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1. Host-symbiont coevolution, cryptic structure, and bleaching susceptibility, in a coral species complex (Scleractinia; Poritidae)

   https://doi.org/10.1038/s41598-020-73501-6  

   Forsman, Z. H. ; Ritson-Williams, R. ; Tisthammer, K. H. ; Knapp, I. S. S. ; Toonen, R. J. ( October 2020 , Scientific Reports)

   The ‘species’ is a key concept for conservation and evolutionary biology, yet the lines between population and species-level variation are often blurred, especially for corals. The ‘Porites lobataspecies complex’ consists of branching and mounding corals that form reefs across the Pacific. We used reduced representation meta-genomic sequencing to examine genetic relationships within this species complex and to identify candidate loci associated with colony morphology, cryptic genetic structure, and apparent bleaching susceptibility. We compared existingPoritesdata with bleached and unbleached colonies of the branching coralP. compressacollected in Kāneʻohe Bay Hawaiʻi during the 2015 coral bleaching event. Loci that mapped to coral, symbiont, and microbial references revealed genetic structure consistent with recent host-symbiont co-evolution. Cryptic genetic clades were resolved that previous work has associated with distance from shore, but no genetic structure was associated with bleaching. We identified many candidate loci associated with morphospecies, including candidate host and symbiont loci with fixed differences between branching and mounding corals. We also found many loci associated with cryptic genetic structure, yet relatively few loci associated with bleaching. Recent host-symbiont co-evolution and rapid diversification suggests that variation and therefore the capacity of these corals to adapt may be underappreciated.

    

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2. Population genetics of the coral Acropora millepora : Toward genomic prediction of bleaching

   https://doi.org/10.1126/science.aba4674  

   Fuller, Zachary L. ; Mocellin, Veronique J. L. ; Morris, Luke A. ; Cantin, Neal ; Shepherd, Jihanne ; Sarre, Luke ; Peng, Julie ; Liao, Yi ; Pickrell, Joseph ; Andolfatto, Peter ; et al ( July 2020 , Science)

   Although reef-building corals are declining worldwide, responses to bleaching vary within and across species and are partly heritable. Toward predicting bleaching response from genomic data, we generated a chromosome-scale genome assembly for the coralAcropora millepora. We obtained whole-genome sequences for 237 phenotyped samples collected at 12 reefs along the Great Barrier Reef, among which we inferred little population structure. Scanning the genome for evidence of local adaptation, we detected signatures of long-term balancing selection in the heat-shock co-chaperonesacsin. We conducted a genome-wide association study of visual bleaching score for 213 samples, incorporating the polygenic score derived from it into a predictive model for bleaching in the wild. These results set the stage for genomics-based approaches in conservation strategies.

    

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3. Population genetics of the coral Acropora millepora: Toward genomic prediction of bleaching

   https://doi.org/10.1101/867754  

   Fuller, Zachary L ; Mocellin, Veronique J ; Morris, Luke A ; Cantin, Neal ; Shepherd, Jihanne ; Sarre, Luke ; Peng, Julie ; Liao, Yi ; Pickrell, Joseph ; Andolfatto, Peter ; et al ( July 2020 , Science)

   Although reef-building corals are declining worldwide, responses to bleaching vary within and across species and are partly heritable. Toward predicting bleaching response from genomic data, we generated a chromosome-scale genome assembly for the coral Acropora millepora We obtained whole-genome sequences for 237 phenotyped samples collected at 12 reefs along the Great Barrier Reef, among which we inferred little population structure. Scanning the genome for evidence of local adaptation, we detected signatures of long-term balancing selection in the heat-shock co-chaperone sacsin We conducted a genome-wide association study of visual bleaching score for 213 samples, incorporating the polygenic score derived from it into a predictive model for bleaching in the wild. These results set the stage for genomics-based approaches in conservation strategies. 

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4. Transcriptomic resilience, symbiont shuffling, and vulnerability to recurrent bleaching in reef‐building corals

   https://doi.org/10.1111/mec.15143  

   Thomas, Luke ; López, Elora H. ; Morikawa, Megan K. ; Palumbi, Stephen R. ( July 2019 , Molecular Ecology)

   As climate change progresses and extreme temperature events increase in frequency, rates of disturbance may soon outpace the capacity of certain species of reef‐building coral to recover from bleaching. This may lead to dramatic shifts in community composition and ecosystem function. Understanding variation in rates of bleaching recovery among species and how that translates to resilience to recurrent bleaching is fundamental to predicting the impacts of increasing disturbances on coral reefs globally. We tracked the response of two heat sensitive species in the genusAcroporato repeated bleaching events during the austral summers of 2015 and 2017. Despite a similar bleaching response, the speciesAcropora gemmiferarecovered faster based on transcriptome‐wide gene expression patterns and had a more dynamic algal symbiont community thanAcropora hyacinthusgrowing on the same reef. Moreover,A. gemmiferahad higher survival to repeated heat extremes, with six‐fold lower mortality thanA. hyacinthus. These patterns suggest that speed of recovery from a first round of bleaching, based on multiple mechanisms, contributes strongly to sensitivity to a second round of bleaching. Furthermore, our data uncovered intragenus variation in a group of corals thought generally to be heat‐sensitive and therefore paint a more nuanced view of the future health of coral reef ecosystems against a backdrop of increasing thermal disturbances.

    

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5. Fine-scale variability in coral bleaching and mortality during a marine heatwave

   https://doi.org/10.3389/fmars.2023.1108365  

   Yadav, Shreya ; Roach, Ty N. ; McWilliam, Michael J. ; Caruso, Carlo ; de Souza, Mariana Rocha ; Foley, Catherine ; Allen, Corinne ; Dilworth, Jenna ; Huckeba, Joel ; Santoro, Erika P. ; et al ( June 2023 , Frontiers in Marine Science)

   Coral bleaching and mortality can show significant spatial and taxonomic heterogeneity at local scales, highlighting the need to understand the fine-scale drivers and impacts of thermal stress. In this study, we used structure-from-motion photogrammetry to track coral bleaching, mortality, and changes in community composition during the 2019 marine heatwave in Kāneʻohe Bay, Hawaiʻi. We surveyed 30 shallow reef patches every 3 weeks for the duration of the bleaching event (August-December) and one year after, resulting in a total of 210 large-area, high-resolution photomosaics that enabled us to follow the fate of thousands of coral colonies through time. We also measured environmental variables such as temperature, sedimentation, depth, and wave velocity at each of these sites, and extracted estimates of habitat complexity (rugosity R and fractal dimension D) from digital elevation models to better understand their effects on patterns of bleaching and mortality. We found that up to 80% of corals experienced moderate to severe bleaching in this period, with peak bleaching occurring in October when heat stress (Degree Heating Weeks) reached its maximum. Mortality continued to accumulate as bleaching levels dropped, driving large declines in more heat-susceptible species (77% loss of Pocillopora cover) and moderate declines in heat-tolerant species (19% and 23% for Porites compressa and Montipora capitata , respectively). Declines in live coral were accompanied by a rapid increase in algal cover across the survey sites. Spatial differences in bleaching were significantly linked to habitat complexity and coral species composition, with reefs that were dominated by Pocillopora experiencing the most severe bleaching. Mortality was also influenced by species composition, fractal dimension, and site-level differences in thermal stress. Our results show that spatial heterogeneity in the impacts of bleaching are driven by a mix of environmental variation, habitat complexity, and differences in assemblage composition. 

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