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Demographic studies that quantify species performances for survival, growth, and reproduction are powerful means to understand and predict how species and communities respond to environmental change through the characterization of population dynamics and sources of demographic bottlenecks. However, demographic studies require fine-scale surveys of populations and are often too effort-intensive to be replicable at large scale and in the long-term. To surpass this obstacle, we describe a digital approach for extracting demographic data on species abundances, sizes, and positions within video-transects, facilitating back-from-the-field data acquisitions on population and community dynamics from video surveys. We report on the characterizations of species demographic performances and investigate community dynamics using surveys of reef-building corals in New Caledonia recorded with underwater cameras, therefore optimizing time spent in the field. The results provide quantitative measures of coral community composition and demographic performances as key ecological indicators of coral reef health, shed light on species life strategies and constraints to their demographics, and open paths for further quantitative investigations. Key findings include the diversity of species life strategies in terms of relative investment in survival, growth, and reproduction found among taxa dominating the coral community, and that several species have adapted mechanisms to prevail under limiting environments. Our approach facilitates image-based extractions of demographic data, helping to accelerate empirical endeavors in ecology and ecosystem management.Author summarySustainable ecosystem management requires comprehension of key ecological processes that affect species resilience. Accurate and reoccurring measurements of species helps us understand how they are responding to various environments and predict what might happen in the future. We developed a digital approach that mimics measurements traditionally performed in the field to measure species abundance, size, and distributions using video records of the ecosystems. This transition to imagery-based surveys helps researchers and managers acquire fine-scale ecological data while optimizing time spent in the field, particularly for studying remote and extreme environments where access is limited. We illustrate the application of our approach by characterizing the dynamics of a coral community in the vast tropical reef system of New Caledonia, where such evaluations of demographic processes controlling coral resilience are inexistent but necessary.

This is an extended version of Figure S2 from the study The War of Corals by Mohsen Kayal & Mehdi Adjeroud. A preprint of the study is available at:https://doi.org/10.1101/2021.12.06.471466 The manuscript is presently under revision for publication in Royal Society Open Science.

Extended Appendix 1 from the studyThe War of Corals by Mohsen Kayal & Mehdi Adjeroud oncoral competitive interactions under changing reef environments.Original posts fromhttps://www.instagram.com/m.o.k.a.y/ Study preprint:https://doi.org/10.1101/2021.12.06.471466 Manuscript in revision for publication in Royal Society Open Science &nbsp

International audience; Predicting whether, how, and to what degree communities recover from disturbance remain major challenges in ecology. To predict recovery of coral communities we applied field survey data of early recovery dynamics to a multi‐species integral projection model that captured key demographic processes driving coral population trajectories, notably density‐dependent larval recruitment. After testing model predictions against field observations, we updated the model to generate projections of future coral communities. Our results indicated that communities distributed across an island landscape followed different recovery trajectories but would reassemble to pre‐disturbed levels of coral abundance, composition, and size, thus demonstrating persistence in the provision of reef habitat and other ecosystem services. Our study indicates that coral community dynamics are predictable when accounting for the interplay between species life‐history, environmental conditions, and density‐dependence. We provide a quantitative framework for evaluating the ecological processes underlying community trajectory and characteristics important to ecosystem functioning.

Preserving coral reef resilience is a major challenge in the Anthropocene, yet recent studies demonstrate failures of reef recovery from disturbance, globally. The wide and vigorous outer-reef system of French Polynesia presents a rare opportunity to assess ecosystem resilience to disturbances at a large-scale equivalent to the size of Europe. In this purpose, we analysed long-term data on coral community dynamics and combine the mixed-effects regression framework with a set of functional response models to evaluate coral recovery trajectories. Analyses of 14 years data across 17 reefs allowed estimating impacts of a cyclone, bleaching event and crown-of-thorns starfish outbreak, which generated divergence and asynchrony in coral community trajectory. We evaluated reef resilience by quantifying levels of exposure, degrees of vulnerability, and descriptors of recovery of coral communities in the face of disturbances. Our results show an outstanding rate of coral recovery, with a systematic return to the pre-disturbance state within only 5 to 10 years. Differences in the impacts of disturbances among reefs and in the levels of vulnerability of coral taxa to these events resulted in diverse recovery patterns. The consistent recovery of coral communities, and convergence toward pre-disturbance community structures, reveals that the processes that regulate ecosystem recovery still prevail in French Polynesia.

Sea-level rise is predicted to cause major damage to tropical coastlines. While coral reefs can act as natural barriers for ocean waves, their protection hinges on the ability of scleractinian corals to produce enough calcium carbonate (CaCO3 ) to keep up with rising sea levels. As a consequence of intensifying disturbances, coral communities are changing rapidly, potentially reducing community-level CaCO3 production. By combining colony-level physiology and long-term monitoring data, we show that reefs recovering from major disturbances can produce 40% more CaCO3 than currently estimated due to the disproportionate contribution of juvenile corals. However, the buffering effect of highly productive juvenile corals is compromised by recruitment failures, which have been more frequently observed after large-scale, repeated bleaching events. While the size structure of corals can bolster a critical ecological function on reefs, climate change impacts on recruitment may undermine this buffering effect, thus further compromising the persistence of reefs and their provision of important ecosystem services.

Marine Recreational Fishing (MRF) is growing worldwide in scientific interest, as evidenced by the increasing number of dedicated publications. Studies on the impacts and benefits to socio-ecosystems and mental health are driving this gradual awareness. In the Mediterranean, MRF is currently responsible for 10% of the catches though, in the context of small-scale fisheries decline, it may become dominant. Sustaining this activity represents a universal challenge for the future of mankind. However, the potential influence of anglers' heterogeneity on both the environment and the Catch Per Unit of Effort (CPUE), used internationally to evaluate fish harvest and as a stocks indicator, hinders MRF management. In addition, little data is available on onshore fishing, while the number of practitioners may increase sharply in the context of a socio-economic crisis, especially in marine protected areas (MPA). We conducted a year-round survey in 2017–2018 in a French Mediterranean MPA subject to tourist flows, during which 144 onshore anglers were polled by semi-directive interviews. We used a typology based on technical and socio-economic data of anglers to describe their behaviour diversity and its influence on CPUE and, more broadly, the marine environment in multi-species fisheries. We characterised four onshore angler profiles, segregated in space and time, including one identified as at risk of causing impacts. Our analyses support using total and per species CPUE independently of profiles to evaluate onshore MRF harvest, except for four species. CPUE seem based on the profiles’ skills and self-regulation which induce similar yields between anglers in the absence of control. This demonstrates the importance of taking into account angler behaviour, as each profile could react differently to management actions and highlights that CPUE could be used to detect the effect of changes in regulation in the framework of adaptive management. Our results support that MRF regulations should be simplified and homogenised at the national level and cooperation with fishing shops in MPA co-management should be promoted.

Communicating new scientific discoveries is key to human progress. Yet, this endeavor has been increasingly hindered by monetary restrictions that restrain scientists from publishing their findings and accessing other scientists’ reports. This process is further exacerbated by a large portion of publishing media owned by private companies that, in contrast with journals from scientific societies, do not reinject academic publishing benefits into the scientific community. As the academic world is not exempt from economic crises and funding restrictions, new alternatives are necessary to support a fair and economically sustainable publishing system for scientists and society as a whole. After summarizing major shortcomings of academic publishing today, we present several solutions that span the levels of the individual scientist, the scientific community, and the publisher to initiate a transformative change towards more sustainable scientific publishing. By providing a voice to the many scientists who are fundamental protagonists, yet often powerless witnesses, of the academic publishing system, as well as a roadmap for implementing solutions, we hope this initiative will go beyond sparking increased awareness and promote a shift towards more sustainable scientific publishing practices.

The authors are particularly grateful to Richard Farman, director ofthe Aquarium des Lagons in Noumea, for hosting our experiments. Wealso wish to express our warmest thanks to the aquarium staff for theirlogistical support, in particular Sylvain Govan and Jeff Dubosc.; International audience; Outbreaks of the coral-eating crown-of-thorns starfish Acanthaster spp. (COTS) have become to be amongst the most severe threats to coral reefs worldwide. Although most research has focused on COTS early development, it remains unclear how COTS populations will keep pace with changing ocean conditions. Since reproduction is a key process contributing to outbreaks, we investigated the reproductive success of adult COTS acclimated for 3-4 months to different treatment combinations of ambient conditions, ocean warming (+2 • C) and acidification (− 0.35 pH). Our results suggest that the optimal breeding season in New Caledonia is concentrated around the end of the calendar year, when water temperature reaches >26 • C. We found negative effects of temperature on egg metrics, fertilisation success, and GSI, conflicting with previously documented effects of temperature on echinoderm reproductive outputs. Fertilisation success dropped drastically (more than threefold) with elevated temperature during the late breeding season. In contrast, we detected no effects of near-future acidification conditions on fertilisation success nor GSI. This is the first time that COTS reproduction is compared among individuals acclimated to different conditions of warming and acidification. Our results highlight the importance of accounting for adult exposure to better understand how COTS reproduction may be impacted in the face of global change.

Marine reserves constitute effective tools for preserving fish stocks and associated human benefits. However, not all reserves perform equally, and predicting the response of marine communities to management actions in the long run is challenging. Our decadal-scale survey of recreational fishing yields at France’s 45-year old Cerbère-Banyuls marine reserve indicated significant protection benefits, with 40-50% higher fishing yields per unit effort in the partial-protection zone of the reserve (where fishing is permitted but at a lower level) than in surrounding non-reserve areas. Over the period 2005-2014, catch per unit effort (CPUE) showed a similar pattern of decline inside and outside the reserve while weight per unit effort (WPUE) increased by 131% inside and decreased by 60% outside. Different CPUE and WPUE trajectories among fish families indicated changing catch assemblages, with yields increasing for the family most valued by fisheries, Sparidae (the ecological winners). However, reserve benefits were restricted to off-shore fishermen (the social winners), as on-shore yields were ∼4 times lower and declining, even inside the reserve. Our study illustrates how surveys of recreational fishing yields can help evaluate the effectiveness of marine protected areas for key social and ecological protagonists. We show that, more than four decades after its establishment, fishing efficiencies at the historical Cerbère-Banyuls marine reserve are still changing, but benefits in terms of catch abundance, weight, and composition remain predominantly restricted to off-shore fishermen. Further regulations appear necessary to guarantee that conservation strategies equitably benefit societal groups.

Este artículo contiene 10 páginas, 5 figuras., Without drastic efforts to reduce carbon emissions and mitigate globalized stressors, tropical coral reefs are in jeopardy. Strategic conservation and management requires identification of the environmental and socioeconomic factors driving the persistence of scleractinian coral assemblages—the foundation species of coral reef ecosystems. Here, we compiled coral abundance data from 2,584 Indo-Pacific reefs to evaluate the influence of 21 climate, social and environmental drivers on the ecology of reef coral assemblages. Higher abundances of framework-building corals were typically associated with: weaker thermal disturbances and longer intervals for potential recovery; slower human population growth; reduced access by human settlements and markets; and less nearby agriculture. We therefore propose a framework of three management strategies (protect, recover or transform) by considering: (1) if reefs were above or below a proposed threshold of >10% cover of the coral taxa important for structural complexity and carbonate production; and (2) reef exposure to severe thermal stress during the 2014–2017 global coral bleaching event. Our findings can guide urgent management efforts for coral reefs, by identifying key threats across multiple scales and strategic policy priorities that might sustain a network of functioning reefs in the Indo-Pacific to avoid ecosystem collapse.

Without drastic efforts to reduce carbon emissions and mitigate globalized stressors, tropical coral reefs are in jeopardy. Strategic conservation and management requires identification of the environmental and socioeconomic factors driving the persistence of scleractinian coral assemblages-the foundation species of coral reef ecosystems. Here, we compiled coral abundance data from 2,584 Indo-Pacific reefs to evaluate the influence of 21 climate, social and environmental drivers on the ecology of reef coral assemblages. Higher abundances of framework-building corals were typically associated with: weaker thermal disturbances and longer intervals for potential recovery; slower human population growth; reduced access by human settlements and markets; and less nearby agriculture. We therefore propose a framework of three management strategies (protect, recover or transform) by considering: (1) if reefs were above or below a proposed threshold of >10% cover of the coral taxa important for structural complexity and carbonate production; and (2) reef exposure to severe thermal stress during the 2014-2017 global coral bleaching event. Our findings can guide urgent management efforts for coral reefs, by identifying key threats across multiple scales and strategic policy priorities that might sustain a network of functioning reefs in the Indo-Pacific to avoid ecosystem collapse.

Outbreaks of the predator crown-of-thorns starfish Acanthaster planci cause widespread coral mortality throughout the Indo-Pacific. Identifying mediating processes that promote coral survival during Acanthaster outbreaks can help support reef resilience. During surveys in French Polynesia, large colonies of the calcifying hydrozoan Millepora platyphylla appeared to be avoided by the starfish, providing predator-free sanctuaries to scleractinian corals in their proximity during an intense Acanthaster outbreak. Such refuges particularly benefited susceptible taxa such as Acropora and Pocillopora, whose populations were normally targeted by the starfish. These biological sanctuaries provided by Millepora to scleractinians are newly identified forms of associational refuges that can increase coral resistance to predation by Acanthaster. Our observations thus identify a key ecological role in reef resilience for Millepora, which is a ubiquitous component of tropical reefs usually seen as a strong competitor of scleractinians.

Coral reefs are increasingly threatened by various types of disturbances, and their recovery is challenged by accelerating, human-induced environmental changes. Recurrent disturbances reduce the pool of mature adult colonies of reef-building corals and undermine post-disturbance recovery from newly settled recruits. Using a long-term interannual data set, we show that coral assemblages on the reef slope of Moorea, French Polynesia, have maintained a high capacity to recover despite a unique frequency of large-scale disturbances which, since the 1990s, have caused catastrophic declines in coral cover and abundance. In 2014, only four years after one of the most extreme cases of coral decline documented, abundance of juvenile and adult colonies had regained or exceeded pre-disturbance levels, and no phase-shift to macroalgal dominance was recorded. This rapid recovery has been achieved despite constantly low coral recruitment rates, suggesting a high post-disturbance survivorship of recruits. However, taxonomic differences in coral susceptibility to disturbances and contrasting recovery trajectories have resulted in changes in the relative composition of species. In the present context of global coral reef decline, our study establishes a new benchmark for the capacity of certain benthic reef communities to sustain and recover their coral cover from repeated, intense disturbances.

International audience; Predicting how communities respond to multiple stressors is challenging because community dynamics, stressors, and animal-stressor interactions can vary with environmental conditions, including the intensity of natural disturbance. Nevertheless, environmental laws stipulate that we predict, measure, and mitigate the ecological effects of some human-induced stressors in the environment, including chemical contaminants in aquatic ecosystems. We conducted an experiment in Antarctica to test how a marine soft-sediment benthic community responded to multiple chemical contaminants and biotic disturbance by manipulating organic carbon enrichment, copper metal contamination, access by large epibenthic animals, and their interaction. Biotic disturbance caused mainly by large echinoderms was manipulated with exclusion cages and cage-control treatments. Colonization patterns in sediment trays revealed that total infaunal abundance and arthropods decreased with toxic Cu (0, 100, and 500 ppm) and total organic carbon (TOC; 0%, 1%, and 2% by wt), as enrichment produced increasing levels of sediment hypoxia/anoxia. Annelids and echinoderms decreased with Cu but increased with TOC because many colonizing polychaete worms, seastars, and epifaunal sea urchins were deposit feeders. Bioturbation by echinoderms disturbed sediments, leading to a substantial decline in total infaunal abundance in uncontaminated sediments, but also an increase in the relative abundance in contaminated sediments, as bioturbation mitigated the effect of both chemical stressors. Biotic disturbance also caused substantial shifts in the species composition of the invertebrate assemblages and an overall increase in species diversity. Prior predictions about the response of benthic marine phyla to the separate and combined effects of Cu and carbon enrichment appear robust to variation in natural biotic disturbance.

Mehdi Adjeroud, Mohsen Kayal, Christophe Peignon, Matthieu Juncker, Suzanne C. Mills, Ricardo Beldade, and Pascal Dumas (2018) Outbreaks of the coral predator Acanthaster spp., the crown-of-thorns seastar (COTS), cause major coral declines across the Indo-Pacific. However, the processes surrounding the initiation and propagation of COTS outbreaks are still unclear. We observed COTS outbreak abundances on several mid-shelf and inner-barrier reefs in the southern section of the New Caledonian lagoon, a multi-location initiation event that we expected to precede a broader region-wide disturbance. However, reef monitoring over 3 years revealed the highly localized and ephemeral character of these outbreaks. Outbreaks that were observed at four reef locations at the beginning of the survey simply faded away, without any specific management actions such as culling efforts. We also found no distinct reef biotope on which COTS outbreaks originated, although mid-shelf and inner-shelf barrier reefs seem to be favoured. New Caledonia has been exempted from to widespread regional COTS outbreaks, which is surprising given its proximity to the Australian Great Barrier Reef - a reef complex particularly vulnerable to COTS - and Vanuatu - where large-scale outbreaks were recorded during the same period. The availability of coral prey is probably not a limiting factor for the propagation of COTS outbreaks on New Caledonian reefs, given the high abundance and diversity of coral assemblages. Our findings reveal that localized and ephemeral COTS outbreaks can be naturally contained and do not necessarily result in widespread disturbances.

Outbreaks of the predator crown-of-thorns seastar (COTS) Acanthaster planci cause widespread coral mortality across the Indo-Pacific. Like many marine invertebrates, COTS is a nocturnal species whose cryptic behaviour during the day can affect its detectability, particularly in structurally complex reef habitats that provide many refuges for benthic creatures. We performed extensive day and night surveys of COTS populations in coral reef habitats showing differing levels of structural complexity and COTS abundance. We tested whether estimations of COTS density varied between day and night observations, and if the differences were related to changes in COTS abundance, reef structural complexity and the spatial scale of observation. Estimations of COTS density were on average 27% higher at night than during the day. Differences in COTS detection varied with changing seastar abundance but not reef structural complexity or scale of observation. Underestimation of COTS abundance in daytime was significant for a broad seastar density range, thus potentially affecting most outbreak events. Our study suggests that portions of COTS populations can be undetected during conventional surveys and control campaigns, which are exclusively conducted by day, and significantly affect the trajectory of coral reefs. Accounting for bias in COTS detection can strengthen coral reef management broadly.

International audience; Ecological signs of Earth's biosphere forewarn an alarming trajectory towards a global mass-extinction. Assessing species performance and susceptibilities to decline is essential to comprehend and reverse this trend. Yet it is challenging, given difficulties associated with quantifying individual and population processes that are variable across time, space, and life-stages. We describe a new approach to estimating and comparing species performances that combines empirical data, a novel theoretical consideration of population dynamics, and modern statistics. Our approach allows for a more realistic continuous representation of individual performances along development stages while taking into account non-linearity, and natural variability as captured by spatio-temporally replicated observations. We illustrate its application in a coral meta-assemblage composed of populations of the three major reef-building taxa Acropora, Pocillopora, Porites. Using a unique set of highly replicated observations of individual coral dynamics under various environmental conditions, we show how taxa differ in their investment in recruitment and size-specific aptitude for growth and survival, notably through different use of clonal shrinkage, fragmentation, fission, and fusion processes. Our results reveal contrasting life-history trade-offs among taxa which, along with differing patterns of density-dependent recruitment, modulate species responses to decline. These differences in coral life history traits reflect opposing life-strategies, imply regulation at differing life-stages, and explain divergence in species trajectories. Our findings indicate a high potential for resilience in Pocillopora and Porites populations, thanks respectively to a sustained recruitment that promotes demographic elasticity through replacement of individuals, and a steady resistance to mortality which confers persistence through lingering of individuals. Resilience in Acropora appears more arbitrary, given high susceptibility to perturbations and dependency of recruitment on presence of established local populations. We identify management actions that can complement Acropora's life history and benefit recovery of its populations following mortality events. Our regression-modelling approach to quantifying and comparing species performances in different population processes is applicable to all taxa, as illustrated even those with complex clonal life histories, and can be implemented at wide spatio-temporal and taxonomic coverage. It can promote more accurate representation of species dynamics in both descriptive and predictive modelling approaches. The semi-parametric contrast curve method we develop facilitates comparing response variables along continuous explicative metrics while accounting for multiple sources of complexity in empirical data. It should widely benefit investigations in ecology and quantitative science.

With ongoing climate change, coral susceptibility to thermal stress constitutes a central concern in reef conservation. In the Persian Gulf, coral reefs are confronted with the most extreme temperatures. Over the last decades, both annual hot and cold peak periods in this region have been associated with episodes of coral bleaching and mortality. Using physiological performance as a measure of coral health, we investigated the thermal susceptibility of the common acroporid coral from the Persian Gulf, Acropora downingi, in Hengam Island where temperature oscillates seasonally in the range 20.2-34.2°C. In a series of two short-term experiments, we exposed corals (1) to the constant temperature levels of summer versus winter, and (2) to progressive temperature deviations from the annual mean toward the two extreme seasonal values and beyond. We monitored four indictors of coral physiological performance: net photosynthesis (Pn), dark respiration (R), autotrophic capability (Pn/R), and survival. Warming revealed detrimental for Pn and survival of corals, while equivalent cooling did not. Pn/R was lower at the warmer thermal level within each season, and during summer compared to winter. Corals exposed to the maximum temperature of summer displayed Pn/R A. downingi are more sensitive to the extreme temperatures endured in summer compared to that experienced in winter; and they may be impacted by future increases in water temperature.

International audience; Recently, attempts to improve decision making in species management have focussed on uncertainties associated with modelling temporal fluctuations in populations. Reducing model uncertainty is challenging; while larger samples improve estimation of species trajectories and reduce statistical errors, they typically amplify variability in observed trajectories. In particular, traditional modelling approaches aimed at estimating population trajectories usually do not account well for nonlinearities and uncertainties associated with multi-scale observations characteristic of large spatio-temporal surveys. We present a Bayesian semi-parametric hierarchical model for simultaneously quantifying uncertainties associated with model structure and parameters, and scale-specific variability over time. We estimate uncertainty across a four-tiered spatial hierarchy of coral cover from the Great Barrier Reef. Coral variability is well described; however, our results show that, in the absence of additional model specifications, conclusions regarding coral trajectories become highly uncertain when considering multiple reefs, suggesting that management should focus more at the scale of individual reefs. The approach presented facilitates the description and estimation of population trajectories and associated uncertainties when variability cannot be attributed to specific causes and origins. We argue that our model can unlock value contained in large-scale datasets, provide guidance for understanding sources of uncertainty, and support better informed decision making.

This study provides a baseline describing natural small scale variability ofSymbiodiniumdensity in the sentinel coralAcropora globicepsduring the summer, under non-bleaching conditions. Spatial scales investigated range from the colony scale (1–10 cm, i.e. among branches of the same colony) to the reef scale (1–10 km, i.e. among stations distributed over several locations and depths), at Moorea Island, French Polynesia. The coral–Symbiodiniumsymbiosis is a key process in scleractinian coral physiology, andSymbiodiniumdensity provides an easy-to-measure and inexpensive biomarker of this symbiosis health. Spatial variability of three major environmental factors: light intensity, sedimentation and water motion was also assessed to evaluate their potential link withSymbiodiniumdensity. Density ofSymbiodiniumdid not significantly differ within colonies or among colonies within a station. However, a marked depth gradient was observed, showing increasing density with increasing depth and decreasing light intensity. These observations provide an interesting reference for forthcoming comparisons with disturbed conditions, such as bleaching events.

International audience; Outbreaks of the coral-killing seastar Acanthaster planci are intense disturbances that can decimate coral reefs. These events consist of the emergence of large swarms of the predatory seastar that feed on reef-building corals, often leading to widespread devastation of coral populations. While cyclic occurrences of such outbreaks are reported from many tropical reefs throughout the Indo-Pacific, their causes are hotly debated, and the spatio-temporal dynamics of the outbreaks and impacts to reef communities remain unclear. Based on observations of a recent event around the island of Moorea, French Polynesia, we show that Acanthaster outbreaks are methodic, slow-paced, and diffusive biological disturbances. Acanthaster outbreaks on insular reef systems like Moorea's appear to originate from restricted areas confined to the ocean-exposed base of reefs. Elevated Acanthaster densities then progressively spread to adjacent and shallower locations by migrations of seastars in aggregative waves that eventually affect the entire reef system. The directional migration across reefs appears to be a search for prey as reef portions affected by dense seastar aggregations are rapidly depleted of living corals and subsequently left behind. Coral decline on impacted reefs occurs by the sequential consumption of species in the order of Acanthaster feeding preferences. Acanthaster outbreaks thus result in predictable alteration of the coral community structure. The outbreak we report here is among the most intense and devastating ever reported. Using a hierarchical, multi-scale approach, we also show how sessile benthic communities and resident coral-feeding fish assemblages were subsequently affected by the decline of corals. By elucidating the processes involved in an Acanthaster outbreak, our study contributes to comprehending this widespread disturbance and should thus benefit targeted management actions for coral reef ecosystems.

Coral reefs in Tonga have been confronted by multiple threats of various origins, including large-scale disturbances and human-induced stressors. These reef communities have been poorly studied, and efficient conservation actions are urgently needed. The aim of this study was to: (1) examine the spatial distribution of coral assemblages in the lagoon of Tongatapu; (2) determine the degree to which spatial heterogeneity of adult corals is influenced by recruitment processes; and (3) examine the implications of these results in terms of conservation actions. We recorded a total of 37 adult and 28 juvenile coral genera, a mean density of 11.6 adult and 5.5 juvenile colonies m(-2), and a dominance of Montipora, Acropora, and Porites. For seven of the 10 dominant genera, spatial patterns of adults were linked to the short-term recruitment pattern history. Despite a reduced diversity and abundance of adult corals in some areas, the lagoon of Tongatapu retains the potential for replenishment through recruitment of young corals. Consequently, we suggest that conservation actions should focus on reducing factors causing coral mortality and maintain suitable conditions for the establishment and growth of juvenile corals, thus increasing the probability that they will reach maturity and participate to the maintenance of local populations. Rather than establishing a large marine protected area, which will almost certainly suffer from a lack of control and poor enforcement, alternative conservation measures could be successfully implemented through the establishment of several small village-based marine reserves, as has been undertaken in other South Pacific islands with promising results.

In nearly all animals, mitochondrial DNA (mtDNA) consists of a single circular molecule that encodes several subunits of the protein complexes involved in oxidative phosphorylation as well as part of the machinery for their expression. By contrast, mtDNA in species belonging to Medusozoa (one of the two major lineages in the phylum Cnidaria) comprises one to several linear molecules. Many questions remain on the ubiquity of linear mtDNA in medusozoans and the mechanisms responsible for its evolution, replication, and transcription. To address some of these questions, we determined the sequences of nearly complete linear mtDNA from 24 species representing all four medusozoan classes: Cubozoa, Hydrozoa, Scyphozoa, and Staurozoa. All newly determined medusozoan mitochondrial genomes harbor the 17 genes typical for cnidarians and map as linear molecules with a high degree of gene order conservation relative to the anthozoans. In addition, two open reading frames (ORFs), polB and ORF314, are identified in cubozoan, schyphozoan, staurozoan, and trachyline hydrozoan mtDNA. polB belongs to the B-type DNA polymerase gene family, while the product of ORF314 may act as a terminal protein that binds telomeres. We posit that these two ORFs are remnants of a linear plasmid that invaded the mitochondrial genomes of the last common ancestor of Medusozoa and are responsible for its linearity. Hydroidolinan hydrozoans have lost the two ORFs and instead have duplicated cox1 at each end of their mitochondrial chromosome(s). Fragmentation of mtDNA occurred independently in Cubozoa and Hydridae (Hydrozoa, Hydroidolina). Our broad sampling allows us to reconstruct the evolutionary history of linear mtDNA in medusozoans.

International audience; Outbreaks of predatory crown-of-thorns seastars (COTS) can devastate coral reef ecosystems, yet some corals possess mutualistic guardian crabs that defend against COTS attacks. However, guarded corals do not always survive COTS outbreaks, with the ecological mechanisms sealing the fate of these corals during COTS infestations remaining unknown. In August 2008 in Moorea (17.539° S, 149.830° W), French Polynesia, an unusually dense multi-species aggregation of predators was observed feeding upon guarded corals following widespread coral decline due to COTS predation. Concurrent assaults from these amplified, mixed-species predator guilds likely overwhelm mutualistic crab defense, ultimately leading to the fall of guarded corals. Our observations indicate that guarded corals can sustain devastating COTS attacks for an extended duration, but eventually concede to intensifying assaults from diverse predators that aggregate in high numbers as alternative prey decays. The fall of guarded corals is therefore suggested to be ultimately driven by an indirect trophic cascade that leads to amplified attacks from diverse starving predators following prey decline, rather than COTS assaults alone

Coral reefs offer natural coastal protection by attenuating incoming waves. Here we combine unique coral disturbance-recovery observations with hydrodynamic models to quantify how structural complexity dissipates incoming wave energy. We find that if the structural complexity of healthy coral reefs conditions is halved, extreme wave run-up heights that occur once in a 100-years will become 50 times more frequent, threatening reef-backed coastal communities with increased waves, erosion, and flooding.