Coral Communities Research Articles

Intersection of coral molecular responses to a localized mortality event and ex situ deoxygenation.

In July 2016, East Bank of Flower Garden Banks (FGB) National Marine Sanctuary experienced a localized mortality event (LME) of multiple invertebrate species that ultimately led to reductions in coral cover. Abiotic data taken directly after the event suggested that acute deoxygenation contributed to the mortality. Despite the large impact of this event on the coral community, there was no direct evidence that this LME was driven by acute deoxygenation, and thus we explored whether gene expression responses of corals to the LME would indicate what abiotic factors may have contributed to the LME. Gene expression of affected and unaffected corals sampled during the mortality event revealed evidence of the physiological consequences of the LME on coral hosts and their algal symbionts from two congeneric species (Orbicella franksi and Orbicella faveolata). Affected colonies of both species differentially regulated genes involved in mitochondrial regulation and oxidative stress. To further test the hypothesis that deoxygenation led to the LME, we measured coral host and algal symbiont gene expression in response to ex situ experimental deoxygenation (control = 6.9 ± 0.08 mg L-1, anoxic = 0.083 ± 0.017 mg L-1) in healthy O. faveolata colonies from the FGB. However, this deoxygenation experiment revealed divergent gene expression patterns compared to the corals sampled during the LME and was more similar to a generalized coral environmental stress response. It is therefore likely that while the LME was connected to low oxygen, it was a series of interconnected stressors that elicited the unique gene expression responses observed here. These insitu and ex situ data highlight how field responses to stressors are unique from those in controlled laboratory conditions, and that the complexities of deoxygenation events in the field likely arise from interactions between multiple environmental factors simultaneously.

Temporal and spatial dynamics of coral symbiont assemblages are affected by local and global impacts

The influence of abiotic variables and anthropogenic pressure on symbiodiniaceans associated with the scleractinian corals Mussismilia hispida and Siderastrea stellata were assessed quarterly at Armação dos Búzios, Brazil, for over 18 months. Thirty-eight Symbiodiniaceae ITS2 rDNA phylotypes were found by metabarcoding, with eight comprising new phylotypes. Both hosts maintained their generalist pattern, with 1–3 dominant lineages. An environmental pressure index and changes in seawater temperature explained the variations in the structure and diversity of Symbiodiniaceae assemblages over time and space. A mild bleaching event affected the photosymbiotic assemblage structure, even in non-bleached colonies. The highly dynamic and diverse photosymbiont assemblages were constantly driven by the influence of environmental variables and human-induced impacts. Furthermore, new strains of Symbiodiniaceae might be associated with lower temperatures caused by upwelling, which is characteristic of this subtropical coral community, highlighting the region's idiosyncrasy and the need for further studies of this coral system.

Modeling the spatial distribution of numbers of coral reef fish species and community types in the Western Indian Ocean faunal province

Predicting and mapping coral reef diversity at moderate scales can assist spatial planning and prioritizing conservation activities. We made coarse-scale (6.25 km2) predictive models for numbers of coral reef fish species and community composition starting with a spatially complete database of 70 environmental variables available for 7039 mapped reef cells in the Western Indian Ocean. An ensemble model was created from a process of variable elimination and selectivity to make the best predictions irrespective of human influences. This best model was compared to models using preselected variables commonly used to evaluate climate change and human fishing and water quality influences. Many variables (~27) contributed to the best number of species and community composition models, but local variables of biomass, depth, and retention connectivity were dominant predictors. The key human-influenced variables included fish biomass and distance to human populations, with weaker associations with sediments and nutrients. Climate-influenced variables were generally weaker and included median sea surface temperature (SST) with contributions in declining order from SST kurtosis, bimodality, excess summer heat, SST skewness, SST rate of rise, and coral cover. Community composition variability was best explained by 2 dominant community richness axes of damselfishes-angelfishes and butterflyfishes-parrotfishes. Numbers of damselfish-angelfish species were ecologically separated by depth, and damselfishes declined with increasing depth, median temperature, cumulative excess heat, rate of temperature rise, and chronic temperature stresses. Species of butterflyfish-parrotfish separated by median temperature, and butterflyfish numbers declined with increasing temperature, chronic and acute temperature variability, and the rate of temperature rise. Several fish diversity hotspots were found in the East African Coastal Current Ecoregion centered in Tanzania, followed by Mayotte, southern Kenya, and northern Mozambique. If biomass can be maintained, the broad distributions of species combined with compensatory community responses should maintain high diversity and ecological resilience to climate change and other human stressors.

The microbiome dynamics and interaction of endosymbiotic Symbiodiniaceae and fungi are associated with thermal bleaching susceptibility of coral holobionts.

The thermal bleaching percentage of coral holobionts shows interspecific differences under heat-stress conditions, which are closely related to the coral-associated microbiome. However, the ecological effects of community dynamics and interactions between Symbiodiniaceae and fungi on coral thermal bleaching susceptibility remain unclear. In this study, we analyzed the diversity, community structure, functions, and potential interaction of Symbiodiniaceae and fungi among 18 coral species from a high thermal bleaching risk atoll using next-generation sequencing. The results showed that heat-tolerant C3u sub-clade and Durusdinium dominated the Symbiodiniaceae community of corals and that there were no core amplicon sequence variants in the coral-associated fungal community. Fungal richness and the abundance of confirmed functional animal-plant pathogens were significantly positively correlated with the coral thermal bleaching percentage. Fungal indicators, including Didymellaceae, Chaetomiaceae, Schizophyllum, and Colletotrichum, were identified in corals. Each coral species had a complex Symbiodiniaceae-fungi interaction network (SFIN), which was driven by the dominant Symbiodiniaceae sub-clades. The SFINs of coral holobionts with low thermal bleaching susceptibility exhibited low complexity and high betweenness centrality. These results indicate that the extra heat tolerance of coral in Huangyan Island may be linked to the high abundance of heat-tolerant Symbiodiniaceae. Fungal communities have high interspecific flexibility, and the increase of fungal diversity and pathogen abundance was correlated with higher thermal bleaching susceptibility of corals. Moreover, fungal indicators were associated with the degrees of coral thermal bleaching susceptibility, including both high and intermediate levels. The topological properties of SFINs suggest that heat-tolerant coral have limited fungal parasitism and strong microbial network resilience.IMPORTANCEGlobal warming and enhanced marine heatwaves have led to a rapid decline in coral reef ecosystems worldwide. Several studies have focused on the impact of coral-associated microbiomes on thermal bleaching susceptibility in corals; however, the ecological functions and interactions between Symbiodiniaceae and fungi remain unclear. We investigated the microbiome dynamics and potential interactions of Symbiodiniaceae and fungi among 18 coral species in Huangyan Island. Our study found that the Symbiodiniaceae community of corals was mainly composed of heat-tolerant C3u sub-clade and Durusdinium. The increase in fungal diversity and pathogen abundance has close associations with higher coral thermal bleaching susceptibility. We first constructed an interaction network between Symbiodiniaceae and fungi in corals, which indicated that restricting fungal parasitism and strong interaction network resilience would promote heat acclimatization of corals. Accordingly, this study provides insights into the role of microorganisms and their interaction as drivers of interspecific differences in coral thermal bleaching.

Deep sea treasures - Insights from museum archives shed light on coral microbial diversity within deepest ocean ecosystems

Deep sea benthic habitats are low productivity ecosystems that host an abundance of organisms within the Cnidaria phylum. The technical limitations and the high cost of deep sea surveys have made exploring deep sea environments and the biology of the organisms that inhabit them challenging. In spite of the widespread recognition of Cnidaria's environmental importance in these ecosystems, the microbial assemblage and its role in coral functioning have only been studied for a few deep water corals. Here, we explored the microbial diversity of deep sea corals by recovering nucleic acids from museum archive specimens. Firstly, we amplified and sequenced the V1–V3 regions of the 16S rRNA gene of these specimens, then we utilized the generated sequences to shed light on the microbial diversity associated with seven families of corals collected from depth in the Coral Sea (depth range 1309 to 2959 m) and Southern Ocean (depth range 1401 to 2071 m) benthic habitats. Surprisingly, Cyanobacteria sequences were consistently associated with six out of seven coral families from both sampling locations, suggesting that these bacteria are potentially ubiquitous members of the microbiome within these cold and deep sea water corals. Additionally, we show that Cnidaria might benefit from symbiotic associations with a range of chemosynthetic bacteria including nitrite, carbon monoxide and sulfur oxidizers. Consistent with previous studies, we show that sequences associated with the bacterial phyla Proteobacteria, Verrucomicrobia, Planctomycetes and Acidobacteriota dominated the microbial community of corals in the deep sea. We also explored genomes of the bacterial genus Mycoplasma, which we identified as associated with specimens of three deep sea coral families, finding evidence that these bacteria may aid the host immune system. Importantly our results show that museum specimens retain components of host microbiome that can provide new insights into the diversity of deep sea coral microbiomes (and potentially other organisms), as well as the diversity of microbes writ large in deep sea ecosystems.

Community-Based Monitoring for Rapid Assessment of Nearshore Coral Reefs Amid Disturbances in Teahupo’o, Tahiti

Nearshore coral reefs at Teahupo’o, Tahiti, are currently threatened by destruction from proposed plans to build a new judging tower in the reef lagoon for the 2024 Olympic surfing event. Local community members were trained to utilize 3D photogrammetry techniques to create high-resolution habitat maps of three sites that will be impacted by dredging and tower construction. The resulting orthomosaics were analyzed to quantify and characterize the coral community structure at each study site. Species diversity, coral colony count, coral colony size, and percent cover of live coral and living benthos were extracted from all survey plots. The resulting data show these sites support healthy and diverse coral communities that contribute to the ecological function of the larger reef system at Teahupo’o. The Hawai’i State Division of Aquatic Resources Penalty Matrix was used to estimate the USD value of the live corals and algae identified among the study sites and the total area that will be impacted by the planned development project. This study highlights the utility of 3D photogrammetry for effective citizen science as well as the large economic and ecological impacts that may occur if this proposed construction occurs.

Early post-settlement events, rather than settlement, drive recruitment and coral recovery at Moorea, French Polynesia.

Understanding population dynamics is a long-standing objective of ecology, but the need for progress in this area has become urgent. For coral reefs, achieving this objective is impeded by a lack of information on settlement versus post-settlement events in determining recruitment and population size. Declines in coral abundance are often inferred to be associated with reduced densities of recruits, which could arise from mechanisms occurring at larval settlement, or throughout post-settlement stages. This study uses annual measurements from 2008 to 2021 of coral cover, the density of coral settlers (S), the density of small corals (SC), and environmental conditions, to evaluate the roles of settlement versus post-settlement events in determining rates of coral recruitment and changes in coral cover at Moorea, French Polynesia. Coral cover, S, SC, and the SC:S ratio (a proxy for post-settlement success), and environmental conditions, were used in generalized additive models (GAMs) to show that: (a) coral cover was more strongly related to SC and SC:S than S, and (b) SC:S was highest when preceded by cool seawater, low concentrations of Chlorophyll a, and low flow speeds, and S showed evidence of declining with elevated temperature. Together, these results suggest that changes in coral cover in Moorea are more strongly influenced by post-settlement events than settlement. The key to understanding coral community resilience may lie in elucidating the factors attenuating the bottleneck between settlers and small corals.

Seasonal bleaching and partial mortality of Pocillopora verrucosa corals of the coast of central Vietnam

IntroductionCoral reefs are one of the most productive yet vulnerable ecosystems on Earth. An important step in understanding the functioning of coral communities is studying the environmental impact on the state of reefs. The present study aimed to assess the impacts of the water temperature, precipitation, wave action, and population density on the seasonal bleaching, partial and complete mortality of shallow-water branching corals of Pocillopora verrucosa species complex. Materials and methodsThe present study was conducted at the Dam Bay research station (Nha Trang Bay) from 26 April 2020 to 2 April 2021. The environmental data was collected from the Dam Bay weather station and using a temperature logger planted on the nursery, coral bleaching and mortality were assessed visually using photos taken underwater. ResultsThroughout the study, the percentage of bleached coral colonies varied from 1 to 41.5% with two peaks, in autumn (October) and spring (April). Rates of the development of partial mortality varied from 0 to 10%, with a pronounced peak in December. At the end of the experiment, the percentage of coral colonies suffering from partial mortality reached 47%. Only 4,5% of experimental colonies died to the end of the study. Analysis revealed that water temperature and partial mortality are the drivers of bleaching, while wave action, temperature and bleaching are the factors with a significant impact on partial mortality.DiscussionThus, both bleaching and patrial mortality are interlinked and the effect of partial mortality on bleaching is stronger than the opposite. Both phenomena have pronounced seasonal variation, but their maximum manifestation does not coincide in time allowing corals to avoid their synergetic effect. Moreover, it was found that the impact of individual stress on mortality prevailed over the impact of seasonal bleaching and partial mortality events. The main cause of coral death was the development of algal fouling at the sites of colony lesions, which gradually spread over the entire surface of the colony.

PENILAIAN TINGKAT REKRUTMEN KARANG KERAS (SCLERACTINIA) DI TAMAN LAUT PANDANAN, LOMBOK UTARA, NUSA TENGGARA BARAT UNTUK MENDUKUNG UPAYA RESTORASI

Pandanan Marine Technopark has a flat topography in the middle, and hills on the southwest and northeast sides. Based on a survey using the manta tow method that has been carried out by researchers in 2021, the sea water conditions are clear but the coral reefs are not good. One of the factors suspected to be the cause of the poor condition of the coral reefs in Pandanan Marine Technoark is that many fishermen used to use bombs to catch fish. One of the efforts made by the government and the local (POKMASWAS) to restore the coral reef ecosystem in the Pandanan Marine Technopark is by creating a program to restore corals asexually using coral transplant media. Until now, no research has been conducted in Pandanan Marine Technopark on the sexual recovery of coral reef ecosystems using a coral recruitment level approach.Coral reefs are a group of zooxanthellae symbiotic animals that are very vulnerable to environmental changes. Meanwhile, coral recruitment is a process in which new individual corals are formed through coral reproduction and then join the coral community by forming new colonies. The method used in this study is Underwater Photo Transect (UPT) method and quantitative analysis method consisting of analysis of coral recruitment data for identification of coral recruitment genus, diversity index, uniformity index and dominance index. The results of the recruitment rate for hard corals (Scleractinia) in Pandanan Marine Technopark showed that the recruitment results were based on the lifeforms found including branching, massive, and free living. Meanwhile, the substrate that recruited coral grows the most is Dead Coral with Algae. There were 7 genus found which included Acropora, Favia, Montastrea, Stylophora, Pocilopora, Leptastrea, and Fungia. Most coral recruitment was found at Station 1 with a depth of 3 meters and the lowest number was at Station 3 with a depth of 3 meters. The average Diversity Index is 1.08, which means that the diversity is relatively small and the ecological pressure is very strong. The average uniformity index was 0.91 which means high uniformity and stable community. Furthermore, the average Dominance Index is 0.51, which means moderate dominance

Mapping the susceptibility of reefs to rubble accumulation across the Great Barrier Reef

Disturbance-induced rubble accumulations are described as “killing fields” on coral reefs as coral recruits suffer high post-settlement mortality, creating a bottleneck for reef recovery. The increasing frequency of coral bleaching events, that can generate rubble once coral dies, has heightened concerns that rubble beds will become more widespread and persistent. But we currently lack the tools to predict where rubble is most likely to accumulate. Here, we developed a modelling framework to identify areas that are likely to accumulate rubble on forereef slopes across the Great Barrier Reef. The algorithm uses new high-resolution bathymetric and geomorphic datasets from satellite remote sensing. We found that 47 km of reef slope (3% of the entire reef surveyed), primarily in the southern region, could potentially reach 50% rubble cover. Despite being statistically significant (p < 0.001), the effects of depth and aspect on rubble cover were minimal, with a 0.2% difference in rubble cover between deeper and shallower regions, as well as a maximum difference of 0.8% among slopes facing various directions. Therefore, we conclude that the effects of depth and aspect were insufficient to influence ecological processes such as larval recruitment and recovery in different coral communities. Maps of potential rubble accumulation can be used to prioritise surveys and potential restoration, particularly after major disturbances have occurred.

Describing the dynamics of recruits and juvenile scleractinian corals using 3d models: a case study from Cayo Mero reef, Morrocoy National Park, Venezuela

Understanding the dynamics of coral recruitment and post-settlement is fundamental to a better comprehension of coral reef dynamics and recovery. We studied the abundance and survivorship of coral recruits and juveniles together with benthic dynamics at a scale of months and centimeters in Playa Mero reef, a disturbed reef in Morrocoy National Park. For this, we used photogrammetry to monitor eight permanent 50x50 cm quadrats haphazardly deployed every 3–4 months over 18 months. Juveniles and recruits of Agaricia spp. were at least four times more abundant than reef builders such as Orbicella spp. A distance-based linear model showed that rugosity, macroalgae, coral cover, and sand were the most important benthic variables and predicted up to 46% of the spatial and temporal variation of recruit and juvenile corals. The mortality of juvenile corals was higher than net recruitment rates, and only a limited number of genera such as Agariciids, Colpophyllia, Porites, and Scolymia were observed as recruits. Using a logit model, we also found a positive relationship between the mean growth rate and survivorship of juvenile corals (Nagelkerke R2= 0.67). We concluded the lack of recruitment of large reef builders, and the rapid mortality of a limited number of juvenile species, might be a sign of a coral community's failure to increase coral cover.

Oasis of the deep: Cold-water corals of the South China Sea

Cold-water coral (CWC) communities are biodiversity hotspots on the world's deep seafloor. Although deep-sea corals in the South China Sea (SCS) have been reported before, they are only sporadic. A comprehensive and systematic understanding of the CWC in the SCS would forge the basis for future protection. Here we conducted the first systematic survey on the CWCs in the following six broad-scale sub-regions, from the northwest and northeast slopes to the seamounts in the western and central basins of the SCS, through twenty-four dives of the human-occupied vehicle ShenhaiYongshi. Statistical analysis provided detailed information on the distribution, abundance, size, diversity, and density of CWCs and the in situ environmental conditions supporting coral habitats. We found that the SCS hosted highly diversified coral communities, including twelve genera of gorgonians, six genera of black corals, and one genus of stony corals. The differences in the spatial distribution patterns of coral communities suggested that several environmental variables (depth, temperature, salinity, substrate, and geomorphology) might influence the development of CWCs in the SCS. The intermediate water layer of the SCS appeared to provide suitable habitat for deep-sea coral communities and potentially promoted connectivity. Furthermore, differences between sub-regions within the SCS may be an important factor responsible for the biogeographic patterns of CWCs. These sub-regions of CWCs were observed to range from 0.004 to 0.622 corals m−2, with an average of 0.139 corals m−2. The mean density of CWCs in the SCS was relatively high compared to well-studied CWC hotspots. Overall, the results revealed the significance of the SCS as an important CWC hotspot in the world. These findings provide a fundamental basis for the protection of deep-sea coral assemblages in the SCS.

Emergent Spatial Patterns Can Indicate Upcoming Regime Shifts in a Realistic Model of Coral Community.

AbstractIncreased stress on coastal ecosystems, such as coral reefs, seagrasses, kelp forests, and other habitats, can make them shift toward degraded, often algae-dominated or barren communities. This has already occurred in many places around the world, calling for new approaches to identify where such regime shifts may be triggered. Theoretical work predicts that the spatial structure of habitat-forming species should exhibit changes prior to regime shifts, such as an increase in spatial autocorrelation. However, extending this theory to marine systems requires theoretical models connecting field-supported ecological mechanisms to data and spatial patterns at relevant scales. To do so, we built a spatially explicit model of subtropical coral communities based on experiments and long-term datasets from Rapa Nui (Easter Island, Chile), to test whether spatial indicators could signal upcoming regime shifts in coral communities. Spatial indicators anticipated degradation of coral communities following increases in frequency of bleaching events or coral mortality. However, they were generally unable to signal shifts that followed herbivore loss, a widespread and well-researched source of degradation, likely because herbivory, despite being critical for the maintenance of corals, had comparatively little effect on their self-organization. Informative trends were found under both equilibrium and nonequilibrium conditions but were determined by the type of direct neighbor interactions between corals, which remain relatively poorly documented. These inconsistencies show that while this approach is promising, its application to marine systems will require detailed information about the type of stressor and filling current gaps in our knowledge of interactions at play in coral communities.

“Accidental” artificial reefs — lessons from Singapore for coral and seagrass restoration along urban shorelines

Due to rapid urbanization and coastline modification, the city-state of Singapore has lost nearly three-quarters of its intertidal coral reef area and nearly 90% of its subtidal coral reef area since pre-colonial times. However, the recolonization of coral communities has occurred naturally on seawall structures despite the lack of any deliberate planning or intervention. Recent mapping of coastal habitats shows that seawall reefs now form a significant proportion of habitat occupied by hard corals in Singapore, including areas where there were historically no coral reefs, and often exist side-by-side with other regenerated marine habitats such as seagrass meadows. These “accidental” artificial reefs demonstrate that new hard coral and seagrass communities can establish along highly urbanized shorelines, albeit in a novel form, and provide ecosystem services to urban communities. Key lessons can be drawn from these case studies, particularly relating to landscape design of urban shorelines to facilitate ecological coral and seagrass restoration. These include constructing seawalls with optimal slope profiles, providing creviced berms at the lower intertidal of seawalls, co-locating seawalls with artificial intertidal lagoons, and considering coral larvae supply in choosing sites for restoration using seawalls. Most importantly, these case studies illustrate the potential of using seawalls to restore hard coral and seagrass habitat on a landscape-scale, especially in urban environments.

Time-series dynamics in symbiont communities of acroporid corals undergoing cold stress in winter

Time-series dynamics in symbiont communities of acroporid corals undergoing cold stress in winter

Typhoon impacts on coral and fish communities revealed by reef soundscape assessment using a low-cost acoustic recording device

Typhoon impacts on coral and fish communities revealed by reef soundscape assessment using a low-cost acoustic recording device

Effectiveness of coral (Bilbunna) relocation as a mitigation strategy for pipeline construction at Hayman Island, Great Barrier Reef

SummaryCoral reef management techniques such as relocation and transplantation are increasingly implemented in the context of increasing coastal development and a global decline of coral reefs over the last 30 years. A 170 m submarine desalination pipeline was constructed in 2020 to discharge wastewater from the desalination plant for Hayman Island resort, Whitsundays, Queensland, Australia. Pre‐construction site assessments were conducted indicating a healthy, diverse and recovering coral community between intertidal and 12 m depth in the proposed route of the pipeline. Mitigation options included the selection of a pipeline route that minimised impact on coral, and relocation and transplantation of hard corals. Two hundred and four corals comprising 35 species from 15 genera, with estimated sizes ranging from small (less than 2 kg) to extra large (over 50 kg), were relocated from the pipeline footprint to a similar nearby site. The estimated total weight of relocated corals was 873–2850 kg. The most common species transplanted were Hump Coral (Porites lutea) (27%), Lesser Star Coral (Goniastrea aspera) (8.3%) and Starflower Coral (Astreopora ocellata) (7.8%). Individual coral survivorship and growth was monitored at zero, one, six, 12 and 24 months. After 24 months total coral survival was 77.5%. The survivorship of relocated coral exceeded the mean for global coral restoration projects and was deemed successful by the regulator. To assist benchmarking of future coral relocation projects we propose a standard of below 50% as poor, 50–60% as below average, over 60% as acceptable and over 80% coral survival at two years as excellent.

Fish and coral communities along the seawall of Sutera Harbour Marina, Kota Kinabalu, Sabah, Malaysia

Coastal development involves the introduction of artificial substrates into the natural marine environment, thereby altering and causing the loss of natural habitat. Nevertheless, such artificial structures are known to provide novel habitat for the recruitment and growth of epifauna. Seawalls serve as hard substrata on which assemblages of benthic communities have been observed to settle and proliferate. This study investigated the diversity and abundance of marine fauna, with a primary focus on fish and hard coral communities along the seawall of Sutera Harbour Marina, Kota Kinabalu, Malaysia, in 2015 and 2022. Additionally, benthic cover was assessed in 2022. The assessment was conducted along 12 belt transects (each measuring 10 m × 3 m) placed on the seawall approximately 2–3 m from the bottom. Data on the diversity and abundance of fish and hard coral colonies were collected. Results showed that 105 fish species from 32 families, and 48 genera of hard corals from 17 families inhabited the marina. There was an increase in the abundance and colony size of hard corals over time. Additional benthic data collected in 2022 indicated that the seawalls had a fair coral cover of 31.7%. Although the taxa richness for fish and hard corals was lower compared to surrounding natural reef habitats, this study presented that artificial structures such as seawalls in the marina can support the natural colonization of marine fauna. This study underscores the importance of artificial structures as possible refuges for marine organisms, particularly in urbanized coastal areas.

Spatial distribution and habitat relationship of sea urchin assemblages (Echinodermata: Echinoidea) in Hong Kong waters

Sea urchins are important components of marine ecosystems and can act as bioindicators, reflecting the health of reefs. The spatial patterns of sea urchins are largely shaped by the type of habitat. In Hong Kong, coral communities are divided into two distinct types: reef-building coral habitats and non-reef-building coral habitats. In summer 2020, a qualitative survey was conducted using SCUBA at 56 sites across eastern and western waters, recording a total of 11 species from 6 families of sea urchins. Out of these 56 sites, 14 were selected for a quantitative survey to investigate the relationship between sea urchin assemblages and the two types of coral habitat. We found that the species composition of sea urchins differed significantly between the two habitats, and the presence of sand was a critical factor influencing the species composition of sea urchins. Sand coverage had a positive effect on Salmasic sphaeroides abundance but a negative effect on the abundance of Diadema setosum and Heliocidaris crassispina. The distribution of sea urchins across different degrees of sand coverage may be associated with food availability or species-specific adaptive behaviour, likely due to niche preferences.

Where are resilience-based management strategies appropriate for coral reefs? Mapping environmental conditions and trends in coral cover in Guam and American Samoa

Resilience-based management strategies are gaining attention as tools to improve coral survival and recovery under increasingly stressful conditions. Prioritizing locations to implement these strategies depends on knowing where corals already show potential signs of resilience and how environmental conditions may shift with climate change. We synthesized environmental conditions and coral cover trends in Guam and American Samoa using present-day climate conditions and 2 future climate scenarios: Representative Concentration Pathways 4.5 and 8.5. We examined the spatial overlap between favorable and unfavorable environmental conditions and locations where coral reefs have maintained or increased coral cover over the past decade. Locations representing 4 combinations of the aforementioned characteristics may be subject to different management strategies: (1) conservation and restoration of robust corals, (2) restoration of declining corals, (3) conservation of genetic material of robust corals and stressor mitigation, and (4) no clear strategy for declining corals. We estimated areas in which multiple management actions could be performed based on these combinations. Under present-day climate conditions, the conservation of genetic material and stressor mitigation were overrepresented in Guam, comprising 23% of the study area; this declined to 15% in future climate scenarios. Coral restoration was at first underrepresented (0%). In American Samoa, the proportional area for each strategy remained consistent regardless of climate. Coral restoration was overrepresented, comprising 54% to 56% of the study area, whereas the conservation of genetic material and stressor mitigation were underrepresented (9% to 11%, respectively). Our approach offers a rapid way to assess where potential management actions could be applied based on data aggregated over large spatial extents, which can complement more detailed, labor-intensive assessments of reef community dynamics, particularly if distinct coral communities inform the boundaries of aggregation units. These results may guide managers in selecting ecologically suitable locations for implementing resilience-based management strategies for coral reefs.