Biology subjects

Baums, I. B.

Publications and source records attributed to Baums, I. B..

16 recordsLinked to original sources

bioRxiv2026

Limited neutral and adaptive genomic divergence suggests Acropora cervicornis can be managed as a single conservation unit across its range

Genomic signatures can provide key insight into the evolutionary history and remaining adaptive potential of threatened populations. As demographic decline erodes both diversity and the processes maintaining it, understanding how remaining variation is distributed becomes increasingly important for conserving species like the staghorn coral, Acropora cervicornis, a foundational but critically endangered Caribbean reef-builder. We analyzed 46 high-coverage A. cervicornis genomes from 10 locations across the tropical western Atlantic to evaluate neutral and adaptive structure, genomic diversity, demographic history, inbreeding, and connectivity, and generated a regional haplotype reference panel for future genomic monitoring. Genome-wide analyses recovered recurring regional substructure, but differentiation was modest and partly explained by isolation-by-distance and spatial variation in effective migration. Subpopulations had similar levels of genomic diversity, shared demographic history, and limited evidence of local adaptation. These patterns support interpreting sampled Caribbean populations as a single evolutionarily significant unit (ESU) containing multiple regional management units (MUs), rather than as deeply divergent evolutionary lineages. Despite substantial retained variation and low current inbreeding, estimated contemporary effective population size was small, suggesting an increased vulnerability to the effects of drift as demographic collapse continues, especially if structure is reinforced by isolated management. Together, our findings emphasize the urgent need for interventions that preserve and enhance genomic diversity, including risk-managed assisted gene flow. Supported by the haplotype reference panel developed here, these strategies will require coordinated efforts across regional entities to conserve and restore A. cervicornis as a jointly managed, single ESU.

genomics↗

bioRxiv2026

Population structure and gene flow in the endangered Caribbean reef-building coral, Acropora palmata

Populations of the Caribbean reef-building coral, Acropora palmata, have declined sharply since their population genetic structure was first characterized in the early 2000s. Previous analyses comprehensively sampled coral colonies across the Caribbean and western North Atlantic but genomic resolution was limited by the number of loci assayed. These analyses indicated extensive asexual reproduction via fragmentation, high outcrossing at the genet level, and a distinct east-west population split. To advance basic research and inform genetic management of this endangered species, we present an updated population genomic assessment using a species-specific microarray to analyze over 4,000 samples representing [~]1,500 genets from 12 geographic regions. Data were contributed by more than 30 research and restoration groups. Our analysis identifies nine spatially structured genetic clusters, with low average pairwise FST values of between 0.01 to 0.125. Interestingly, legacy genets from the Florida Reef Tract were admixed between two clusters, one dominant in the Mesoamerican Reef Tract on the western flank and the other cluster appearing in genets from Cuba to the south. Migration surface analyses highlight the influence of major current systems on gene flow. Isolation by distance was evident along the Greater Antilles but weak along the Florida Reef Tract. Kinship among wild genets was low across sites, suggesting limited local relatedness; however, assisted sexual reproduction in restoration efforts may disrupt natural kinship patterns. These findings refine our understanding of A. palmatas genetic architecture and underscore the importance of incorporating genomic data into conservation strategies.

genomics↗

bioRxiv2026

Baseline microbiomes of the pillar coral Dendrogyra cylindrus reveal novel taxa and regional differences

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 Floridas 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 Curacao. 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 Curacao 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. IMPORTANCETropical corals face global extinction if average temperatures rise by 2{degrees}C (3.6{degrees}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.

microbiology↗

bioRxiv2026

Maintenance cost of photosynthesis sets key ecological constraints on zooxanthellate corals

Ecological models using light limitation to explain coral depth distribution have largely disregarded the energetic cost of sustaining photosynthetic activity. Here, we quantified photosystem II (PSII) turnover across a depth-simulated light gradient in a zooxanthellate coral, measuring PSII half-life, D1 protein abundance, and PSII-complex gene expression. Maximum photosynthetic capacity remained stable across irradiance levels while respiration rose and PSII turnover accelerated as a power law, imposing increasing ATP demand at the shallowest depths. Declining D1 protein abundance alongside stable transcript levels demonstrated that this escalating maintenance cost operates through post-transcriptional regulation. Consequently, a decreasing fraction of photosynthetic usable energy is available for translocation to the coral host at high irradiance, as the energy required for PSII repair increases. Integrating these physiological constraints into a bio-optical model revealed that the balance between photosynthetic capacity and its maintenance cost defines an optimal depth, the Photosynthetic Usable Energy Supply (PUES) maximum, where host energetic returns are maximized. This framework provides a mechanistic basis for understanding depth distributions in symbiotic corals and extends as a predictive tool for any photosynthetic organism operating under variable irradiance, including forecasting how environmental degradation contracts viable depth ranges.

ecology↗

bioRxiv2025

Heat-responsive dynamic shifts in alternative splicing of the coral Acropora cervicornis

Climate change has caused drastic declines in corals. As sessile organisms, response to shifting environmental conditions may include changes in gene expression, epigenetic modifications, or the microbiome, but as of yet, a common mechanism of stress response, alternative splicing (AS), has been underexplored in corals. Using short-term acute thermal stress assays, we investigated patterns of AS in the scleractinian coral Acropora cervicornis during response to and a subsequent overnight recovery phase from low (33), medium (35), and high (37) levels of heat stress. We find that 40% of the genomic gene set is subject to AS. Our findings demonstrate conserved and dynamic shifts in splicing profiles during the heat treatment and subsequent recovery phase. AS increased in response to heat stress and was primarily dominated by intron retention in specific classes of transcripts, including those related to splicing regulation itself. While AS returned to baseline levels post-exposure to low heat, AS persisted even after reprieve from higher levels of heat stress. Partial overlap of AS transcripts with differentially expressed genes suggests that AS may represent a distinct and previously underappreciated regulatory mechanism for thermal stress response in corals.

genomics↗

bioRxiv2024

Genomes of the Caribbean reef-building corals Colpophyllia natans, Dendrogyra cylindrus, and Siderastrea siderea

Corals populations worldwide are declining rapidly due to elevated ocean temperatures and other human impacts. The Caribbean harbors a high number of threatened, endangered, and critically endangered coral species compared to reefs of the larger Indo-Pacific. The reef corals of the Caribbean are also long diverged from their Pacific counterparts and may have evolved different survival strategies. Most genomic resources have been developed for Pacific coral species which may impede our ability to study the changes in genetic composition of Caribbean reef communities in response to global change. To help fill the gap in genomic resources, we used PacBio HiFi sequencing to generate the first genome assemblies for three Caribbean, reef-building corals, Colpophyllia natans, Dendrogyra cylindrus, and Siderastrea siderea. We also explore the genomic novelties that shape scleractinian genomes. Notably, we find abundant gene duplications of all classes (e.g., tandem and segmental), especially in S. siderea. This species has one of the largest genomes of any scleractinian coral (822Mb) which seems to be driven by repetitive content and gene family expansion and diversification. As the genome size of S. siderea was double the size expected of stony corals, we also evaluated the possibility of an ancient whole genome duplication using Ks tests and found no evidence of such an event in the species. By presenting these genome assemblies, we hope to develop a better understanding of coral evolution as a whole and to enable researchers to further investigate the population genetics and diversity of these three species.

evolutionary biology↗

bioRxiv2023

Genome assemblies and genetic maps highlight chromosome-scale macrosynteny in Atlantic acroporids

BackgroundCorals belong to the Cnidaria, an early branching phylum of metazoans. Over the course of their long evolutionary history, they have adapted to changing environments, such as rising sea levels and increasing ocean temperatures. While their history speaks to their evolutionary capacity, it is less clear how quickly they may respond to rapid changes. A critical aspect of adaptive capacity is the structure of their genome and the genetic diversity contained within. FindingsHere, we present chromosome-scale genome assemblies and genetic linkage maps of two critically endangered coral species, Acropora palmata and A. cervicornis, the two extant Atlantic acroporid corals. Genomes of both species were resolved into 14 chromosomes with comparable assembly sizes (A. palmata, 287Mb; A. cervicornis, 305Mb). Gene content, repeat content, gene collinearity and macrosynteny were largely preserved between the Atlantic acroporids but a 2.5 Mb inversion and 1.4 Mb translocation were detected between two of the chromosome pairs. Macrosynteny and gene collinearity decreased when comparing Atlantic with Pacific acroporids. Paracentric inversions of whole chromosome arms characterized A. hyacinthus, specifically. In the larger context of cnidarian evolution, the four acroporids and another scleractinian coral with chromosome-resolved genome assemblies retained six of 21 cnidarian ancestral linkage groups, while also privately sharing numerous ALG fission and fusion events compared to other distantly related cnidarians. Genetic linkage maps were built using a 30K genotyping array with 105 offspring in one family for A. palmata and 154 offspring across 16 families for A. cervicornis. The A. palmata consensus linkage map spans 1,013.42 cM and includes 2,114 informative markers. The A. cervicornis consensus map spans 927.36 cM across 4,859 markers. A. palmata and A. cervicornis exhibited similarly high sex-averaged genome-wide recombination rates (3.53 cM/Mb and 3.04 cM/Mb, respectively) relative to other animals. In our gamete-specific maps, we found pronounced sex-based differences in recombination, known as heterochiasmy, in this simultaneous hermaphrodite, with both species showing recombination rates 2-2.5X higher in eggs compared to sperm. ConclusionsThe genomic resources presented here are the first of their kind available for Atlantic coral species. These data sets revealed that adaptive capacity of endangered Atlantic corals is not limited by their recombination rates, with both species exhibiting high recombination rates and heterochiasmy. Nevertheless, the two sister species maintain high levels of macrosynteny and gene collinearity between them. The few large-scale rearrangements detected deserve further study as a potential cause of fertilization barriers between the species. Together, the assemblies and genetic maps presented here now enable genome-wide association studies and discovery of quantitative trait loci; tools that can aid in the conservation of these endangered corals.

evolutionary biology↗

bioRxiv2023

Precise age estimation in clonal species using a somatic genetic clock

Age and longevity are key parameters for demography and life-history evolution of organisms. In clonal species, a widespread life history among animals, plants, algae and fungi, the sexually produced offspring (the genet) grows indeterminately by producing iterative modules, or ramets. The age of large genets often remains elusive, while estimates based on their spatial extent as proxy for age are unreliable. Here, we present a method for age estimation using a molecular clock based on the accumulation of fixed somatic genetic variation (SoGV) that segregates among ramets of the same genet. Using a stochastic model of a generic clonal organism, we demonstrate that the accumulation of fixed SoGV via somatic genetic drift will approach linearity after a short lag phase, and is determined by the mitotic mutation rate, without direct dependence on asexual generation time. The lag phase decreased with lower stem cell population size (N), number of founder cells for the formation of new modules (N0), and the ratio of symmetric vs. asymmetric stem cell divisions. We apply the somatic genetic clock to the clonal plant model Zostera marina (eelgrass) and show that linearity is approached within a few years. Taking advantage of two long-term cultivation experiments for Z. marina (4 and 17 years respectively) as calibration points, we find genet ages up to 1,403 years in a global data set of 20 eelgrass populations. The somatic genetic clock is applicable to any multicellular clonal species where a small number of founder cells are recruited to form new ramets, opening novel research avenues to study longevity and hence, demography and population dynamics of clonal species.

evolutionary biology↗

bioRxiv2023

Genomic comparison of the temperate coral Astrangia poculata with tropical corals yields insights into winter quiescence, innate immunity, and sexual reproduction

Facultatively symbiotic corals provide important experimental models to explore the establishment, maintenance, and breakdown of the mutualism between corals and members of the algal family Symbiodiniaceae. The temperate coral Astrangia poculata is one such model as it is not only facultatively symbiotic, but also occurs across a broad temperature and latitudinal gradient. Here, we report the de novo chromosome-scale assembly and annotation of the A. poculata genome. Though widespread segmental/tandem duplications of genomic regions were detected, we did not find strong evidence of a whole genome duplication (WGD) event. Comparison of the gene arrangement between A. poculata and the tropical coral Acropora millepora revealed 56.38% of the orthologous genes were conserved in syntenic blocks despite [~]415 million years of divergence. Gene families related to sperm hyperactivation and innate immunity, including lectins, were found to contain more genes in A. millepora relative to A. poculata. Sperm hyperactivation in A. millepora is expected given the extreme requirements of gamete competition during mass spawning events in tropical corals, while lectins are important in the establishment of coral-algal symbiosis. By contrast, gene families involved in sleep promotion, feeding suppression, and circadian sleep/wake cycle processes were expanded in A. poculata. These expanded gene families may play a role in A. poculatas ability to enter a dormancy-like state ("winter quiescence") to survive freezing temperatures at the northern edges of the species range.

genomics↗

bioRxiv2022

The deep-sea coral, Callogorgia delta, associates with bacteria belonging to a novel marine branch of the Mollicutes

Microbes perform critical functions in corals yet most knowledge is derived from the photic zone. Here, we discovered two mollicutes that dominate the microbiome of the deep-sea octocoral, Callogorgia delta, and reside in the mesoglea. These symbionts were abundant across the hosts range, absent in the water, and rare in sediments. The symbionts lack all known fermentative capabilities including glycolysis and can only generate energy from arginine provided by the coral host. Their genomes feature extensive mechanisms to interact with foreign DNA which may be indicative of their role in symbiosis. We erect the novel family Oceanoplasmataceae which includes these symbionts and others associated with four marine invertebrate phyla. Its exceptionally broad host range suggests that the diversity of this enigmatic family remains largely undiscovered. Oceanoplasmataceae genomes are the most highly reduced among mollicutes providing new insight into their reductive evolution and the roles of coral symbionts.

microbiology↗

bioRxiv2021

An algal symbiont (Breviolum psygmophilum) responds more strongly to chronic high temperatures than its facultatively symbiotic coral host (Astrangia poculata)

Scleractinian corals form the foundation of coral reefs by secreting skeletons of calcium carbonate. Their intracellular algal symbionts (Symbiodiniaceae) translocate a large proportion of photosynthate to the coral host, which is required to maintain high rates of calcification. Global warming is causing dissociation of coral host and algal symbiont, visibly presented as coral bleaching. Despite decades of study, the precise mechanisms of coral bleaching remain unknown. Separating the thermal stress response of the coral from the algal symbiont is key to understanding bleaching in tropical corals. The facultatively symbiotic northern star coral, Astrangia poculata, naturally occurs as both symbiotic and aposymbiotic (lacking algal symbionts) polyps - sometimes on the same coral colony. Thus, it is possible to separate the heat stress response of the coral host alone from the coral in symbiosis with its symbiont Breviolum psygmophilum. Using replicate symbiotic and aposymbiotic ramets of A. poculata, we conducted a chronic heat stress experiment to increase our understanding of the cellular mechanisms resulting in coral bleaching. Sustained high temperature stress resulted in photosynthetic dysfunction in B. psygmophilum, including a decline in maximum photosynthesis rate, maximum photochemical efficiency, and the absorbance peak of chlorophyll a. Interestingly, the metabolic rates of symbiotic and aposymbiotic corals were differentially impacted. RNAseq analysis revealed more differentially expressed genes between heat-stressed and control aposymbiotic colonies than heat-stressed and control symbiotic colonies. Notably, aposymbiotic colonies increased the expression of inflammation-associated genes such as nitric oxide synthases. Unexpectedly, the largest transcriptional response was observed between heat-stressed and control B. psygmophilum, including genes involved in photosynthesis, response to oxidative stress, and meiosis. Thus, it appears that the algal symbiont suppresses the immune response of the host, potentially increasing the vulnerability of the host to pathogens. The A. poculata-B. psygmophilum symbiosis provides a tractable model system for investigating thermal stress and immune challenge in scleractinian corals.

ecology↗

bioRxiv2021

The coral symbiont Candidatus Aquarickettsia is variably abundant in threatened Caribbean acroporids and transmitted horizontally.

The aquatic symbiont "Candidatus Aquarickettsia rohweri" infects a diversity of non-bilaterian metazoan phyla. In the threatened coral Acropora cervicornis, Aquarickettsia proliferates in response to increased nutrient exposure, resulting in suppressed growth and increased disease susceptibility and mortality. This study evaluated the extent, as well as the ecology and evolution of Aquarickettsia infecting the Caribbean corals: Ac. cervicornis and Ac. palmata and their hybrid ( Ac. prolifera). The bacterial parasite Aquarickettsia was found in all acroporids, with host and sampling location impacting infection magnitude. Phylogenomic and genome-wide single nucleotide variant analysis found Aquarickettsia clustering by region, not by coral taxon. Fixation analysis suggested within coral colonies, Aquarickettsia are genetically isolated to the extent that reinfection is unlikely. Relative to other Rickettsiales, Aquarickettsia is undergoing positive selection, with Florida populations experiencing greater positive selection relative to the other Caribbean locations. This may be due to Aquarickettsia response to increased nutrient stress in Florida, as indicated by greater in situ replication rates in these corals. Aquarickettsia did not significantly codiversify with either coral animal nor algal symbiont, and qPCR analysis of gametes and juveniles from susceptible coral genotypes indicated absence in early life stages. Thus, despite being an obligate parasite, Aquarickettsia must be horizontally transmitted via coral mucocytes, an unidentified secondary host, or a yet unexplored environmentally mediated mechanism. Importantly, the prevalence of Aquarickettsia in Ac. cervicornis and high abundance in Florida populations suggests that disease mitigation efforts in the US and Caribbean should focus on preventing early infection via horizontal transmission.

microbiology↗

bioRxiv2020

Juvenile corals inherit mutations acquired during the parents lifespan

128 years ago, August Weismann proposed that the only source of inherited genetic variation in animals is the germline1. Julian Huxley reasoned that if this were true, it would falsify Jean-Baptiste Lamarcks theory that acquired characteristics are heritable2. Since then, scientists have discovered that not all animals segregate germline cells from somatic cells permanently and early in development3. In fact, throughout their lives, Cnidaria4-6 and Porifera7 maintain primordial stem cells that continuously give rise to both germline and somatic cells. The fate of mutations generated in this primordial stem cell line during adulthood remains an open question. It was unknown whether post-embryonic mutations could be heritable in animals8-10--until now. Here we use two independent genetic marker analyses to show that post-embryonic mutations are inherited in the coral Acropora palmata (Cnidaria, Anthozoa). This discovery upends the long-held supposition that post-embryonic genetic mutations acquired over an animals lifetime in non-germline tissues are not heritable2. Over the centuries-long lifespan of a coral, the inheritance of post-embryonic mutations may not only change allele frequencies in the local larval pool but may also spread novel alleles across great distances via larval dispersal. Thus, corals may have the potential to adapt to changing environments via heritable somatic mutations10. This mechanism challenges our understanding of animal adaptation and prompts a deeper examination of both the process of germline determination in Cnidaria and the role of post-embryonic genetic mutations in adaptation and epigenetics of modular animals. Understanding the role of post-embryonic mutations in animal adaptation will be crucial as ecological change accelerates in the Anthropocene.

evolutionary biology↗

bioRxiv2020

Genotypic similarity among algal symbionts corresponds to associations with closely related coral hosts

Mutualisms where hosts are coupled metabolically to their symbionts often exhibit high partner fidelity. Most reef-building corals form obligate symbioses with specific species of photosymbionts, dinoflagellates in the family Symbiodiniaceae, despite needing to acquire symbionts early in their development from environmental sources. Three Caribbean acroporids (Acropora palmata, A. cervicornis, and their hybrid A. prolifera) are geographically sympatric across much of their range in the greater Caribbean, but often occupy different depth and light habitats. Both species and their hybrid associate with Symbiodinium fitti, a genetically diverse species of symbiont that is specific to these hosts. Since the physiology of the dinoflagellate partner is strongly influenced by light (and therefore depth), we investigated whether S. fitti populations from each host source were differentiated genetically. We generated shallow genome sequences of acroporid colonies sampled from across the Caribbean. Single Nucleotide Polymorphisms (SNPs) among S. fitti strains were identified by aligning sequences to a ~600 Mb draft assembly of the S. fitti genome, assembled from an A. cervicornis metagenome. Phylogenomic and multivariate analyses revealed that allelic variation among S. fitti partitioned to each host species, as well as their hybrid, rather than by biogeographic origin. This is particularly noteworthy because the hybrid, A. prolifera, has a sparse fossil record and may be of relatively recent origin. Many of the SNPs putatively under selection were non-synonymous mutations predicted to alter protein efficiency. Differences in allele frequency among S. fitti populations from each host taxon may correspond to distinct phenotypes that thrive in the different cellular environments found in each acroporid. The non-random sorting among genetically diverse strains, or genotypes, to different hosts could be the basis for lineage diversification via disruptive selection, leading to ecological specialization and ultimately speciation.

genomics↗

bioRxiv2020

Deep-sea corals near cold seeps associate with chemoautotrophic bacteria that are related to the symbionts of cold seep and hydrothermal vent mussels.

Cnidarians are known for their symbiotic relationships, yet no known association exists between corals and chemoautotrophic microbes. Deep-sea corals, which support diverse animal communities in the Gulf of Mexico, are often found on authigenic carbonate in association with cold seeps. Sulfur-oxidizing chemoautotrophic bacteria of the SUP05 cluster are dominant symbionts of bathymodiolin mussels at cold seeps and hydrothermal vents around the world and have also been found in association with sponges. Therefore, we investigated whether other basal metazoans, corals, also associate with bacteria of the SUP05 cluster and report here that such associations are widespread. This was unexpected because it has been proposed that cnidarians would not form symbioses with chemoautotrophic bacteria due to their high oxygen demand and their lack of specialized respiratory structures. We screened corals, water, and sediment for SUP05 using 16S metabarcoding and found SUP05 phylotypes associated with corals at high relative abundance (10 - 91%). These coral-associated SUP05 phylotypes were coral host specific, absent in water samples, and rare or not detected in sediment samples. The genome of one SUP05 phylotype associated with Paramuricea sp. type B3, contained the genetic potential to oxidize reduced sulfur compounds and fix carbon and these pathways were transcriptionally active. Finally, the relative abundance of this SUP05 phylotype was positively correlated with chemoautotrophically-derived carbon and nitrogen input into the coral holobiont based on stable carbon and nitrogen isotopic compositions. We propose that SUP05 may supplement the diet of its host coral through chemoautotrophy or may provide nitrogen, essential amino acids, or vitamins. This is the first documented association between a chemoautotrophic symbiont and a cnidarian, broadening the known symbioses of corals and may represent a novel interaction between coral communities and cold seeps.

microbiology↗

bioRxiv2018

Fallen Pillars: The Past, Present, and Future Population Dynamics of a Rare, Specialist Coral-Algal Symbiosis

Rare and ecologically specialized species are at greater risk of extinction. Rarity in terms of low genotypic diversity may be obscured in sessile foundation species that can reproduce asexually. Asexual propagules are often only distinguishable from sexual recruits through molecular tools. Thus, molecular markers are necessary to assess genotypic variation and population structure in clonal organisms such as corals. The global decline of corals has necessitated marker development for improved conservation of rare coral species. We infer past demographic changes, describe modern population structure, and quantify asexual reproduction of the uncommon Caribbean pillar coral, Dendrogyra cylindrus and its endosymbiotic dinoflagellate, Symbiodinium dendrogyrum using de novo microsatellite markers. Results show that D. cylindrus comprises three distinct populations in the Caribbean whereas the symbiont was differentiated into four populations. Thus, barriers to gene flow differ between host and symbiont. In Florida, host and symbiont reproduced mainly asexually, yielding lower genotypic diversity than predicted from census size. Models of past demographic events revealed no evidence of historical changes in population size, consistent with the geological record of D. cylindrus indicating it has been rare for hundreds of thousands of years. The most recent global thermal stress event triggered a severe disease outbreak among D. cylindrus in Florida, resulting in a precipitous population decline. Projections indicate a high likelihood that this species may become locally extinct within the coming decades. The ecosystem consequences of losing rare coral species and their symbionts with increasingly frequent extreme warming events are not known but require urgent study.

genetics↗