using observations from 19 other localities across the Wider Caribbean Region, we confirmed the co-occurrence of discrete spring- and autumn-spawning populations across a range of latitudes. Thus, we show that seasonal, temporal reproductive isolation (allochrony), but not gametic reproductive isolation, is a strong barrier to gene flow in sympatric lineages of this critically endangered reef-building coral. More broadly, our findings underscore the role of allochrony in the creation and maintenance of cryptic coral lineages and the urgency of identifying, quantifying, and conserving this diversity before it is lost.