Uncovering a mitochondrial unfolded protein response in corals and its role in adapting to a changing world.

The Anthropocene will be characterized by increased environmental disturbances, leading to the survival of stress-tolerant organisms, particularly in the oceans, where novel marine diseases and elevated temperatures are re-shaping ecosystems. These environmental changes underscore the importance of identifying mechanisms which promote stress tolerance in ecologically important non-model species such as reef-building corals. Mitochondria are central regulators of cellular stress and have dedicated recovery pathways including the mitochondrial unfolded protein response, which increases the transcription of protective genes promoting protein homeostasis, free radical detoxification and innate immunity. In this investigation, we identify a mitochondrial unfolded protein response in the endangered Caribbean coral Orbicella faveolata, by performing in vivo functional replacement using a transcription factor (Of-ATF5) originating from a coral in the model organism Caenorhabditis elegans. In addition, we use RNA-seq network analysis and transcription factor-binding predictions to identify a transcriptional network of genes likely to be regulated by Of-ATF5 which is induced during the immune challenge and temperature stress. Overall, our findings uncover a conserved cellular pathway which may promote the ability of reef-building corals to survive increasing levels of environmental stress.

Life or death: disease-tolerant coral species activate autophagy following immune challenge.

Global climate change has increased the number and severity of stressors affecting species, yet not all species respond equally to these stressors. Organisms may employ cellular mechanisms such as apoptosis and autophagy in responding to stressful events. These two pathways are often mutually exclusive, dictating whether a cell adapts or dies. In order to examine differences in cellular response to stress, we compared the immune response of four coral species with a range of disease susceptibility. Using RNA-seq and novel pathway analysis, we were able to identify differences in response to immune stimulation between these species. Disease-susceptible species Orbicella faveolata activated pathways associated with apoptosis. By contrast, disease-tolerant species Porites porites and Porites astreoides activated autophagic pathways. Moderately susceptible species Pseudodiploria strigosa activated a mixture of these pathways. These findings were corroborated by apoptotic caspase protein assays, which indicated increased caspase activity following immune stimulation in susceptible species. Our results indicate that in response to immune stress, disease-tolerant species activate cellular adaptive mechanisms such as autophagy, while susceptible species turn on cell death pathways. Differences in these cellular maintenance pathways may therefore influence the organismal stress response. Further study of these pathways will increase understanding of differential stress response and species survival in the face of changing environments.

Structural and Evolutionary Relationships of Melanin Cascade Proteins in Cnidarian Innate Immunity.

Melanin is an essential product that plays an important role in innate immunity in a variety of organisms across the animal kingdom. Melanin synthesis is performed by many organisms using the tyrosine metabolism pathway, a general pathway that utilizes a type-three copper oxidase protein, called PO-candidates (phenoloxidase candidates). While melanin synthesis is well characterized in organisms like arthropods and humans, it is not as well understood in non-model organisms such as cnidarians. With the rising anthropomorphic climate change influence on marine ecosystems, cnidarians, specifically corals, are under an increased threat of bleaching and disease. Understanding innate immune pathways, such as melanin synthesis, is vital to gaining insights into how corals may be able to fight these threats. In this study, we use comparative bioinformatic approaches to provide a comprehensive analysis of genes involved in tyrosine-mediated melanin synthesis in cnidarians. Eighteen PO-candidates representing five phyla were studied to identify their evolutionary relationship. Cnidarian species were most similar to chordates due to domain presents in the amino acid sequences. From there, functionally conserved domains in coral proteins were identified in a coral disease dataset. Five stony corals exposed to stony coral tissue loss disease were leveraged to identify eighteen putative tyrosine metabolism genes, genes with functionally conserved domains to their Homo sapiens counterpart. To put this pathway the context of coral health, putative genes were correlated to melanin concentration from tissues of stony coral species in the disease exposure dataset. In this study, tyrosinase was identified in stony corals as correlated to melanin concentrations and likely plays a key role in immunity as a resistance trait. In addition, stony coral genes were assigned to all modules within the tyrosine metabolism pathway, indicating an evolutionary conservation of this pathway across phyla. Overall, this study provides a comprehensive analysis of the genes involved in tyrosine-mediated melanin synthesis in cnidarians.

Genetics of coral resilience.

Genome-wide study in staghorn coral identifies markers of disease resistance.

Stony coral tissue loss disease induces transcriptional signatures of in situ degradation of dysfunctional Symbiodiniaceae.

Stony coral tissue loss disease (SCTLD), one of the most pervasive and virulent coral diseases on record, affects over 22 species of reef-building coral and is decimating reefs throughout the Caribbean. To understand how different coral species and their algal symbionts (family Symbiodiniaceae) respond to this disease, we examine the gene expression profiles of colonies of five species of coral from a SCTLD transmission experiment. The included species vary in their purported susceptibilities to SCTLD, and we use this to inform gene expression analyses of both the coral animal and their Symbiodiniaceae. We identify orthologous coral genes exhibiting lineage-specific differences in expression that correlate to disease susceptibility, as well as genes that are differentially expressed in all coral species in response to SCTLD infection. We find that SCTLD infection induces increased expression of rab7, an established marker of in situ degradation of dysfunctional Symbiodiniaceae, in all coral species accompanied by genus-level shifts in Symbiodiniaceae photosystem and metabolism gene expression. Overall, our results indicate that SCTLD infection induces symbiophagy across coral species and that the severity of disease is influenced by Symbiodiniaceae identity.

Variations in reactive oxygen release and antioxidant activity in multiple Symbiodinium types in response to elevated temperature.

As ocean temperatures rise, investigations into what the physiological effects will be on the symbiotic microalga Symbiodinium, and how these may play into the cnidarian bleaching response, have highlighted the contribution of reactive oxygen species (ROS). Previous studies have laid this groundwork using a limited number of Symbiodinium phylotypes, and so this study aims to expand this understanding by exploring the effects of sub-lethal elevated temperatures on the physiological response of seven genetically distinct types of Symbiodinium, including A1, B1, B2, C1, D, E1, and F2. The production of ROS (at 26 °C, 29 °C, 30 °C, and 31 °C) and activity of the antioxidants catalase (CAT) and superoxide dismutase (SOD) (at 26 °C and 31 °C) were measured as indicators of sensitivity or tolerance to heat stress. Symbiodinium types B1 and C1 were the most thermally sensitive, with C1 producing the highest amount of ROS at elevated temperatures. Types A1 and F2 were tolerant, having no increase in ROS production, and were the only types to increase both CAT and SOD activity with temperature stress. Type B2 had decreased ROS production and elevation of CAT activity, while type E1 had decreased levels of ROS production at elevated temperatures. Type D was the only Symbiodinium type to remain unaffected by elevated temperatures. These results are consistent with previous findings of relative sensitivity or tolerance to elevated temperatures, specifically with regards to types A1, B1, and F2. The inclusion of types B2, C1, D, and E1 provides further new evidence of how types differ in their thermal responses, suggesting differing mechanisms exist in the Symbiodnium response to higher temperature and highlighting the importance of establishing symbiont identity when exploring the response of intact associations to this type of stress.

Microbial regulation in gorgonian corals.

Gorgonian corals possess many novel natural products that could potentially mediate coral-bacterial interactions. Since many bacteria use quorum sensing (QS) signals to facilitate colonization of host organisms, regulation of prokaryotic cell-to-cell communication may represent an important bacterial control mechanism. In the present study, we examined extracts of twelve species of Caribbean gorgonian corals, for mechanisms that regulate microbial colonization, such as antibacterial activity and QS regulatory activity. Ethanol extracts of gorgonians collected from Puerto Rico and the Florida Keys showed a range of both antibacterial and QS activities using a specific Pseudomonas aeruginosa QS reporter, sensitive to long chain AHLs and a short chain N-acylhomoserine lactones (AHL) biosensor, Chromobacterium violaceium. Overall, the gorgonian corals had higher antimicrobial activity against non-marine strains when compared to marine strains. Pseudopterogorgia americana, Pseusopterogorgia acerosa, and Pseudoplexuara flexuosa had the highest QS inhibitory effect. Interestingly, Pseudoplexuara porosa extracts stimulated QS activity with a striking 17-fold increase in signal. The stimulation of QS by P. porosa or other elements of the holobiont may encourage colonization or recruitment of specific microbial species. Overall, these results suggest the presence of novel stimulatory QS, inhibitory QS and bactericidal compounds in gorgonian corals. A better understanding of these compounds may reveal insight into coral-microbial ecology and whether a therapeutic potential exists.

Disease resistance in coral is mediated by distinct adaptive and plastic gene expression profiles.

Infectious diseases are an increasing threat to coral reefs, resulting in altered community structure and hindering the functional contributions of disease-susceptible species. We exposed seven reef-building coral species from the Caribbean to white plague disease and determined processes involved in (i) lesion progression, (ii) within-species gene expression plasticity, and (iii) expression-level adaptation among species that lead to differences in disease risk. Gene expression networks enriched in immune genes and cytoskeletal arrangement processes were correlated to lesion progression rates. Whether or not a coral developed a lesion was mediated by plasticity in genes involved in extracellular matrix maintenance, autophagy, and apoptosis, while resistant coral species had constitutively higher expression of intracellular protein trafficking. This study offers insight into the process involved in lesion progression and within- and between-species dynamics that lead to differences in disease risk that is evident on current Caribbean reefs.

Experimental transmission of Stony Coral Tissue Loss Disease results in differential microbial responses within coral mucus and tissue.

Stony coral tissue loss disease (SCTLD) is a widespread and deadly disease that affects nearly half of Caribbean coral species. To understand the microbial community response to this disease, we performed a disease transmission experiment on US Virgin Island (USVI) corals, exposing six species of coral with varying susceptibility to SCTLD. The microbial community of the surface mucus and tissue layers were examined separately using a small subunit ribosomal RNA gene-based sequencing approach, and data were analyzed to identify microbial community shifts following disease acquisition, potential causative pathogens, as well as compare microbiota composition to field-based corals from the USVI and Florida outbreaks. While all species displayed similar microbiome composition with disease acquisition, microbiome similarity patterns differed by both species and mucus or tissue microhabitat. Further, disease exposed but not lesioned corals harbored a mucus microbial community similar to those showing disease signs, suggesting that mucus may serve as an early warning detection for the onset of SCTLD. Like other SCTLD studies in Florida, Rhodobacteraceae, Arcobacteraceae, Desulfovibrionaceae, Peptostreptococcaceae, Fusibacter, Marinifilaceae, and Vibrionaceae dominated diseased corals. This study demonstrates the differential response of the mucus and tissue microorganisms to SCTLD and suggests that mucus microorganisms may be diagnostic for early disease exposure.

Patterns of coral ecological immunology: variation in the responses of Caribbean corals to elevated temperature and a pathogen elicitor.

Disease epizootics are increasing with climatic shifts, yet within each system only a subset of species are identified as the most vulnerable. Understanding ecological immunology patterns as well as environmental influences on immune defenses will provide insight into the persistence of a functional system through adverse conditions. Amongst the most threatened ecosystems are coral reefs, with coral disease epizootics and thermal stress jeopardizing their survival. Immune defenses were investigated within three Caribbean corals, Montastraea faveolata, Stephanocoenia intersepta and Porites astreoides, which represent a range of disease and bleaching susceptibilities. Levels of several immune parameters were measured in response to elevated water temperature and the presence of a commercial pathogen-associated molecular pattern (PAMP) - lipopolysaccharide (LPS) - as an elicitor of the innate immune response. Immune parameters included prophenoloxidase (PPO) activity, melanin concentration, bactericidal activity, the antioxidants peroxidase and catalase, and fluorescent protein (FP) concentration. LPS induced an immune response in all three corals, although each species responded differently to the experimental treatments. For example, M. faveolata, a disease-susceptible species, experienced significant decreases in bactericidal activity and melanin concentration after exposure to LPS and elevated temperature alone. Porites astreoides, a disease-resistant species, showed increased levels of enzymatic antioxidants upon exposure to LPS independently and increased PPO activity in response to the combination of LPS and elevated water temperature. This study demonstrates the ability of reef-building corals to induce immune responses in the presence of PAMPs, indicating activation of PAMP receptors and the transduction of appropriate signals leading to immune effector responses. Furthermore, these data address the emerging field of ecological immunology by highlighting interspecific differences in immunity and immunocompetences among Caribbean corals, which are reflected in their life-history characteristics, disease susceptibilities and bleaching-induced mortality.

The presence of multiple phenoloxidases in Caribbean reef-building corals.

The melanin-synthesis pathways, phenoloxidase (PO) and laccases, are staple components of invertebrate immunity and have been shown to be vital in disease resistance. The importance of this pathway in immunity is a consequence of the release of oxygen radicals with cytotoxic effects and the production of insoluble melanin, which aids in the encapsulation of pathogens and parasites. Recently, melanization has been demonstrated as a critical immune response in several coral systems, although the biochemical components have not been thoroughly investigated. Coral diseases are posing a serious threat to coral reef survival, necessitating a full understanding of resistance mechanisms. In this study, we take a comparative approach to probe potential pathway components of melanin-synthesis in seven species from four different families of healthy Caribbean reef-building corals. Using different quinone substrates, we tested for the activity of the POs catecholase and cresolase, as well as laccase activity in each coral species. Since many invertebrate POs demonstrate some dependence on cations such as copper, calcium and magnesium, we treated the coral extracts with the chelators EDTA and EGTA to test the reliance of coral catecholase on these cations. The activity of the antioxidants peroxidase, superoxide dismutase and catalase was also tested in each coral and correlated to PO activity. All corals had demonstrable catecholase, cresolase and laccase activities, but only catecholase and cresolase activities varied significantly among species. Catecholase activity in each coral species was reduced by treatment with EDTA and EGTA, although some coral species were less affected than the others. Overall, these data show remarkable heterogeneity among the seven coral species of boulder-like reef building Caribbean coral. These differences may originate from the level of investment of each coral species into immunity and may explain disease ecology on the reef.

Cnidarian Pattern Recognition Receptor Repertoires Reflect Both Phylogeny and Life History Traits.

Pattern recognition receptors (PRRs) are evolutionarily ancient and crucial components of innate immunity, recognizing danger-associated molecular patterns (DAMPs) and activating host defenses. Basal non-bilaterian animals such as cnidarians must rely solely on innate immunity to defend themselves from pathogens. By investigating cnidarian PRR repertoires we can gain insight into the evolution of innate immunity in these basal animals. Here we utilize the increasing amount of available genomic resources within Cnidaria to survey the PRR repertoires and downstream immune pathway completeness within 15 cnidarian species spanning two major cnidarian clades, Anthozoa and Medusozoa. Overall, we find that anthozoans possess prototypical PRRs, while medusozoans appear to lack these immune proteins. Additionally, anthozoans consistently had higher numbers of PRRs across all four classes relative to medusozoans, a trend largely driven by expansions in NOD-like receptors and C-type lectins. Symbiotic, sessile, and colonial cnidarians also have expanded PRR repertoires relative to their non-symbiotic, mobile, and solitary counterparts. Interestingly, cnidarians seem to lack key components of mammalian innate immune pathways, though similar to PRR numbers, anthozoans possess more complete immune pathways than medusozoans. Together, our data indicate that anthozoans have greater immune specificity than medusozoans, which we hypothesize to be due to life history traits common within Anthozoa. Overall, this investigation reveals important insights into the evolution of innate immune proteins within these basal animals.

Microbial dysbiosis reflects disease resistance in diverse coral species.

Disease outbreaks have caused significant declines of keystone coral species. While forecasting disease outbreaks based on environmental factors has progressed, we still lack a comparative understanding of susceptibility among coral species that would help predict disease impacts on coral communities. The present study compared the phenotypic and microbial responses of seven Caribbean coral species with diverse life-history strategies after exposure to white plague disease. Disease incidence and lesion progression rates were evaluated over a seven-day exposure. Coral microbiomes were sampled after lesion appearance or at the end of the experiment if no disease signs appeared. A spectrum of disease susceptibility was observed among the coral species that corresponded to microbial dysbiosis. This dysbiosis promotes greater disease susceptiblity in coral perhaps through different tolerant thresholds for change in the microbiome. The different disease susceptibility can affect coral's ecological function and ultimately shape reef ecosystems.

What are the physiological and immunological responses of coral to climate warming and disease?

Coral mortality due to climate-associated stress is likely to increase as the oceans get warmer and more acidic. Coral bleaching and an increase in infectious disease are linked to above average sea surface temperatures. Despite the uncertain future for corals, recent studies have revealed physiological mechanisms that improve coral resilience to the effects of climate change. Some taxa of bleached corals can increase heterotrophic food intake and exchange symbionts for more thermally tolerant clades; this plasticity can increase the probability of surviving lethal thermal stress. Corals can fight invading pathogens with a suite of innate immune responses that slow and even arrest pathogen growth and reduce further tissue damage. Several of these responses, such as the melanin cascade, circulating amoebocytes and antioxidants, are induced in coral hosts during pathogen invasion or disease. Some components of immunity show thermal resilience and are enhanced during temperature stress and even in bleached corals. These examples suggest some plasticity and resilience to cope with environmental change and even the potential for evolution of resistance to disease. However, there is huge variability in responses among coral species, and the rate of climate change is projected to be so rapid that only extremely hardy taxa are likely to survive the projected changes in climate stressors.

Investigating the roles of transforming growth factor-beta in immune response of Orbicella faveolata, a scleractinian coral.

Symbiotic relationships range from parasitic to mutualistic, yet all endosymbionts face similar challenges, including evasion of host immunity. Many symbiotic organisms have evolved similar mechanisms to face these challenges, including manipulation of the host's transforming growth factor-beta (TGFß) pathway. Here we investigate the TGFß pathway in scelaractinian corals which are dependent on symbioses with dinoflagellates from the family Symbiodiniaceae. Using the Caribbean coral, Orbicella faveolata, we explore the effects of enhancement and inhibition of the TGFß pathway on host gene expression. Following transcriptomic analyses, we demonstrated limited effects of pathway manipulation in absence of immune stimulation. However, manipulation of the TGFß pathway significantly affects the subsequent ability of host corals to mount an immune response. Enhancement of the TGFß pathway eliminates transcriptomic signatures of host coral immune response, while inhibition of the pathway maintains the response. This is, to our knowledge, the first evidence of an immunomodulatory role for TGFß in a scelaractinian coral. These findings suggest variation in TGFß signaling may have implications in the face of increasing disease prevelance. Our results suggest that the TGFß pathway can modulate tradeoffs between symbiosis and immunity. Further study of links between symbiosis, TGFß, and immunity is needed to better understand the ecological implications of these findings.

Immune defenses of healthy, bleached and diseased Montastraea faveolata during a natural bleaching event.

One prominent hypothesis regarding climate change and scleractinian corals is that thermal stress compromises immune competence. To test this hypothesis we tracked how the immune defenses of bleached, apparently healthy and yellow band disease (YBD) diseased Montastraea faveolata colonies varied with natural thermal stress in southwestern Puerto Rico. Colonies were monitored for 21 mo from the peak of the bleaching event in October 2005 to August 2007. Since sea surface temperature was significantly higher in summer and fall 2005 than 2006, year of collection was used as a proxy for temperature stress, and colony fragments collected in 2005 were compared with those collected in 2006. Mortality rate was high (43% overall) and all colonies (except one) either died or became infected with YBD by August 2007. YBD-infected tissue did not bleach (i.e. expel zooxanthellae) during the 2005 bleaching event, even when healthy tissue of these colonies bleached. Immune activity was assayed by measuring prophenoloxidase (PPO), peroxidase (POX), lysozyme-like (LYS) and antibacterial (AB) activity. Immune activity was variable between all coral samples, but there was a significant elevation of PPO activity in bleached colonies collected in 2005 relative to apparently healthy and YBD-diseased corals in 2006. In YBD-diseased colonies, LYS and AB activity were elevated in both healthy and infected tissue, indicating a systemic response; activity levels in these colonies were higher compared to those that appeared healthy. In both these immune parameters, there was a trend for suppression of activity in corals that were bleached in 2005. These data, while complicated by between-genet variability, illustrate the complex interaction between disease and temperature stress on immune function.

Chemical Responses to the Biotic and Abiotic Environment by Early Diverging Metazoans Revealed in the Post-Genomic Age.

For many years methodological constraints limited insights on the molecular biology of non-model organisms. However, the development of various sequencing platforms has led to an explosion of transcriptomic and genomic data on non-model systems. As a consequence the molecular drivers of organismal phenotypes are becoming clearer and the chemicals that animals use to detect and respond to their environments are increasingly being revealed-this latter area inspired our symposium theme. The papers in this volume broadly address this theme by their more specific focus in one of the following general areas: 1) sensory biology and the molecular basis of perception, 2) chemicals deployed to deal with the biotic and abiotic environment, and 3) chemical interactions along the parasite-mutualist continuum. Here we outline and synthesize the content of these papers-an exercise which demonstrates that sophisticated gene repertoires enable early diverging metazoans to encode many of the signaling, sensory, defensive, and offensive capacities typically associated with animals that have complex nervous systems. We then consider opportunities and associated challenges that may delay progress in comparative functional biochemistry, a reinvigorated field that can be expected to rapidly expand with new 'omics data. Future knowledge of chemical adaptations should afford new perspectives on the comparative evolution of chemical mediators.

Responding to Threats Both Foreign and Domestic: NOD-Like Receptors in Corals.

Historically mechanisms with which basal animals such as reef-building corals use to respond to changing and increasingly stressful environments have remained elusive. However, the increasing availability of genomic and transcriptomic data from these organisms has provided fundamental insights into the biology of these critically important ecosystem engineers. Notably, insights into cnidarians gained in the post-genomics age have revealed a surprisingly complex immune system which bears a surprising level of similarity with the vertebrate innate immune system. This system has been critically linked to how corals respond to the two most prominent threats on a global scale, emerging coral diseases and increasing water temperature, which are recognized cellularly as either foreign or domestic threats, respectively. These threats can arise from pathogenic microbes or internal cellular dysfunction, underscoring the need to further understand mechanisms corals use to sense and respond to threats to their cellular integrity. In this investigation and meta-analysis, we utilize resources only recently available in the post-genomic era to identify and characterize members of an underexplored class of molecules known as NOD-like receptors in the endangered Caribbean coral Orbicella faveolata. We then leverage these data to identify pathways possibly mediated by NLRs in both O. faveolata and the ecologically important branching coral Acropora digitifera. Overall, we find support that this class of proteins may provide a mechanistic link to how reef-building corals respond to threats both foreign and domestic.

Proteomic Investigation of a Diseased Gorgonian Coral Indicates Disruption of Essential Cell Function and Investment in Inflammatory and Other Immune Processes.

As scleractinian coral cover declines in the face of increased frequency in disease outbreaks, future reefs may become dominated by octocorals. Understanding octocoral disease responses and consequences is therefore necessary if we are to gain insight into the future of ecosystem services provided by coral reefs. In Florida, populations of the octocoral Eunicea calyculata infected with Eunicea black disease (EBD) were observed in the field in the fall of 2011. This disease was recognized by a stark, black pigmentation caused by heavy melanization. Histological preparations of E. calyculata infected with EBD demonstrated granular amoebocyte (GA) mobilization, melanin granules in much of the GA population, and the presence of fungal hyphae penetrating coral tissue. Previous transcriptomic analysis also identified immune trade-offs evidenced by increased immune investment at the expense of growth. Our investigation utilized proteogenomic techniques to reveal decreased investment in general cell signaling while increasing energy production for immune responses. Inflammation was also prominent in diseased E. calyculata and sheds light on factors driving the extreme phenotype observed with EBD. With disease outbreaks continuing to increase in frequency, our results highlight new targets within the cnidarian immune system and provide a framework for understanding transcriptomics in the context of an organismal disease phenotype and its protein expression.

Evidence of an inflammatory-like response in non-normally pigmented tissues of two scleractinian corals.

Increasing evidence of links between climate change, anthropogenic stress and coral disease underscores the importance of understanding the mechanisms by which reef-building corals resist infection and recover from injury. Cellular inflammation and melanin-producing signalling pathway are two mechanisms employed by invertebrates to remove foreign organisms such as pathogens, but they have not been recorded previously in scleractinian corals. This study demonstrates the presence of the phenoloxidase (PO) activating melanin pathway in two species of coral, Acropora millepora and a massive species of Porites, which both develop local pigmentation in response to interactions with a variety of organisms. L-DOPA (3-(3,4-dihydroxyphenyl)-L-alanine) substrate-based enzyme activation assays demonstrated PO activity in healthy tissues of both species and upregulation in pigmented tissues of A. millepora. Histological staining conclusively identified the presence of melanin in Porites tissues. These results demonstrate that the PO pathway is active in both coral species. Moreover, the upregulation of PO activity in areas of non-normal pigmentation in A. millepora and increased melanin production in pigmented Porites tissues suggest the presence of a generalized defence response to localized stress. Interspecific differences in the usage of pathways involved in innate immunity may underlie the comparative success of massive Porites sp. as long-lived stress tolerators.