Coral Communities Research Articles

High survival following bleaching underscores the resilience of a frequently disturbed region of the Great Barrier Reef

AbstractNatural bleaching events provide an opportunity to examine how local‐scale environmental variation influences bleaching severity and recovery. During the 2020 marine heat wave, we documented widespread and severe coral bleaching affecting 75%–98% of coral cover throughout the Keppel Islands in the southern inshore Great Barrier Reef. Acropora, Pocillopora, and Porites were the most severely affected genera, while Montipora was comparatively less susceptible. Site‐specific heat‐exposure metrics were not correlated with Acropora bleaching severity, but recovery was faster at sites that experienced lower heat exposure. Despite severe bleaching and exposure to accumulated heat that often results in coral mortality (degree heating weeks ~4–8), cover remained stable. Approximately 94% of fate‐tracked Acropora millepora colonies survived, perhaps due to reduced irradiance stress from high turbidity, heterotrophic feeding, and large tidal flows that can increase mass transfer. Severe bleaching followed by rapid recovery and the continuing dominance of Acropora populations in the Keppel Islands is indicative of high resilience. These coral communities have survived a 0.8°C increase in average temperatures over the last 150 years. However, recovery following the 2020 bleaching was driven by the easing of thermal stress, which may challenge their recovery potential under further warming.

An assessment of Ryukyu Archipelago's coral communities over a wide latitudinal range

Japan's coral reefs have changed dramatically over the last several decades due to climate change and anthropogenic impacts. Due to its dynamic location, the islands of Ryukyu Archipelago offer a unique environmental gradient to study the interactions between environmental variations, connectivity, resilience, climate change and adaptation of marine biodiversity along a wide latitudinal gradient. To obtain detailed baseline assessments of the reefs of Ryukyu Islands, we carried out Point Intercept Transect surveys to compare the assemblages of benthic communities in six regions of Ryukyu Archipelago, examining the influence of latitude gradient and emphasising on regional features. A total of 69 individual reef sites were surveyed for this study. The overall hard coral cover for Ryukyu archipelago was 22.41% ± 7.98 (Mean ± SD) from pooled dataset. The most abundant benthic assemblage for Ryukyu Archipelago was turf algae (39.72% ± 7.21) with all regions except Amami Oshima Island having overall cover above 40%. A total of 2607 individual colonies of hermatypic corals were recorded and identified up to genus level for this study where 55 genera of corals belonging to 17 families were recorded for Ryukyu Archipelago. The overall relative abundance was highest for Acroporidae family (32.22%), followed by Merulinidae family (27.69%) and Poritidae family (14.38%). Our results highlight the current condition of the reefs of Ryukyu archipelago and offers baseline data on broad scale ecosystem where future ecological change could be compared.

Coral disease outbreak at the remote Flower Garden Banks, Gulf of Mexico

East and West Flower Garden Bank (FGB) are part of Flower Garden Banks National Marine Sanctuary (FGBNMS) in the northwest Gulf of Mexico. This geographically-isolated reef system contains extensive coral communities with the highest coral cover (>50%) in the continental United States due, in part, to their remoteness and depth, and have historically exhibited low incidence of coral disease and bleaching despite ocean warming. Yet in late August 2022, disease-like lesions on seven coral species were reported during routine monitoring surveys on East and West FGB (2.1–2.6% prevalence). A series of rapid response cruises were conducted in September and October 2022 focused on 1) characterizing signs and epidemiological aspects of the disease across FGB and within long-term monitoring sites, 2) treating affected coral colonies with Base 2B plus amoxicillin, and 3) collecting baseline images through photostations and photomosaics. Marginal and/or multi-focal lesions and tissue loss were observed, often associated with substantial fish and invertebrate predation, affecting the dominant coral species Pseudodiploria strigosa (7–8% lesion prevalence), Colpophyllia natans (11–18%), and Orbicella spp. (1%). Characterizing this disease event during its early epidemic phase at East and West FGB provides a critical opportunity to observe how coral disease functions in a relatively healthy coral ecosystem versus on reefs chronically affected by various stressors (e.g., Caribbean reefs adjacent to urban centers). Insights into the etiology, spread, and impacts of the disease can ultimately inform efforts to mitigate its effects on coral communities.

The Red Coral Community in the Messina Strait: New Findings from the 1700s Lazzaro Spallanzani Collection

The precious red coral (Corallium rubrum L.) represents one of the most fascinating marine species of the Mediterranean Sea. Several samples, including red coral together with its accompanying species, were found in the zoological collection of the Italian pioneer biologist Lazzaro Spallanzani (1729–1799), collected in the Messina Strait during his voyage in Sicily (1788). The study of these samples allowed the inclusion of numerous additional species in the traditional red coral facies as the large oyster Neopycnodonte cochlear, the giant barnacle Pachylasma giganteum, the mesophotic scleractinian Caryophyllia (Caryophyllia) cyathus. These specimens proved to be very useful in describing the diversity of the paleo-community including red coral, shedding light on its formation processes. In particular, some specimens are composed of red coral rubble consolidated and cemented with other carbonatic remains Probably, these peculiar specimens have a similar origin to those of the Sciacca Banks already known from the Sicily Channel. In fact, the two areas are prone to intense seismic activity that periodically causes mass mortalities of red coral from nearby rocky reliefs and the formation of biogenic detritus, while the resulting chemical environment of the water and the sediments allows the consolidation of the carbonatic remains.

Occurrence, bioaccumulation and ecological risk of organic ultraviolet absorbers in multiple coastal and offshore coral communities of the South China Sea

The occurrence of organic ultraviolet absorbers (OUVAs) in coral reef regions has aroused widespread concern. This study focused on the occurrence, distribution, bioaccumulation and ecological risk of ten OUVAs in both coastal and offshore coral reef regions in the South China Sea. While the Σ10OUVAs was 85 % lower in the offshore seawater (15.1 ng/L) than in the coastal seawater (102.1 ng/L), the Σ10OUVAs was 21 % lower in the offshore corals (1.82 μg/g dry weight (dw)) than in the coastal corals (2.31 μg/g dw). This difference was speculated to relate to the high intensity of human activities in the coastal regions. Moreover, the offshore corals showed higher bioaccumulative capability toward OUVAs (log bioaccumulation factors (BAFs): 1.22–5.07) than the coastal corals (log BAFs: 0.17–4.38), which was presumably the influence of varied physiological status under different environmental conditions. The results of the ecological risk assessment showed that BP-3 resulted in 73 % of coastal corals and 20 % of offshore corals at a risk of bleaching. Therefore, the usage and discharge of BP-3 should be managed and controlled by the countries adjacent to the South China Sea for the protection of coral reefs.

Highly conserved thermal performance strategies may limit adaptive potential in corals.

Increasing seawater temperatures are expected to have profound consequences for reef-building corals' physiology. Understanding how demography changes in response to chronic exposure to warming will help forecast how coral communities will respond to climate change. Here, we measure growth rates of coral fragments of four common species, while exposing them to temperatures ranging from 19°C to 31°C for one month to calibrate their thermal-performance curves (TPCs). Our results show that, while there are contrasting differences between species, the shape of the TPCs was remarkably consistent among individuals of the same species. The low variation in thermal sensitivity within species may imply a reduced capacity for rapid adaptive responses to future changes in thermal regimes. Additionally, interspecific differences in thermal responses show a negative relationship between maximum growth and thermal optima, contradicting expectations derived from the classic 'warmer-is-better' hypothesis. Among species, there was a trade-off between current and future growth, whereby most species perform well under current thermal regimes but are susceptible to future increases in temperature. Increases in water temperature with climate change are likely to reduce growth rates, further hampering future coral reef recovery rates and potentially altering community composition.

Characteristics of The Bleached Microbiome of The Generalist Coral Pocillopora damicornis from Two Distinct Reef Habitats.

Generalist coral species may play an important role in predicting, managing, and responding to the growing coral reef crisis as sea surface temperatures are rising and reef wide bleaching events are becoming more common. Pocilloporids are amongst the most widely distributed and studied of generalist corals, characterized by a broad geographic distribution, phenotypic plasticity, and tolerance of sub-optimal conditions for coral recruitment and survival. Emerging research indicates that microbial communities associated with Pocilloporid corals may be contributing to their persistence on coral reefs impacted by thermal stress; however, we lack detailed information on shifts in the coral-bacterial symbiosis during bleaching events across many of the reef habitats these corals are found. Here, we characterized the bacterial communities of healthy and bleached Pocillopora damicornis corals during the bleaching events that occurred during the austral summer of 2020 on Heron Island, on the southern Great Barrier Reef, and the austral summer of 2019 on Lord Howe Island, the most southerly coral reef in Australia. Regardless of reef location, significant differences in α and β diversities, core bacterial community, and inferred functional profile of the bleached microbiome of P.damicornis were not detected. Consistent with previous reports, patterns in the Pocilloporid coral microbiome, including no increase in pathogenic taxa or evidence of dysbiosis, are conserved during bleaching responses. We hypothesize that the resilience of holobiont interactions may aid the Pocilloporids to survive Symbiodiniaceae loss and contribute to the success of Pocilloporids.

Deep-sea biology in undergraduate classrooms: Open access data from remotely operated vehicles provide impactful research experiences

Opportunities for students to conduct research in courses increase feelings of belonging in science, retention in STEM majors, and sense of ownership in a student’s educational experience. However, many research fields are challenging to bring to students: for example, deep-sea biology often requires expensive expeditions, restricting participation and accessibility. Access to deep-sea systems has been expanded by programs such as the National Oceanic and Atmospheric Administration's Ocean Exploration (NOAA Ocean Exploration), which uses telepresence to bring deep-sea exploration to a global audience. Here, we present one example of how remotely operated vehicles can engage students in original research. Students in an undergraduate Marine Biology lab at at the State University of New York at Geneseo investigated the relationship between substrate rugosity and biodiversity of cold-water coral communities and associate fauna. The study site in the Pacific Remote Islands Marine National Monument was explored by NOAA Ocean Exploration using NOAA Ship Okeanos Explorer as part of the 2017 Mountains in the Deep Expedition, EX1705. Organism density was greater at higher rugosity levels, suggesting that complex substrates support higher abundances in deep-sea communities. This research experience enhanced student’s understanding of the scientific process, appreciation for deep-sea communities, and engagement in the course. Open access deep-sea data provide impactful opportunities for students to participate in original research, increasing the accessibility and reach of deep-sea science.

Diversity of coral-associated pit crabs (Crustacea: Decapoda: Cryptochiridae) from Hong Kong, with description of two new species of Lithoscaptus A. Milne-Edwards, 1862

Highly specialized cryptochirid crabs are obligate symbionts of scleractinian corals in tropical and subtropical seas. General morphologies of cryptochirid crabs remain poorly described due to their small size and difficulties in collection; thus, the current inventory is probably an underestimation. In the present study, we sampled cryptochirid crabs from coral communities in Hong Kong. In the literature, onlyCryptochirus hongkongensis(nowNeotroglocarcinus hongkongensis) with unknown hosts had been recorded in Hong Kong since 1936. In addition to morphological examination, identification in the present study is further supported by sequence divergence of mitochondrial cytochrome c oxidase I (COI) and 16S ribosomal DNA markers. Six operative taxonomic units (OTUs), representing four species and one species complex with two species, were revealed among our material:Cryptochirus coralliodytes,Lithoscaptus paradoxus,Lithoscaptus doughnutsp. nov.,Lithoscaptus scottaesp. nov., andXynomaia shenispecies complex. Morphological description of these species is provided, including description of the two new pseudocryptic species. The hosts of the genusLithoscaptusbelong largely to the Merulinidae, whileL. doughnutsp. nov. inhabits the Plesiastreidae.

Bacterial diversity associated with Millepora alcicornis and Phyllogorgia dilatata corals and prospection for genes encoding bioactive molecules

Coral reefs are marine environments with notable biodiversity, considered rich in organisms that produce active molecules. The microorganisms present in coral tissue play different roles that are not yet fully understood. The study of the microbiota can explain how microorganisms influence the health of corals and their participation in defense mechanisms. In this study, the cultivable bacterial communities of Millepora alcicornis and Phyllogorgia dilatata corals were analyzed following the steps of isolation, morphological characterization, and molecular identification by amplification of the gene encoding 16S rRNA. Of the 135 bacterial isolates, 121 of which were from M. alcicornis and 14 from P. dilatata, most belonged to the phyla Proteobacteria and Firmicutes, with the majority from the genera Vibrio and Bacillus. Coral metagenomic DNA was extracted and the gene encoding 16S rRNA was analyzed. The isolates were evaluated, in vitro, for antimicrobial activity against ATCC strains of Escherichia coli and Staphylococcus aureus, and 29 isolates from M. alcicornis showed activity against at least one of the bacteria evaluated. Using specific pairs of primers, 27 isolates showed amplification for at least one pair of primers designed for amplification of genes involved in the synthesis of non-ribosomal peptides (NRPs) and polyketides (PKs). However, only in some cases was a relationship detected between the isolates that showed antimicrobial activity in vitro and the isolates where amplification of genes involved in the synthesis of NRPs and PKs was observed.

Diversity and Biogeography of Coral Mucus-Associated Bacterial Communities: The Case of Acropora formosa

The role of microorganisms in coral health, disease, and nutrition has been demonstrated in various studies. Environmental factors including pH, temperature, and dissolved oxygen also play crucial roles in maintaining sustainable coral ecosystems. However, how geographical and environmental factors influence bacterial diversity and community composition is unclear. Here, bacterial communities associated with Acropora formosa coral were sampled from four different locations—Phu Quoc Islands (Vietnam), Nha Trang (Vietnam), Ujung Gelam (Indonesia), and Bourake (New Caledonia)—and compared using tagged 16S rRNA sequencing. We identified 24 bacterial phyla, 47 classes, 114 orders, and 495 genera from 18 samples. Overall, Proteobacteria (1039 distant amplicon sequence variants [ASVs]) and Firmicutes (589 ASVs) were predominant, while Verrucomicrobiota (75 ASVs) and Planctomycetota (46 ASVs) were minor taxa. Alpha diversity analyses revealed that the bacterial community associated with Acropora formosa from Ujung Gelam had the highest indexes (Observed and Chao1), while the figures for Bourake were the lowest. Non-metric multidimensional scaling analysis (NMDS) showed significant differences in bacterial communities among locations (ADONIS, p = 1 × 10−4). Temperature was strongly correlated with the distribution of bacterial communities in Bourake, whereas pH and dissolved oxygen were significantly correlated with the presence of coral-associated bacterial communities in Phu Quoc and Nha Trang. Across all samples, 28 potential biological markers and 95 core ASVs were found, revealing significant differences in coral-associated bacterial communities. Collectively, these findings provide a comprehensive understanding of bacterial communities living in coral reefs across different geographic sites, which could be useful springboards for further studies.

Temporal functional changes in coral and fish communities on subtropical coastal coral reefs

Context Global and local stressors can drive phase shifts from zooxanthellate scleractinian coral communities to macroalgae-dominated ecosystems. However, our understanding of altered ecosystem functioning, productivity and stability remains limited as pre-shift data are typically lacking for degraded coral-reef sites. Aims Here, we assessed functional changes in fish communities in Nakagusuku Bay, Okinawa, Japan, over 45 years, by comparing pre-disturbance (1975) to post-disturbance (2018–2020) datasets, and identified possible drivers of changes. Methods We analysed data for 393 fish species and 26 coral genera at 13 sites, measured at four-time points (1975, 2018, 2019 and 2020). Analyses were performed using a range of ordination techniques. Key results We found reductions in functional richness and trait space contraction over time for fishes. Changes in coral functional groups over time correlated with changes in the functional diversity of reef-fish communities; a reduction in branching corals reduced habitat availability for coral-reliant fishes. Conclusions Increasing sedimentation and eutrophication as a result of construction along the Nakagusuku Bay coast likely reduced living coral cover and fish diversity, and thermal stress likely facilitated the simplification and shifts of both coral communities and coral specialist fishes away from shore. Implications Both global and local threats need to be considered when assessing functioning of coral-reef ecosystems for coral-reef conservation efforts.

Impacts of selective feeding of crown-of-thorns starfish on the coral community in the South China Sea

Context Crown-of-thorns starfish (Acanthaster spp., hereafter COTS), the most destructive predator of corals, selectively consume specific corals. However, their feeding selectivity varies among reef regions and differs from their feeding preferences in laboratory experiments. Aims In this study, the impacts of selective feeding of Acanthaster sp. on the coral community in the South China Sea were explored. Methods COTS predation on 701 coral colonies and 24 coral genera during 2018−2019 was evaluated. Key results Montipora had a greater scar cover (57%) than did Pocillopora (27%) because of COTS predation. Additionally, the relative abundance of encrusting Montipora (heat-tolerant corals) decreased from 40 to 14%, whereas that of branching Pocillopora (less heat-tolerant corals) increased from 40 to 60%. Moreover, the rare genera Millepora and Astreopora had the greatest scar cover (from 78 to ~100%) and feeding electivity values (from 0.66 to ~0.98), and were thus positively selected by COTS for predation. Further regional comparisons showed four types of feeding selection worldwide. Conclusions Less heat-tolerant coral became predominant in the coral community owing to selective feeding of COTS. COTS could consume rare, less-preferred corals by changing feeding selections. Implications These findings suggest that COTS not only directly consume coral individuals but also indirectly change the coral community composition, which is potentially related to the vulnerability of the coral community to global warming.

Decadal-scale variation in coral calcification on coral-depleted Caribbean reefs

Over recent decades, many Caribbean reefs have transitioned to states where stony corals are no longer spatially dominant. The community dynamics culminating in this outcome are well known, but its functional implications remain incompletely understood. Here we used annual surveys from 1992 to 2019 to describe coral communities at 6 sites off St. John, US Virgin Islands, and explored how their ecological dynamics interact with their capacity to sustain estimated coral community calcification (G, kg CaCO3 m-2 yr-1). These communities had low coral cover (≤4.4%), but they changed through small and incremental events that summed to a slight decline in coral cover and changes in species assemblages favoring biotic homogenization and weedy species. Estimated coral G remained low, between 0.3 and 1.3 kg CaCO3 m-2 yr-1 (8.2-35.6 mmol CaCO3 m-2 d-1), but it differed among sites and years. The dominant contributors to G were Siderastrea siderea (1 site), Porites astreoides (1 site), and Orbicella spp. (4 sites), but higher G only occurred where Orbicella spp. remained relatively common; G dramatically declined at 1 site when the abundance of this genus decreased. These results suggest that some coral-depleted reefs may maintain low G that could be sufficient to avoid transitions into net negative budget states, provided that biological and physical erosion and dissolution of CaCO3 (not recorded here) are minimal. Further mortalities of the few coral species remaining on these reefs through disturbances like stony coral tissue loss disease would compromise this delicate production-erosion balance, and likely see transitions of such reefs into negative carbonate budget states.

Dynamics of the Benthic Communities of Pigeon Islets (Guadeloupe Island, Lesser Antilles) from 2012 to 2021 Monitored by a Photo-Quadrats Technique

Since 2012, the benthic communities of the Pigeon Islets (Guadeloupe Island, Lesser Antilles) have been monitored biannually by the National Park of Guadeloupe using photo—quadrats. This monitoring was carried out at 12 coral reef stations distributed around the islets. The data collected from 2012 to 2021 highlight the significant events that disrupted the coral communities of Guadeloupe. Among these remarkable phenomena, an episode of coral bleaching occurred in 2019, followed by the appearance of a new coral disease, Stony Coral Tissue Loss Disease (SCTLD), in 2020. As a result of these threats, a drop of 54% of coral cover was observed in the monitored sites, accompanied by a rise of 16% of the surface occupied by macroalgae. The protocol set up by the National Park of Guadeloupe was found to be efficient to follow the temporal dynamics of benthic reef communities.

Varied effects of Lobophora chemistry on settlement of larvae from five coral genera

High macroalgae cover on degraded coral reefs may reduce settlement of coral larvae via both direct and indirect mechanisms, limiting coral recovery potential following disturbances. We tested the allelopathic effect of a Lobophora sp., a common fast-growing brown macroalga, on the settlement of 9 coral species from 5 taxonomically diverse genera (Acropora, Coelastrea, Goniastrea, Montipora and Platygyra) common on inshore reefs of the Great Barrier Reef. Larval settlement of 2 species, Montipora aequituberculata and Platygyra daedalea, was reduced at the highest concentration of Lobophora-conditioned seawater (equivalent to ~15 g Lobophora per 100 cm2). Lobophora surface extracts were mildly toxic and elicited varied responses, increasing settlement in Coelastrea aspera, Acropora hyacinthus and M. aequituberculata and decreasing settlement in A. millepora, A. elseyi and A. intermedia, but overall had minimal effects. Lobophora non-polar extracts inhibited larval settlement in 7 species at high concentrations, whereas lower concentrations increased settlement for Goniastrea retiformis and C. aspera. Polar chemical extracts of Lobophora thalli reduced larval settlement in all 9 species and proved lethal at high concentrations for 7 species. The amplified effects of extracts compared with conditioned seawater suggest that experimental studies of extracts may overestimate allelopathic effects on coral settlement. This study adds to the growing evidence that Lobophora can reduce coral settlement via chemical allelopathy in laboratory experiments. Further efforts are required to confirm the transferability of these results to the field. Our results highlight coral inter-specific variation in Lobophora sensitivity, which may contribute to future changes in coral community composition.

Coral early life history dynamics: conspecific facilitation or limitation are dependent on distinct life stage interactions

Replenishment of species within a community is largely determined by early life history dynamics, yet the relative contributions of settlement versus post-settlement processes in establishing coral community structure are not well understood. Here, we examined the effect of larval density, number of settlers and presence of conspecific adults on settlement and post-settlement success for 3 broadcast-spawning scleractinian corals under aquarium conditions. We found that settlement and post-settlement survival are dependent on larval or settler densities but with contrasting effects among the studied coral species. Settlement of Anacropora spinosa larvae was inversely density-dependent, while settlement of Acropora gemmifera and Acropora digitifera larvae was density-independent. A. spinosa had high post-settlement survivorship, while A. digitifera underwent significant density-dependent post-settlement mortality within 14 d after settlement. A. gemmifera showed no significant trend in post-settlement survival by number of initial settlers, but had higher mortality in the second week than the first week, as did A. digitifera. For all species, settler spatial patterns were under-dispersed; the probability of contact between settlers positively correlated to initial settler numbers, and the extent of under-dispersion showed little change over time. The presence of conspecific adults led to a decrease in larval settlement of A. spinosa and A. gemmifera but had only a weak influence on settler survival for A. spinosa. These data indicate, in an experimental setting, that settlement success of larvae can be reduced when in close proximity to a conspecific adult and may be mediated by a waterborne agent. Overall, our results demonstrate the potential for self-limitation via conspecific larval-adult interactions and facilitation among early life stages of a cohort.

Marine animal forests in turbid environments are overlooked seascapes in urban areas

Marginal reefs can provide meaningful information about the structure and dynamics of ecosystems under suboptimal environmental conditions. In addition to their different characteristics, these environments can also occur in urbanized areas. In this note, we characterize marine animal forests (MAFs) on turbid-zone reefs on an urban coast in the equatorial southwestern Atlantic. Overall, the sandstone ferruginous reefs (6-10 m depth) exhibited a flat topography and gentle slope (1-2 m above the seabed). Benthic cover is composed mainly of sponge gardens and ascidians. In addition, we found a low-relief coral carpet with only one massive reef-building coral (Siderastrea stellata) and Zoantharia. The ascidians and sponges had a higher diversity (at least 15 species) than the cnidarians (two species) in these forests. The main animals forming this seascape are weedy and stress-tolerant species adapted to challenging environmental conditions, such as swell waves, mesotidal regimes, moderate turbidity waters, and periodic burial. In this regard, these conditions and human impacts have shaped a unique MAF. Remarkably, the studied formations seem similar to high-latitude marginal reefs or low-latitude reefs under the influence of upwelling, which sustains soft corals and non-framework building coral communities along with sponges and ascidians. In particular, the shallow MAFs along the semi-arid coast of Brazil seem to lack some of the characteristics of low-latitude reefs under high sedimentation, whose structure was described as coral rubble within sedimentary matrices. This suggests that factors other than periodic burials and low light availability affected these MAFs. These overlooked forests are widespread in this area and have been neglected in studies, despite their richness (> 31 taxa) and valuable ecosystem goods and services. In the context of urbanized areas subject to climate change and pollution impacts, jetties, and dredging activities, it is necessary to consider these lush forests in impact assessments and conservation policies.

Corals at the Edge of Environmental Limits: A New Conceptual Framework to Re-Define Marginal and Extreme Coral Communities

Corals at the Edge of Environmental Limits: A New Conceptual Framework to Re-Define Marginal and Extreme Coral Communities

Palau’s warmest reefs harbor thermally tolerant corals that thrive across different habitats

Ocean warming is killing corals, but heat-tolerant populations exist; if protected, they could replenish affected reefs naturally or through restoration. Palau’s Rock Islands experience consistently higher temperatures and extreme heatwaves, yet their diverse coral communities bleach less than those on Palau’s cooler outer reefs. Here, we combined genetic analyses, bleaching histories and growth rates of Porites cf. lobata colonies to identify thermally tolerant genotypes, map their distribution, and investigate potential growth trade-offs. We identified four genetic lineages of P. cf. lobata. On Palau’s outer reefs, a thermally sensitive lineage dominates. The Rock Islands harbor two lineages with enhanced thermal tolerance; one of which shows no consistent growth trade-off and also occurs on several outer reefs. This suggests that the Rock Islands provide naturally tolerant larvae to neighboring areas. Finding and protecting such sources of thermally-tolerant corals is key to reef survival under 21st century climate change.