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When tropical systems lose species, they are often assumed to be buffered against declines in functional diversity by the ability of the species-rich biota to display high functional redundancy: i.e., a high number of species performing similar functions. We tested this hypothesis using a ninefold richness gradient in global fish faunas on tropical reefs encompassing 6,316 species distributed among 646 functional entities (FEs): i.e., unique combinations of functional traits. We found that the highest functional redundancy is located in the Central Indo-Pacific with a mean of 7.9 species per FE. However, this overall level of redundancy is disproportionately packed into few FEs, a pattern termed functional over-redundancy (FOR). For instance, the most speciose FE in the Central Indo-Pacific contains 222 species (out of 3,689) whereas 38% of FEs (180 out of 468) have no functional insurance with only one species. Surprisingly, the level of FOR is consistent across the six fish faunas, meaning that, whatever the richness, over a third of the species may still be in overrepresented FEs whereas more than one third of the FEs are left without insurance, these levels all being significantly higher than expected by chance. Thus, our study shows that, even in high-diversity systems, such as tropical reefs, functional diversity remains highly vulnerable to species loss. Although further investigations are needed to specifically address the influence of redundant vs. vulnerable FEs on ecosystem functioning, our results suggest that the promised benefits from tropical biodiversity may not be as strong as previously thought.

Data from over 2,500 reefs worldwide is used to identify 15 bright spots—sites where reef biomass is significantly higher than expected—and surveys of local experts in these areas suggest that strong sociocultural institutions and high levels of local engagement are among the factors supporting higher fish biomass. Bright spots among the world's coral reefs The health of the world's coral reefs, which provide goods and services for millions of people, is declining. Effective management of these ecosystems requires an understanding of the underlying drivers of reef decline. In a study that spans the gap between ecology and the social sciences, Joshua Cinner and colleagues develop a Bayesian hierarchical model, using data from more than 2,500 reefs worldwide, to predict reef fish biomass based on various socioeconomic drivers and environmental conditions. They identify 15 bright spots — sites where reef biomass is significantly higher than expected. The bright spots are found not only among iconic remote and pristine areas, but also where there are strong sociocultural institutions and high levels of local engagement. On the basis of this analysis, the authors argue for a refocus of coral reef conservation efforts away from locating and protecting remote, pristine sites, towards unlocking potential solutions from sites that have successfully confronted the coral reef crisis. Ongoing declines in the structure and function of the world’s coral reefs 1 , 2 require novel approaches to sustain these ecosystems and the millions of people who depend on them 3 . A presently unexplored approach that draws on theory and practice in human health and rural development 4 , 5 is to systematically identify and learn from the ‘outliers’—places where ecosystems are substantially better (‘bright spots’) or worse (‘dark spots’) than expected, given the environmental conditions and socioeconomic drivers they are exposed to. Here we compile data from more than 2,500 reefs worldwide and develop a Bayesian hierarchical model to generate expectations of how standing stocks of reef fish biomass are related to 18 socioeconomic drivers and environmental conditions. We identify 15 bright spots and 35 dark spots among our global survey of coral reefs, defined as sites that have biomass levels more than two standard deviations from expectations. Importantly, bright spots are not simply comprised of remote areas with low fishing pressure; they include localities where human populations and use of ecosystem resources is high, potentially providing insights into how communities have successfully confronted strong drivers of change. Conversely, dark spots are not necessarily the sites with the lowest absolute biomass and even include some remote, uninhabited locations often considered near pristine 6 . We surveyed local experts about social, institutional, and environmental conditions at these sites to reveal that bright spots are characterized by strong sociocultural institutions such as customary taboos and marine tenure, high levels of local engagement in management, high dependence on marine resources, and beneficial environmental conditions such as deep-water refuges. Alternatively, dark spots are characterized by intensive capture and storage technology and a recent history of environmental shocks. Our results suggest that investments in strengthening fisheries governance, particularly aspects such as participation and property rights, could facilitate innovative conservation actions that help communities defy expectations of global reef degradation.

The relationship between species and the functional diversity of assemblages is fundamental in ecology because it contains key information on functional redundancy, and functionally redundant ecosystems are thought to be more resilient, resistant and stable. However, this relationship is poorly understood and undocumented for species-rich coastal marine ecosystems. Here, we used underwater visual censuses to examine the patterns of functional redundancy for one of the most diverse vertebrate assemblages, the coral reef fishes of New Caledonia, South Pacific. First, we found that the relationship between functional and species diversity displayed a non-asymptotic power-shaped curve, implying that rare functions and species mainly occur in highly diverse assemblages. Second, we showed that the distribution of species amongst possible functions was significantly different from a random distribution up to a threshold of ∼90 species/transect. Redundancy patterns for each function further revealed that some functions displayed fast rates of increase in redundancy at low species diversity, whereas others were only becoming redundant past a certain threshold. This suggested non-random assembly rules and the existence of some primordial functions that would need to be fulfilled in priority so that coral reef fish assemblages can gain a basic ecological structure. Last, we found little effect of habitat on the shape of the functional-species diversity relationship and on the redundancy of functions, although habitat is known to largely determine assemblage characteristics such as species composition, biomass, and abundance. Our study shows that low functional redundancy is characteristic of this highly diverse fish assemblage, and, therefore, that even species-rich ecosystems such as coral reefs may be vulnerable to the removal of a few keystone species.

Delineating regions is an important first step in understanding the evolution and biogeography of faunas. However, quantitative approaches are often limited at a global scale, particularly in the marine realm. Reef fishes are the most diversified group of marine fishes, and compared to most other phyla, their taxonomy and geographical distributions are relatively well known. Based on 169 checklists spread across all tropical oceans, the present work aims to quantitatively delineate biogeographical entities for reef fishes at a global scale. Four different classifications were used to account for uncertainty related to species identification and the quality of checklists. The four classifications delivered converging results, with biogeographical entities that can be hierarchically delineated into realms, regions and provinces. All classifications indicated that the Indo-Pacific has a weak internal structure, with a high similarity from east to west. In contrast, the Atlantic and the Eastern Tropical Pacific were more strongly structured, which may be related to the higher levels of endemism in these two realms. The \"Coral Triangle\", an area of the Indo-Pacific which contains the highest species diversity for reef fishes, was not clearly delineated by its species composition. Our results show a global concordance with recent works based upon endemism, environmental factors, expert knowledge, or their combination. Our quantitative delineation of biogeographical entities, however, tests the robustness of the results and yields easily replicated patterns. The similarity between our results and those from other phyla, such as corals, suggests that our approach may be of broad utility in describing and understanding global marine biodiversity patterns.

Mesophotic coral ecosystems (MCEs) have gained considerable attention this last decade but the paucity of knowledge on these ecosystems is pronounced, particularly in the Southwestern Indian Ocean region. We explore the spatial variation in macro-benthic and scleractinian communities along a wide depth gradient (15–95 m) and among contrasted sites around Reunion Island. Values for percent cover of macro-benthic and scleractinian communities varied significantly along depth, resulting in a vertical zonation of communities. We recorded a transition of light-dependent communities towards heterotrophic organisms between shallow and upper mesophotic zones at 30–45 m, and a community shift in the lower mesophotic zone at 75 m. Despite overlaps in scleractinian genera distribution along the depth gradient, predominant genera of shallow depths were in low abundance in MCEs (> 30 m). Our findings highlight the importance of MCEs as distinct ecosystems sheltering diverse, unique habitats and harboring abundant cnidarian-habitat forming organisms. Supporting the ‘Deep Reef Refuge Hypothesis’, 56% of scleractinian genera spanned shallow to mesophotic depths, while one-third were depth specialists, either shallow or mesophotic. This highlights the limited refuge potential of mesophotic reefs for Southwestern Indian Ocean coral communities. Our findings establish baseline data for monitoring and conserving Reunion Island’s MCEs.

Communities of coral reef fishes are changing due to global warming and overfishing. To understand these changes and inform conservation, knowledge of species diversity and distributions is needed. The western Indian Ocean (WIO) contains the second highest coral reef biodiversity hotspot globally, yet a detailed analysis of the diversity of coral reef fishes is lacking. This study developed a timed visual census method and recorded 356 species from 19 families across four countries in the WIO to examine patterns in species diversity. Species richness and composition differed most between the island countries of Madagascar and Comoros and both these locations differed from locations in Tanzania and Mozambique which were similar. These three regional groupings helped define WIO ecoregions for conservation planning. The highest species richness was found in Tanzania and Mozambique, and the lowest and most different species composition was found in Comoros. Biogeography explains these differences with naturally lower species diversity expected from the small, oceanic, and isolated islands of Comoros. Present day ocean currents maintain these diversity patterns and help explain the species composition in northeast Madagascar. Species distributions were driven by 46 of the 356 species; these provide guidance on important species for ongoing monitoring. The results provide a benchmark for testing future changes in reef fish species richness.

Context: The coronavirus pandemic (COVID-19) has caused a major health crisis, requiring the implementation of various public health measures in order to slow the spread of the virus and reduce the associated mortality. However, the success of these measures depends on people's acceptance of them. This research aimed at understanding people's representations of COVID-19 and its crisis management, and ultimately at understanding their attitudes toward health measures for counteracting the spread of COVID-19 in Reunion Island together with the behaviours expected of them. Method: Using Random Digit Dialling, a qualitative study was conducted with 53 inhabitants between February and May 2021. The COREQ checklist was followed. A dual textometric and manual thematic analysis was adopted in order to identify representations of COVID and the management of the crisis. Results: Some respondents perceived COVID-19 as a serious disease, while others saw it as a banal virus or even doubted its existence. A perceived ineffectiveness of public health measures and the incompetency of public actors predominated in the participants' discourse. Conclusions: Thus, there was a considerable lack of trust and negative attitudes toward health measures, possibly influencing people's acceptance and explaining numerous controversies. This research examines the importance of considering people's representations of the health situation in order to improve people's acceptance of protective measures.

The western Indian Ocean (WIO) is recognized as a marine biodiversity hotspot with complex oceanographic circulation resulting in limited connectivity between remote islands. This ocean region comprises several subregions of varying biodiversity, with the northern Mozambique Channel standing out as the core of this hotspot. Although the hydroids in this region are known to include cryptic species and show contrasting connectivity patterns, the mesophotic depths remain largely unexplored. The Deep Reef Refuge Hypothesis suggests that mesophotic coral ecosystems may act as refuges. However, this hypothesis is based on several prerequisites that could be affected by the presence of cryptic species. We investigated the genetic diversity and connectivity of seven hydroid species by collecting samples at euphotic and mesophotic depths around the islands of Mayotte and Reunion. Population genetic patterns were investigated using multivariate analyses and Bayesian clustering, with 8–18 microsatellite markers per species. The results revealed greater genetic diversity in Mayotte than in Reunion, even though fewer samples were collected there. This is in line with the location of the heart of the hotspot in the northern part of the WIO. In addition, all species exhibited strong genetic differentiation between samples from the two islands, supporting the “one island, one species” hypothesis previously proposed for hydroids in the region. However, contrasting values were obtained among depths depending on the species and the island, demonstrating the importance of a multi‐species approach. The inclusion of mesophotic samples from the Taxella eximia/gracilicaulis and Macrorhynchia phoenicea species complexes provides new insights into the true biodiversity of these genera, revealing additional cryptic species and putative hybridization. Furthermore, the genetic connectivity estimation performed here among depths highlights several species that could be evaluated in terms of the vertical connectivity prerequisite of the Deep Reef Refuge Hypothesis in Mayotte and Reunion. To assess whether hydroids (Cnidaria, Hydrozoa) are good candidates to test the Deep Reef Refuge Hypothesis and particularly the vertical connectivity component, we analysed the genetic diversity and structure of seven species sampled across euphotic and mesophotic depths around Mayotte and Reunion in the southwestern Indian Ocean. Consistent with the position of the western Indian Ocean's biodiversity hotspot, genetic diversity was higher in Mayotte than in Reunion, while all species showed strong genetic differentiation between islands. Incorporating mesophotic samples not only uncovered additional cryptic lineages, but also revealed depth‐related genetic patterns that varied sharply among species and between islands.

The scleractinian coral Porites lutea, an important reef-building coral on western Indian Ocean reefs (WIO), is affected by a newly-reported white syndrome (WS) the Porites white patch syndrome (PWPS). Histopathology and culture-independent molecular techniques were used to characterise the microbial communities associated with this emerging disease. Microscopy showed extensive tissue fragmentation generally associated with ovoid basophilic bodies resembling bacterial aggregates. Results of 16S rRNA sequence analysis revealed a high variability between bacterial communities associated with PWPS-infected and healthy tissues in P. lutea, a pattern previously reported in other coral diseases such as black band disease (BBD), white band disease (WBD) and white plague diseases (WPD). Furthermore, substantial variations in bacterial communities were observed at the different sampling locations, suggesting that there is no strong bacterial association in Porites lutea on WIO reefs. Several sequences affiliated with potential pathogens belonging to the Vibrionaceae and Rhodobacteraceae were identified, mainly in PWPS-infected coral tissues. Among them, only two ribotypes affiliated to Shimia marina (NR043300.1) and Vibrio hepatarius (NR025575.1) were consistently found in diseased tissues from the three geographically distant sampling localities. The role of these bacterial species in PWPS needs to be tested experimentally.

Mesophotic coral ecosystems (MCEs), occurring between 30 and 150 m depth, are increasingly recognized for their ecological importance, yet they remain underexplored, particularly in the southwestern Indian Ocean. During benthic surveys conducted at depths ranging from 15 to 75 m off northwestern Reunion Island, we documented a dense and extensive marine animal forest (MAF) dominated by large arborescent octocorals Subergorgia cf. suberosa. This monospecific community formed a complex three‐dimensional habitat spanning a substantial area at mesophotic depths over the northwestern abrupt slopes of Reunion Island. Colonies reached over 1.5 m in height, providing structural habitat for diverse fish and macroinvertebrate assemblages. A total of 53 fish species from 22 families were recorded in association with this MAF, alongside numerous epibionts and understory anthozoans. Despite its ecological value, this habitat faces significant anthropogenic pressures, particularly from fishing activities. Nearly 25% of photographed S. cf. suberosa colonies were entangled in fishing lines, with visible damage including tissue necrosis and polyp loss affecting up to 47% of colonies. These impacts, along with sedimentation from runoff, raise concerns for the long‐term persistence of this mesophotic habitat‐forming community, as long‐lived, slow‐growing organisms like gorgonians may be more impacted by these types of disturbances and take longer to recover than fast‐growing organisms. Our findings represent the first quantitative assessment of Subergorgia dominance and associated biodiversity in this region and highlight its role as a potential refuge for shallow reef taxa. The ecological significance, spatial extent, and fragility of this MAF underscore the urgent need for spatially replicated surveys, targeted conservation strategies, and regulation of damaging activities such as fishing. We advocate for increased research efforts focused on mesophotic habitats, with particular attention to the population dynamics and ecological functions of large gorgonians, to better understand the role of MCEs in reef resilience under accelerating environmental changes.