Persistence and change in community composition of reef corals through present, past, and future climates

Decadal stability in coral cover could mask hidden changes on reefs in the East Asian Seas

Coral reefs in the Central Indo-Pacific region comprise some of the most diverse and yet threatened marine habitats. While reef monitoring has grown throughout the region in recent years, studies of coral reef benthic cover remain limited in spatial and temporal scales. Here, we analysed 24,365 reef surveys performed over 37 years at 1972 sites throughout East Asia by the Global Coral Reef Monitoring Network using Bayesian approaches. Our results show that overall coral cover at surveyed reefs has not declined as suggested in previous studies and compared to reef regions like the Caribbean. Concurrently, macroalgal cover has not increased, with no indications of phase shifts from coral to macroalgal dominance on reefs. Yet, models incorporating socio-economic and environmental variables reveal negative associations of coral cover with coastal urbanisation and sea surface temperature. The diversity of reef assemblages may have mitigated cover declines thus far, but climate change could threaten reef resilience. We recommend prioritisation of regionally coordinated, locally collaborative long-term studies for better contextualisation of monitoring data and analyses, which are essential for achieving reef conservation goals.

Warmer temperatures favor slower-growing bacteria in natural marine communities

Earth's life-sustaining oceans harbor diverse bacterial communities that display varying composition across time and space. While particular patterns of variation have been linked to a range of factors, unifying rules are lacking, preventing the prediction of future changes. Here, analyzing the distribution of fast- and slow-growing bacteria in ocean datasets spanning seasons, latitude, and depth, we show that higher seawater temperatures universally favor slower-growing taxa, in agreement with theoretical predictions of how temperature-dependent growth rates differentially modulate the impact of mortality on species abundances. Changes in bacterial community structure promoted by temperature are independent of variations in nutrients along spatial and temporal gradients. Our results help explain why slow growers dominate at the ocean surface, during summer, and near the tropics and provide a framework to understand how bacterial communities will change in a warmer world.

Deep resilience: An evolutionary perspective on calcification in an age of ocean acidification

The success of today's calcifying organisms in tomorrow's oceans depends, in part, on the resilience of their skeletons to ocean acidification. To the extent this statement is true there is reason to have hope. Many marine calcifiers demonstrate resilience when exposed to environments that mimic near-term ocean acidification. The fossil record similarly suggests that resilience in skeletons has increased dramatically over geologic time. This "deep resilience" is seen in the long-term stability of skeletal chemistry, as well as a decreasing correlation between skeletal mineralogy and extinction risk over time. Such resilience over geologic timescales is often attributed to genetic canalization-the hardening of genetic pathways due to the evolution of increasingly complex regulatory systems. But paradoxically, our current knowledge on biomineralization genetics suggests an opposing trend, where genes are co-opted and shuffled at an evolutionarily rapid pace. In this paper we consider two possible mechanisms driving deep resilience in skeletons that fall outside of genetic canalization: microbial co-regulation and macroevolutionary trends in skeleton structure. The mechanisms driving deep resilience should be considered when creating risk assessments for marine organisms facing ocean acidification and provide a wealth of research avenues to explore.

Co-occurring anthropogenic stressors reduce the timeframe of environmental viability for the world's coral reefs

Anthropogenic disturbances are posing unprecedented challenges to the persistence of ecosystems worldwide. The speed at which these disturbances reach an ecosystem's tolerance thresholds will determine the time available for adaptation and conservation. Here, we aim to calculate the year after which a given environmental stressor permanently exceeds the bounds of an ecosystem's tolerance. Ecosystem thresholds are here defined as limits in a given stressor beyond which ecosystems have showed considerable changes in community assembly and functioning, becoming remnants of what they once were, but not necessarily leading to species extirpation or extinction. Using the world's coral reefs as a case example, we show that the projected effects of marine heatwaves, ocean acidification, storms, land-based pollution, and local human stressors are being underestimated considerably by looking at disturbances independently. Given the spatial complementarity in which numerous disturbances impact the world's coral reefs, we show that the timelines of environmental suitability are halved when all disturbances are analyzed simultaneously, as opposed to independently. Under business-as-usual scenarios, the median year after which environmental conditions become unsuitable for the world's remaining coral reefs was, at worse, 2050 for any one disturbance alone (28 years left); but when analyzed concurrently, this date was shortened to 2035 (13 years left). When analyzed together, disturbances reduced the date of environmental suitability because areas that may remain suitable under one disturbance could become unsuitable by any of several other variables. The significance of co-occurring disturbances at reducing timeframes of environmental suitability was evident even under optimistic scenarios. The best-case scenario, characterized by strong mitigation of greenhouse gas emissions and optimistic human development, resulted in 41% of global coral reefs with unsuitable conditions by 2100 under any one disturbance independently; yet when analyzed in combination up to 64% of the world's coral reefs could face unsuitable environmental conditions by one disturbance or another. Under the worst-case scenario, nearly all coral reef ecosystems worldwide (approximately 99%) will permanently face unsuitable conditions by 2055 in at least one of the disturbances analyzed. Prior studies have indicated the projected dire effects of climate change on coral reefs by mid-century; by analyzing a multitude of projected disturbances, our study reveals a much more severe prognosis for the world's coral reefs as they have significantly less time to adapt while highlighting the urgent need to tackle available solutions to human disturbances.

Rapid recovery of coral communities from a mass bleaching event in the summer of 2016, observed in Amitori Bay, Iriomote Island, Japan

Devastating bleaching of coral communities at Amitori Bay, Iriomote Island, Japan, occurred in 2016 during the third global mass bleaching event in 2014-2017. The present study documented changes in coral communities in Amitori Bay from just before until after the 2016 bleaching event (2016-2020), by measuring coral cover and recruitment at nine sites (with two additional sites in 2018) in the bay. Spawning rates of acroporid corals were also monitored from 2017 to 2019 by visual observation and using bundle collectors to observe how long the effect of bleaching persisted. Reductions of 64.7 and 89.5% from 2016 to 2017 were observed in cover and recruitment of all coral families, respectively. Coral cover of all coral families recovered to pre-bleaching levels by 2020 and recruitment in 2020 was about two times greater than the pre-bleaching level. These results mirrored those of acroporids. Spawning rates of Acropora corals increased significantly from 40.6% in 2017 to 90.0% in 2019. Recovery of coral cover 4 years after the severe bleaching event was likely related to regrowth of remnants and of surviving juveniles of < 5 cm. The sudden increase in recruitment was likely driven by a combination of larval supply from other populations, increased numbers of reproductive adults, increases in spawning rates, and increased larval retention in the bay due to wind conditions in 2020. This study suggests that coral communities as in Amitori Bay will be critical for local-scale community persistence, serving as both source and sink populations.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s00227-022-04091-2.

Trait-based approaches to global change ecology: moving from description to prediction

Trait-based approaches are increasingly recognized as a tool for understanding ecosystem re-assembly and function under intensifying global change. Here we synthesize trait-based research globally (n = 865 studies) to examine the contexts in which traits may be used for global change prediction. We find that exponential growth in the field over the last decade remains dominated by descriptive studies of terrestrial plant morphology, highlighting significant opportunities to expand trait-based thinking across systems and taxa. Very few studies (less than 3%) focus on predicting ecological effects of global change, mostly in the past 5 years and via singular traits that mediate abiotic limits on species distribution. Beyond organism size (the most examined trait), we identify over 2500 other morphological, physiological, behavioural and life-history traits known to mediate environmental filters of species' range and abundance as candidates for future predictive global change work. Though uncommon, spatially explicit process models—which mechanistically link traits to changes in organism distributions and abundance—are among the most promising frameworks for holistic global change prediction at scales relevant for conservation decision-making. Further progress towards trait-based forecasting requires addressing persistent barriers including (1) matching scales of multivariate trait and environment data to focal processes disrupted by global change, and (2) propagating variation in trait and environmental parameters throughout process model functions using simulation.

Micro-CT reconstruction reveals the colony pattern regulations of four dominant reef-building corals

Colonies are the basic geometric building blocks of coral reefs. However, the forming regulations of both colonies and reefs are still not understood adequately. Therefore, in this study, we reconstructed 25 samples using high-resolution micro-computed tomography to investigate coral growth patterns and parameters. Our skeleton and canal reconstructions revealed the characteristics of different coral species, and we further visualized the growth axes and growth rings to understand the coral growth directions. We drew a skeleton grayscale map and calculated the coral skeleton void ratios to ascertain the skeletal diversity, devising a method to quantify coral growth. On the basis of the three-dimensional (3D) reconstructions and growth parameters, we investigated the growth strategies of different coral species. This research increases the breadth of knowledge on how reef-building corals grow their colonies, providing information on reef-forming regulations. The data in this paper contain a large amount of coral growth information, which can be used in further research on reef-forming patterns under different conditions. The method used in this study can also be applied to animals with porous skeletons.

Microstructural characteristics of the stony coral genus Acropora useful to coral reef paleoecology and modern conservation

Identification of fossil corals is often limited due to poor preservation of external skeleton morphology, especially in the genus Acropora which is widespread across the Indo-Pacific. Based on skeleton characteristics from thin section, we here develop a link between the internal skeleton structure and external morphology. Ten characteristics were summarized to distinguish Acropora and five related genera, including the type and differentiation of corallites, the skeleton nature of corallites (septa, columellae, dissepiments, wall), and calcification centers within septa. Acropora is distinctive for its dimorphic corallites: axial and radial. Isopora is similar to Acropora but possess more than a single axial corallites. Montipora and Astreopora (family Acroporidae) have monomorphic corallites and a synapticular ring wall, with clustered calcification center in the former and medial lines in the latter. Pocillopora and Porties are classified by distinctive dissepiments, columellae and septa. These microstructural skeleton characteristics were effective in the genus identification of fossil corals from drilled cores in the South China Sea. Eighteen detailed characteristics (ten of axial corallites, four of radial corallites, and four of coenosteum) were used in the Acropora species classification. The axial corallites size and structure (including corallite diameter, synapticular rings, and septa), the septa of radial corallites, and the arrangement of coenosteum were critical indicators for species identification. This identification guide can help paleoenvironmental and paleoecological analyses and modern coral reef conservation and restoration.

Diversity, structure and demography of coral assemblages on underwater lava flows of different ages at Reunion Island and implications for ecological succession hypotheses

Understanding colonization of new habitats and ecological successions is key to ecosystem conservation. However, studies on primary successions are scarce for reef-building corals, due to the rarity of newly formed substratum and the long-term monitoring efforts required for their long life cycle and slow growth rate. We analysed data describing the diversity, structure and demography of coral assemblages on lava flows of different ages and coral reefs at Reunion Island, to evaluate the strength and mechanisms of succession, and its agreement to the theoretical models. No significant differences were observed between the two habitats for most structure and demographic descriptors. In contrast, species richness and composition differentiated coral reefs from lava flows, but were not related to the age of the lava flow. We observed a strong dominance of Pocillopora colonies, which underline the opportunistic nature of this taxa, with life-history traits advantageous to dominance on primary and secondary successional stages. Although some results argue in favor of the tolerance model of succession, the sequences of primary successions as theorized in other ecosystems were difficult to observe, which is likely due to the high frequency and intensity of disturbances at Reunion, that likely distort or set back the expected successional sequences.

Interactions between coral propagules in aquarium and field conditions

The effects of intraspecific and interspecific interactions between three species of scleractinian coral micro-colonies, namely Lithophyllon undulatum, Turbinaria mesenterina and Platygyra sinensis were evaluated for their survivorship, tissue loss and growth in both field (in-situ) and aquarium (ex-situ) conditions over 12 weeks. Regardless of environmental conditions and interactions, L. undulatum survived better (91.7 ± 6.2%) than T. mesenterina (75.0 ± 25.0%) and P. sinensis (60.4 ± 39.5%). Similarly, L. undulatum registered the lowest tissue loss (0.5 ± 0.7%) as compared to T. mesenterina (14.3 ± 19.4%) and P. sinensis (22.0 ± 30.0%). However, P. sinensis gained more weight (3.2 ± 5.2 g) than either T. mesenterina (2.7 ± 2.4 g) or L. undulatum (0.8 ± 1.1 g). In both environments, all three species in intraspecific interaction generally had higher survivorship, lower tissue loss and better growth than those in interspecific interaction except the latter in in-situ conditions had a twofold increase in growth (5.8 ± 3.7 g) than the former in-situ conditions (2.8 ± 3.7 g). Hence, all three species are potentially suitable for transplantation and mariculture except perhaps for P. sinensis which performed poorly in ex-situ conditions. Corals can be transplanted either with different colonies of the same species or together with other coral taxa. This study demonstrated that L. undulatum should be transplanted between T. mesenterina and P. sinensis for optimal growth and survival.

Evolutionary Traits that Enable Scleractinian Corals to Survive Mass Extinction Events

Scleractinian “stony” corals are major habitat engineers, whose skeletons form the framework for the highly diverse, yet increasingly threatened, coral reef ecosystem. Fossil coral skeletons also present a rich record that enables paleontological analysis of coral origins, tracing them back to the Triassic (~241 Myr). While numerous invertebrate lineages were eradicated at the last major mass extinction boundary, the Cretaceous-Tertiary/K-T (66 Myr), a number of Scleractinian corals survived. We review this history and assess traits correlated with K-T mass extinction survival. Disaster-related “survival” traits that emerged from our analysis are: (1) deep water residing (>100 m); (2) cosmopolitan distributions, (3) non-symbiotic, (4) solitary or small colonies and (5) bleaching-resistant. We then compared these traits to the traits of modern Scleractinian corals, using to IUCN Red List data, and report that corals with these same survival traits have relatively stable populations, while those lacking them are presently decreasing in abundance and diversity. This shows corals exhibiting a similar dynamic survival response as seen at the last major extinction, the K-T. While these results could be seen as promising, that some corals may survive the Anthropocene extinction, they also highlight how our relatively-fragile Primate order does not possess analogous “survival” characteristics, nor have a record of mass extinction survival as some corals are capable.

Defining variation in pre-human ecosystems can guide conservation: An example from a Caribbean coral reef

Many Caribbean coral reefs are heavily degraded, yet their pre-human, natural states are often assumed or estimated using space-for-time substitution approaches. Here we use an 11-hectare suite of fossilised mid-Holocene (7.2-5.6 ka) fringing reefs in Caribbean Panama to define natural variation in hard coral community structure before human-impact to provide context to the states of the same reefs today. We collected bulk samples from four trenches dug into the mid-Holocene fossil reef and surficial bulk samples from 2-10 m depths on five adjacent modern reefs extending over 5 km. Analysis of the abundances of coral taxa in fossil bulk samples define the Historical Range of Variation (HRV) in community structure of the reefs. When compared to the community structure of adjacent modern reefs, we find that most coral communities today fall outside the HRV, identifying them as novel ecosystems and corroborating the well-documented transition from acroporid-dominated Caribbean reefs to reefs dominated by stress-tolerant taxa (Porites and Agaricia). We find one modern reef, however, whose community composition remains within the HRV showing that it has not transitioned to a novel state. Reef-matrix cores extracted from this reef reveal that the coral community has remained in this state for over 800 years, suggesting long-term stability and resistance to the region-wide shift to novel states. Without these data to provide historical context, this potentially robust and stable reef would be overlooked since it does not fulfil expectations of what a Caribbean coral reef should look like in the absence of humans. This example illustrates how defining past variation using the fossil record can improve our understanding of modern degradation and guide conservation.

Warm seawater temperature promotes substrate colonization by the blue coral, Heliopora coerulea

Background

Heliopora coerulea, the blue coral, is a reef building octocoral that is reported to have a higher optimum temperature for growth compared to most scleractinian corals. This octocoral has been observed to grow over both live and dead scleractinians and to dominate certain reefs in the Indo-Pacific region. The molecular mechanisms underlying the ability of H. coerulea to tolerate warmer seawater temperatures and to effectively compete for space on the substrate remain to be elucidated.

Methods

In this study, we subjected H. coerulea colonies to various temperatures for up to 3 weeks. The growth and photosynthetic efficiency rates of the coral colonies were measured. We then conducted pairwise comparisons of gene expression among the different coral tissue regions to identify genes and pathways that are expressed under different temperature conditions.

Results

A horizontal growth rate of 1.13 ± 0.25 mm per week was observed for corals subjected to 28 or 31 °C. This growth rate was significantly higher compared to corals exposed at 26 °C. This new growth was characterized by the extension of whitish tissue at the edges of the colony and was enriched for a matrix metallopeptidase, a calcium and integrin binding protein, and other transcripts with unknown function. Tissues at the growth margin and the adjacent calcified encrusting region were enriched for transcripts related to proline and riboflavin metabolism, nitrogen utilization, and organic cation transport. The calcified digitate regions, on the other hand, were enriched for transcripts encoding proteins involved in cell-matrix adhesion, translation, receptor-mediated endocytosis, photosynthesis, and ion transport. Functions related to lipid biosynthesis, extracellular matrix formation, cell migration, and oxidation-reduction processes were enriched at the growth margin in corals subjected for 3 weeks to 28 or 31 °C relative to corals at 26 °C. In the digitate region of the coral, transcripts encoding proteins that protect against oxidative stress, modify cell membrane composition, and mediate intercellular signaling pathways were enriched after just 24 h of exposure to 31 °C compared to corals at 28 °C. The overall downregulation of gene expression observed after 3 weeks of sustained exposure to 31 °C is likely compensated by symbiont metabolism.

Discussion

These findings reveal that the different regions of H. coerulea have variable gene expression profiles and responses to temperature variation. Under warmer conditions, the blue coral invests cellular resources toward extracellular matrix formation and cellular migration at the colony margins, which may promote rapid tissue growth and extension. This mechanism enables the coral to colonize adjacent reef substrates and successfully overgrow slower growing scleractinian corals that may already be more vulnerable to warming ocean waters.

Multiscale variability in coral recruitment in the Mascarene Islands: From centimetric to geographical scale

Coral recruitment refers to the processes allowing maintenance and renewal of coral communities. Recruitment success is therefore indispensable for coral reef recovery after disturbances. Recruitment processes are governed by a variety of factors occurring at all spatial and temporal scales, from centimetres to hundreds of kilometres. In the present context of rising disturbances, it is thus of major importance to better understand the relative importance of different scales in this variation, and when possible, the factors associated with these scales. Multiscale spatio-temporal variability of scleractinian coral recruitment was investigated at two of the Mascarene Islands: Reunion and Rodrigues. Recruitment rates and taxonomic composition were examined during three consecutive six-month periods from regional to micro-local scales (i.e. from hundreds of kilometres to few centimetres) and between two protection levels (no-take zones and general protection zones). Very low recruitment rates were observed. Rodrigues displayed lower recruitment rates than Reunion. Recruit assemblage was dominated by Pocilloporidae (77.9%), followed by Acroporidae (9.9%) and Poritidae (5.2%). No protection effect was identified on coral recruitment, despite differences in recruitment rates among sites within islands. Recruits were patchily distributed within sites but no aggregative effect was detected, i.e. the preferentially colonised tiles were not spatially grouped. Recruits settled mainly on the sides of the tiles, especially at Rodrigues, which could be attributed to the high concentration of suspended matter. The variability of recruitment patterns at various spatial scales emphasises the importance of micro- to macro-local variations of the environment in the dynamics and maintenance of coral populations. High temporal variability was also detected, between seasons and years, which may be related to the early 2016 bleaching event at Rodrigues. The low recruitment rates and the absence of protection effect raise questions about the potential for recovery from disturbances of coral reefs in the Mascarene Islands.

Spatial Patterns and Short-term Changes of Coral Assemblages Along a Cross-shelf Gradient in the Southwestern Lagoon of New Caledonia

Coral reef assemblages generally form gradients of spatial structures which are governed by a variety of interacting physical and biological processes that vary in intensity, frequency, and spatial scale. Assessing the structure of contemporary reef assemblages may help to understand future changes and to identify appropriate conservation actions. The spatial distribution and interannual variability (from 2006 to 2008) of coral assemblages were investigated at 10 stations in the southwestern lagoon of New Caledonia, and the strength of the cross-shelf gradient was evaluated. Coral cover, generic richness, and abundance of adult and juvenile assemblages were highly variable within and among the three major reef habitats (fringing, mid-shelf, and barrier reefs). Abundance increased with distance from shore, whereas generic richness and cover were not correlated with shelf position. Assemblage composition was generally related to habitat, even though some mid-shelf and fringing reef assemblages resembled those observed on other habitats. A significant correlation between juvenile and adult distributions was recorded, suggesting that adult assemblages are partly controlled by the short-term history of recruitment patterns. The interannual variation of coral assemblages was far less pronounced, with significant changes only detected at some mid-shelf and barrier reefs, for a few genera characterised by high turn-over.

Research gaps of coral ecology in a changing world

Coral reefs have long inspired marine ecologists and conservationists around the world due to their ecological and socioeconomic importance. Much knowledge on the anthropogenic impacts on coral species has been accumulated, but relevant research gaps on coral ecology remain underappreciated in human-modified seascapes. In this review we assessed 110 studies on coral responses to five major human disturbances- acidification, climate change, overfishing, pollution and non-regulated tourism -to identify geographic and theoretical gaps in coral ecology and help to guide further researches on the topic. We searched for papers in Web of Science published from 2000 to 2016 and classified them according to the ocean, ecoregion, human threat, level of biological organization, study approach, method of data collection, depth of data collected, and type of coral response. Most studies were carried out in the Indo-Pacific and Caribbean (36.3 and 31.9%, respectively) and used observational approach (60%) with scuba diving (36.3%) to assess the impact of ocean warming (55.4%) on coral communities (58.2%). Only 37 of the 141 global ecoregions that contain coral reefs were studied. All studies were restricted to shallow waters (0.5-27 m depth) and reported negative responses of corals to human disturbance. Our results reinforce the notion that corals are sensitive to anthropogenic changes. They reveal the scarcity of information on coral responses to pollution, tourism, overfishing and acidification, particularly in mesophotic ecosystems (>30 m depth) and in ecoregions outside the Indo-Pacific and Caribbean. Experimental studies at the individual and population levels should be also encouraged.

Delayed coral recovery in a warming ocean

Climate change threatens coral reefs across the world. Intense bleaching has caused dramatic coral mortality in many tropical regions in recent decades, but less obvious chronic effects of temperature and other stressors can be equally threatening to the long-term persistence of diverse coral-dominated reef systems. Coral reefs persist if coral recovery rates equal or exceed average rates of mortality. While mortality from acute destructive events is often obvious and easy to measure, estimating recovery rates and investigating the factors that influence them requires long-term commitment. Coastal development is increasing in many regions, and sea surface temperatures are also rising. The resulting chronic stresses have predictable, adverse effects on coral recovery, but the lack of consistent long-term data sets has prevented measurement of how much coral recovery rates are actually changing. Using long-term monitoring data from 47 reefs spread over 10 degrees of latitude on Australia's Great Barrier Reef (GBR), we used a modified Gompertz equation to estimate coral recovery rates following disturbance. We compared coral recovery rates in two periods: 7 years before and 7 years after an acute and widespread heat stress event on the GBR in 2002. From 2003 to 2009, there were few acute disturbances in the region, allowing us to attribute the observed shortfall in coral recovery rates to residual effects of acute heat stress plus other chronic stressors. Compared with the period before 2002, the recovery of fast-growing Acroporidae and of "Other" slower growing hard corals slowed after 2002, doubling the time taken for modest levels of recovery. If this persists, recovery times will be increasing at a time when acute disturbances are predicted to become more frequent and intense. Our study supports the need for management actions to protect reefs from locally generated stresses, as well as urgent global action to mitigate climate change.

Shifting paradigms in restoration of the world's coral reefs

Many ecosystems around the world are rapidly deteriorating due to both local and global pressures, and perhaps none so precipitously as coral reefs. Management of coral reefs through maintenance (e.g., marine-protected areas, catchment management to improve water quality), restoration, as well as global and national governmental agreements to reduce greenhouse gas emissions (e.g., the 2015 Paris Agreement) is critical for the persistence of coral reefs. Despite these initiatives, the health and abundance of corals reefs are rapidly declining and other solutions will soon be required. We have recently discussed options for using assisted evolution (i.e., selective breeding, assisted gene flow, conditioning or epigenetic programming, and the manipulation of the coral microbiome) as a means to enhance environmental stress tolerance of corals and the success of coral reef restoration efforts. The 2014-2016 global coral bleaching event has sharpened the focus on such interventionist approaches. We highlight the necessity for consideration of alternative (e.g., hybrid) ecosystem states, discuss traits of resilient corals and coral reef ecosystems, and propose a decision tree for incorporating assisted evolution into restoration initiatives to enhance climate resilience of coral reefs.

Spatial competition dynamics between reef corals under ocean acidification

Climate change, including ocean acidification (OA), represents a major threat to coral-reef ecosystems. Although previous experiments have shown that OA can negatively affect the fitness of reef corals, these have not included the long-term effects of competition for space on coral growth rates. Our multispecies year-long study subjected reef-building corals from the Gulf of Aqaba (Red Sea) to competitive interactions under present-day ocean pH (pH 8.1) and predicted end-of-century ocean pH (pH 7.6). Results showed coral growth is significantly impeded by OA under intraspecific competition for five out of six study species. Reduced growth from OA, however, is negligible when growth is already suppressed in the presence of interspecific competition. Using a spatial competition model, our analysis indicates shifts in the competitive hierarchy and a decrease in overall coral cover under lowered pH. Collectively, our case study demonstrates how modified competitive performance under increasing OA will in all likelihood change the composition, structure and functionality of reef coral communities.

A geological perspective on the degradation and conservation of western Atlantic coral reefs

Continuing coral-reef degradation in the western Atlantic is resulting in loss of ecological and geologic functions of reefs. With the goal of assisting resource managers and stewards of reefs in setting and measuring progress toward realistic goals for coral-reef conservation and restoration, we examined reef degradation in this region from a geological perspective. The importance of ecosystem services provided by coral reefs-as breakwaters that dissipate wave energy and protect shorelines and as providers of habitat for innumerable species-cannot be overstated. However, the few coral species responsible for reef building in the western Atlantic during the last approximately 1.5 million years are not thriving in the 21st century. These species are highly sensitive to abrupt temperature extremes, prone to disease infection, and have low sexual reproductive potential. Their vulnerability and the low functional redundancy of branching corals have led to the low resilience of western Atlantic reef ecosystems. The decrease in live coral cover over the last 50 years highlights the need for study of relict (senescent) reefs, which, from the perspective of coastline protection and habitat structure, may be just as important to conserve as the living coral veneer. Research is needed to characterize the geological processes of bioerosion, reef cementation, and sediment transport as they relate to modern-day changes in reef elevation. For example, although parrotfish remove nuisance macroalgae, possibly promoting coral recruitment, they will not save Atlantic reefs from geological degradation. In fact, these fish are quickly nibbling away significant quantities of Holocene reef framework. The question of how different biota covering dead reefs affect framework resistance to biological and physical erosion needs to be addressed. Monitoring and managing reefs with respect to physical resilience, in addition to ecological resilience, could optimize the expenditure of resources in conserving Atlantic reefs and the services they provide.

Exploring the Symbiodinium rare biosphere provides evidence for symbiont switching in reef-building corals

Reef-building corals possess a range of acclimatisation and adaptation mechanisms to respond to seawater temperature increases. In some corals, thermal tolerance increases through community composition changes of their dinoflagellate endosymbionts (Symbiodinium spp.), but this mechanism is believed to be limited to the Symbiodinium types already present in the coral tissue acquired during early life stages. Compelling evidence for symbiont switching, that is, the acquisition of novel Symbiodinium types from the environment, by adult coral colonies, is currently lacking. Using deep sequencing analysis of Symbiodinium rDNA internal transcribed spacer 2 (ITS2) PCR amplicons from two pocilloporid coral species, we show evidence consistent with de novo acquisition of Symbiodinium types from the environment by adult corals following two consecutive bleaching events. Most of these newly detected symbionts remained in the rare biosphere (background types occurring below 1% relative abundance), but one novel type reached a relative abundance of ~33%. Two de novo acquired Symbiodinium types belong to the thermally resistant clade D, suggesting that this switching may have been driven by consecutive thermal bleaching events. Our results are particularly important given the maternal mode of Symbiodinium transmission in the study species, which generally results in high symbiont specificity. These findings will cause a paradigm shift in our understanding of coral-Symbiodinium symbiosis flexibility and mechanisms of environmental acclimatisation in corals.

Temporal comparison and predictors of fish species abundance and richness on undisturbed coral reef patches

Large disturbances can cause rapid degradation of coral reef communities, but what baseline changes in species assemblages occur on undisturbed reefs through time? We surveyed live coral cover, reef fish abundance and fish species richness in 1997 and again in 2007 on 47 fringing patch reefs of varying size and depth at Mersa Bareika, Ras Mohammed National Park, Egypt. No major human or natural disturbance event occurred between these two survey periods in this remote protected area. In the absence of large disturbances, we found that live coral cover, reef fish abundance and fish species richness did not differ in 1997 compared to 2007. Fish abundance and species richness on patches was largely related to the presence of shelters (caves and/or holes), live coral cover and patch size (volume). The presence of the ectoparasite-eating cleaner wrasse, Labroides dimidiatus, was also positively related to fish species richness. Our results underscore the importance of physical reef characteristics, such as patch size and shelter availability, in addition to biotic characteristics, such as live coral cover and cleaner wrasse abundance, in supporting reef fish species richness and abundance through time in a relatively undisturbed and understudied region.