Palau’s warmest reefs harbor a thermally tolerant coral lineage that thrives 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 chronically 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 P cf. lobata genetic lineages. On Palau’s outer reefs, a thermally sensitive lineage dominates. The Rock Islands harbor two lineages with enhanced thermal tolerance and no consistent growth trade-off. One of these lineages 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.

Do Oil and Gas Lease Stipulations in the Northwestern Gulf of Mexico Need Expansion to Better Protect Vulnerable Coral Communities? How Low Relief Habitats Support High Coral Biodiversity

The continental shelf of the northwestern Gulf of Mexico harbors extensive reefs and banks that support diverse coral reefs and mesophotic communities. Mesophotic communities range in depth from 40 to 200 m and, in this region, foster some of the densest coral forests [aggregations of mesophotic octocoral, antipatharian, and branching stony coral communities] reported in published literature (10.23 ± 9.31 col/m2). The geologic features underlying the exposed substrates that harbor mesophotic communities are targeted for extensive hydrocarbon exploration and extraction, as they often contain oil and/or natural gas. The Bureau of Ocean Energy Management regulates offshore energy development in the United States and is tasked with protecting sensitive biological communities from impacts related to oil and gas activities. This study analyzed alpha and beta diversity of mesophotic coral forests on fourteen topographic banks in the northwestern Gulf of Mexico. The objective of the study was to examine differences in structure and community in relation to lease stipulations established by the Bureau of Ocean and Energy Management. It was determined that dense and diverse mesophotic coral forests and carbonate producers exist in present regulatory zones that prohibit oil and gas activities; however, the coral communities exist in higher densities, diversity, and richness in low relief substrates outside of these regulatory zones. Our findings suggest low relief hard substrates serve as important habitat for mesophotic coral forests; thus, we suggest the expansion of current stipulations should be considered to provide better protection to vulnerable coral communities on low relief features. Furthermore, additional studies to refine the relationship between low relief structures and biodiversity are needed to develop more meaningful habitat definitions to support resource management and improve resource protection in the future.

Community similarity and species overlap between habitats provide insight into the deep reef refuge hypothesis

The deep reef refuge hypothesis (DRRH) postulates that mesophotic coral ecosystems (MCEs) may provide a refuge for shallow coral reefs (SCRs). Understanding this process is an important conservation tool given increasing threats to coral reefs. To establish a better framework to analyze the DRRH, we analyzed stony coral communities in American Sāmoa across MCEs and SCRs to describe the community similarity and species overlap to test the foundational assumption of the DRRH. We suggest a different approach to determine species as depth specialists or generalists that changes the conceptual role of MCEs and emphasizes their importance in conservation planning regardless of their role as a refuge or not. This further encourages a reconsideration of a broader framework for the DRRH. We found 12 species of corals exclusively on MCEs and 183 exclusively on SCRs with another 63 species overlapping between depth zones. Of these, 19 appear to have the greatest potential to serve as reseeding species. Two additional species are listed under the U.S. Endangered Species Act, Acropora speciosa and Fimbriaphyllia paradivisa categorized as an occasional deep specialist and a deep exclusive species, respectively. Based on the community distinctiveness and minimal species overlap of SCR and MCE communities, we propose a broader framework by evaluating species overlap across coral reef habitats. This provides an opportunity to consider the opposite of the DRRH where SCRs support MCEs.

Novel Mitochondrial DNA Markers for Scleractinian Corals and Generic-Level Environmental DNA Metabarcoding

Coral reefs, the most biodiverse habitats in the ocean, are formed by anthozoan cnidarians, the scleractinian corals. Recently, however, ongoing climate change has imperiled scleractinian corals and coral reef environments are changing drastically. Thus, convenient, high-density monitoring of scleractinian corals is essential to understand changes in coral reef communities. Environmental DNA (eDNA) metabarcoding is potentially one of the most effective means of achieving it. Using publicly available scleractinian mitochondrial genomes, we developed high-specificity primers to amplify mitochondrial 12S ribosomal RNA (12S) and cytochrome oxidase-1 (CO1) genes of diverse scleractinian corals, which could be used for genus-level metabarcoding analyses, using next-generation sequencing technologies. To confirm the effectiveness of these primers, PCR amplicon sequencing was performed using eDNA isolated along the seashore of Okinawa, Japan. We successfully amplified all eDNA samples using PCR. Approximately 93 and 72% of PCR amplicon sequences of 12S and CO1 primers originated from scleractinian 12S and CO1 genes, respectively, confirming higher specificities for coral mitochondrial genes than primers previously used for coral eDNA metabarcoding. We also found that hierarchical clustering, based on the percentage of mapped reads to each scleractinian genus, discriminates between sampling locations, suggesting that eDNA surveys are sufficiently powerful to reveal differences between coral communities separated by <1 km. We conclude that the method reported here is a powerful tool for conducting efficient eDNA surveys targeting scleractinian corals.

Persistence of Corals in Marginal Habitats: the Role of the Environment, and Symbiont Diversity and Ecophysiology

<p>Many corals live in marginal habitats, close to their survival thresholds of water temperature, light penetration and aragonite saturation. Living under these highly variable and extreme conditions is likely facilitated by specific physiological adaptations and/or the presence of unique species of coral and their symbionts but data on these factors are limited. The specific objectives of the study were to: (1) examine the diversity and distribution patterns of corals in marginal environments, (2) investigate the diversity, distribution patterns and host specificity of symbionts in corals in marginal environments, (3) assess the influence of environmental variables on host and symbiont distribution in marginal environments, in comparison to 'optimal' environments, and (4) examine the physiological responses to changing environmental conditions and stress of corals and their symbionts in marginal environments. Surveys of coral community patterns were conducted at the Kermadec Islands (KI), New Zealand, and Palmyra Atoll, USA, with local scale environmental parameters (i.e. wave exposure and sedimentation) found to control the diversity and distribution of the coral communities. Symbiodinium types were identified to subcladal level in a range of coral species at each of the survey sites, using ITS2-DGGE. A high diversity of C type symbionts (19 types in 13 host genera), and reduced host specificity was observed at the high latitude site of Lord Howe Island (LHI), Australia, with similarly high diversity at the KI (10 types in 9 genera). Thirteen novel clade C types were identified in corals at LHI, with two of these types also present in hosts at the KI. The reduced host specificity of symbionts at LHI, compared to tropical sites, implies that the evolution of novel holobionts may be an important mechanism whereby corals can cope with variable and stressful conditions. Further, physiological assessment of the novel LHI symbionts led to the suggestion that Symbiodinium at LHI may be specialised for cooler and more variable temperatures, so contributing to the success of corals at this marginal location. In contrast, a low diversity of generalist symbionts (C and D types) were uncovered at the equatorial site of Palmyra Atoll (10 types in 13 genera), attributed to the stressful environmental regime resulting in a reduced population of stresstolerant symbionts. The variation in environmental parameters, particularly sedimentation, around Palmyra Atoll has led to diversification of coral communities, however this environmental variation has not affected the symbiont communities. While it has been suggested that marginal coral communities might be better adapted for survival in an environment modified by global climate change, the local scale environmental factors are also important drivers of both coral and symbiont distributions, and should be considered when making predictions for the future. Further, assessment of the physiological tolerance ranges of both the multiple, novel symbionts at high latitudes, and the few, potentially stress-tolerant symbionts at Palmyra should be conducted, to help determine whether they have the ability to adjust to new environmental conditions.</p>

Persistence of Corals in Marginal Habitats: the Role of the Environment, and Symbiont Diversity and Ecophysiology

<p>Many corals live in marginal habitats, close to their survival thresholds of water temperature, light penetration and aragonite saturation. Living under these highly variable and extreme conditions is likely facilitated by specific physiological adaptations and/or the presence of unique species of coral and their symbionts but data on these factors are limited. The specific objectives of the study were to: (1) examine the diversity and distribution patterns of corals in marginal environments, (2) investigate the diversity, distribution patterns and host specificity of symbionts in corals in marginal environments, (3) assess the influence of environmental variables on host and symbiont distribution in marginal environments, in comparison to 'optimal' environments, and (4) examine the physiological responses to changing environmental conditions and stress of corals and their symbionts in marginal environments. Surveys of coral community patterns were conducted at the Kermadec Islands (KI), New Zealand, and Palmyra Atoll, USA, with local scale environmental parameters (i.e. wave exposure and sedimentation) found to control the diversity and distribution of the coral communities. Symbiodinium types were identified to subcladal level in a range of coral species at each of the survey sites, using ITS2-DGGE. A high diversity of C type symbionts (19 types in 13 host genera), and reduced host specificity was observed at the high latitude site of Lord Howe Island (LHI), Australia, with similarly high diversity at the KI (10 types in 9 genera). Thirteen novel clade C types were identified in corals at LHI, with two of these types also present in hosts at the KI. The reduced host specificity of symbionts at LHI, compared to tropical sites, implies that the evolution of novel holobionts may be an important mechanism whereby corals can cope with variable and stressful conditions. Further, physiological assessment of the novel LHI symbionts led to the suggestion that Symbiodinium at LHI may be specialised for cooler and more variable temperatures, so contributing to the success of corals at this marginal location. In contrast, a low diversity of generalist symbionts (C and D types) were uncovered at the equatorial site of Palmyra Atoll (10 types in 13 genera), attributed to the stressful environmental regime resulting in a reduced population of stresstolerant symbionts. The variation in environmental parameters, particularly sedimentation, around Palmyra Atoll has led to diversification of coral communities, however this environmental variation has not affected the symbiont communities. While it has been suggested that marginal coral communities might be better adapted for survival in an environment modified by global climate change, the local scale environmental factors are also important drivers of both coral and symbiont distributions, and should be considered when making predictions for the future. Further, assessment of the physiological tolerance ranges of both the multiple, novel symbionts at high latitudes, and the few, potentially stress-tolerant symbionts at Palmyra should be conducted, to help determine whether they have the ability to adjust to new environmental conditions.</p>

An emerging coral disease outbreak decimated Caribbean coral populations and reshaped reef functionality

Diseases are major drivers of the deterioration of coral reefs, linked to major declines in coral abundance, reef functionality, and reef-related ecosystems services1-3. An outbreak of a new disease is currently rampaging through the populations of the remaining reef-building corals across the Caribbean region. The outbreak was first reported in Florida in 2014 and reached the northern Mesoamerican reef by summer 2018, where it spread across the ~ 450-km reef system only in a few months4. Rapid infection was generalized across all sites and mortality rates ranged from 94% to < 10% among the 21 afflicted coral species. This single event further modified the coral communities across the region by increasing the relative dominance of weedy corals and reducing reef functionality, both in terms of functional diversity and calcium carbonate production. This emergent disease is likely to become the most lethal disturbance ever recorded in the Caribbean, and it will likely result in the onset of a new functional regime where key reef-building and complex branching acroporids (a genus apparently unaffected) will once again become conspicuous structural features in reef systems with yet even lower levels of physical functionality.

Mesophotic coral communities escape thermal coral bleaching in French Polynesia

Climate change and consequent coral bleaching are causing the disappearance of reef-building corals worldwide. While bleaching episodes significantly impact shallow waters, little is known about their impact on mesophotic coral communities. We studied the prevalence of coral bleaching two to three months after a heat stress event, along an extreme depth range from 6 to 90 m in French Polynesia. Bayesian modelling showed a decreasing probability of bleaching of all coral genera over depth, with little to no bleaching observed at lower mesophotic depths (greater than or equal to 60 m). We found that depth-generalist corals benefit more from increasing depth than depth-specialists (corals with a narrow depth range). Our data suggest that the reduced prevalence of bleaching with depth, especially from shallow to upper mesophotic depths (40 m), had a stronger relation with the light-irradiance attenuation than temperature. While acknowledging the geographical and temporal variability of the role of mesophotic reefs as spatial refuges during thermal stress, we ought to understand why coral bleaching reduces with depth. Future studies should consider repeated monitoring and detailed ecophysiological and environmental data. Our study demonstrated how increasing depth may offer a level of protection and that lower mesophotic communities could escape the impacts of a thermal bleaching event.

Coral Communities on Marginal High-Latitude Reefs in West Australian Marine Parks

Many temperate reefs are experiencing a shift towards a greater abundance of tropical species in response to marine heatwaves and long-term ocean warming worldwide. Baseline data for coral communities growing in high-latitude reefs is required to better understand ecosystem changes over time. In this study, we explore spatial and temporal trends in the distribution of coral communities from 1999 to 2019 at 118 reef sites within the five marine parks located in the south-west of Western Australia (WA) between 30° to 35 °S. Our estimates of coral cover were generally low (< 5%), except for a few sites in Jurien Bay Marine Park and Rottnest Island Marine Reserve where coral cover was 10% to 30%. Interannual changes in genera assemblages were detected but were not consistent over time, whereas significant temporal increases in coral cover estimates were found at the lowest latitude site in Jurien Bay. Coral assemblages were primarily distinguished by Turbinaria spp. at Marmion Marine Park and Ngari Capes Marine Park, and Pocillopora spp. and Dipsastraea spp. at Rottnest Island and Jurien Bay. Our findings suggest that conditions in south-west WA are favorable to the ongoing survival of existing genera and there were minimal signs of expansion in coral cover at most study sites. Coral cover and composition on these reefs may, however, change with ongoing ocean warming and increased occurrence of marine heatwaves. This study provides a valuable benchmark for assessing future changes in coral assemblages and highlights the need for targeted hard-coral surveys to quantify subtle changes in high-latitude coral community assemblages.