JournalTOCs

Coral Reefs
Journal Prestige (SJR): 1.307

Citation Impact (citeScore): 3
Number of Followers: 22

Hybrid journal (It can contain Open Access articles)
ISSN (Print) 1432-0975 - ISSN (Online) 0722-4028
Published by Springer-Verlag

[2469 journals]

Decadal stability of coral reef benthic communities on Palmyra Atoll,
central Pacific, through two bleaching events
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  Abstract: Abstract The prevalence of coral bleaching due to thermal stress has been increasing on coral reefs worldwide. While many studies have documented how corals respond to warming, fewer have focused on benthic community responses over longer time periods or on the response of non-coral taxa (e.g., crustose coralline algae, macroalgae, or turf). Here, we quantify spatial and temporal changes in benthic community composition over a decade using image analysis of permanent photoquadrats on Palmyra Atoll in the central Pacific Ocean. Eighty permanent plots were photographed annually between 2009 and 2018 on both the wave-exposed fore reef (FR, 10 m depth, n = 4 sites) and the wave-sheltered reef terrace (RT, 5 m depth, n = 4 sites) habitats. The El Niño events of 2009–2010 and 2015–2016 resulted in acute thermal stress and coral bleaching was observed at both reef habitats during these events. Across 10 yr and two bleaching events, the benthic community structure on Palmyra shows evidence of long-term stability. Communities on the RT exhibited minimal change in percent cover of the dominant functional groups, while the FR had greater variability and minor declines in hard coral cover. There was also spatial variation in the trajectory of each site through time. Coral cover decreased at some sites 1 yr following both bleaching events and was replaced by different algal groups depending on the site, yet returned to pre-bleaching levels within 2 yr. Overall, our data reveal the resilience of calcifier-dominated coral reef communities on Palmyra Atoll that have persisted over the last decade despite two bleaching events, demonstrating the capacity for these reefs to recover from and/or withstand disturbances in the absence of local stressors.
  PubDate: 2022-05-21
Coralline Hills: high complexity reef habitats on seamount summits of the
Vitória-Trindade Chain
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  Abstract: Abstract Seamounts and oceanic islands play an important role as biodiversity hotspots amid the vastness of the oligotrophic open ocean. While island ecology and evolution have received a lot of attention in the last decades, the exploration and understanding of community and habitat dynamics of seamounts remain challenging. Here, we investigate the ecology and biogeography of fish and benthic communities of a recently discovered southwestern Atlantic reef system at Davis seamount. This seamount belongs to the Vitória-Trindade Chain and is located in international waters off the Brazilian coast. We present this reef system, that also occurs on other shallow seamounts of the chain, as a new reef habitat named “Coralline Hills”: Its hill-shaped structure is mainly built by crustose coralline algae and rises up from the seamount summit at 60–70 m to 17 m depth. The benthic community is mainly composed by coralline algae and sponges. Fish biomass at Davis coralline hill is dominated by carnivores, mainly top predators such as nurse sharks and large groupers. The relatively shallow reef top presents higher species richness, abundance and distinct trophic structure (mostly omnivore and planktivore species) than the mesophotic zone (with higher abundance of carnivorous fishes). A biogeographic analysis revealed that the reef fish community structure is greatly influenced by a set of dispersal and establishment traits that strongly differs from that encountered on coastal reefs of the central Brazilian coast and on insular reefs of Trindade Island. Gathering information about the ecology and structure of such unique and remote habitat is timely, since the region is under imminent threat such as fishing and mining and lacks international attention.
  PubDate: 2022-05-20
Natural variability in seawater temperature compromises the metabolic
performance of a reef-forming cold-water coral with implications for
vulnerability to ongoing global change
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  Abstract: Abstract Seawater temperature is one of the main variables that determines cold-water coral distribution worldwide. As part of an initiative to explore new areas of deep-sea habitats along the Southeast United States (SEUS) continental margin, a series of expeditions were carried out as part of the Deep-Sea Exploration to Advance Research on Corals/Canyons/Cold seeps (DEEP SEARCH) project. During these explorations, a cold-water coral reef complex composed mainly of Lophelia pertusa was located off the coast of South Carolina at 650–850 m depth. In this geographic area the species normally has a thermal tolerance between 6 and 12 °C with the capacity to form extensive calcium carbonate structures, thus creating complex habitat for a variety of associated species. Owing to the paucity of these structures and the unusual environmental conditions of this geographic area, with regular arrival of warm surface waters from the Gulf Stream, the main aim of this study was to understand the physiological response of L. pertusa to the variation in extreme temperature events in this region. Short-term experiments simulated the rate of temperature increase from the ambient temperature (8 °C) to the environmental maximum (14 °C) (heat-wave treatment). We found that temperature had a significant effect on the metabolic functions through an increase in respiration (0.108 to 0.247 µmol O2 g−1DW h−1) and excretion rates (0.002 to 0.011 µmol NH3 g−1DW h−1) at 14 °C. Oxygen to Nitrogen ratios (O:N) also showed an effect of temperature where corals switched from lipid-dominated toward a mix of lipid-protein and protein-dominated catabolism. To further characterize the metabolic response, feeding assays (capture rate of Artemia) were performed at the same temperature range with an overall three-fold decrease in capture rates under 14 °C compared to ambient temperature, thus increasing the probability of temperature-induced metabolic stress. Our results suggest that temperature variations affect the metabolic response of cold-water corals, particularly along the SEUS continental margin. Since the incursion of warm surface water to deeper zones is predicted to increase in frequency and duration due to climate change, L. pertusa may be implicated negatively, followed by ecological consequences for the survival and functionality for the ecosystem it supports.
  PubDate: 2022-05-13
Climate impacts alter fisheries productivity and turnover on coral reefs
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  Abstract: Abstract Alteration of benthic reef habitat after coral bleaching and mortality induces changes in fish assemblages, with implications for fisheries. Our understanding of climate impacts to coral reef fisheries is largely based on fish abundance and biomass. The rates at which biomass is produced and replenished (productivity and turnover) are also important to sustaining fisheries, yet the responses of these metrics following bleaching are largely unknown. Here, we examine changes in fish productivity and turnover after mass coral bleaching events in Seychelles, on reefs that were recovering to coral-dominated habitats and those that shifted to macroalgae-dominated regimes. Productivity of fish assemblages increased on all recovering reefs, particularly on fished reefs resulting in levels similar to protected reefs 19 years after bleaching. Herbivore-detritivores, such as scraping and excavating parrotfish, appeared to drive biomass production through increased abundance on recovering reefs. Productivity on regime-shifted reefs remained stable at 1994 levels in fished areas, with increases observed on protected reefs. Large increases in browser productivity (particularly on protected reefs), combined with increases for invertivores, maintained post-bleaching productivity on macroalgal reefs. For all diet groups, net turnover was generally higher on fished regime-shifted reefs than on recovering reefs, suggesting fish biomass is more readily replenished on macroalgal reefs. Reef structural complexity was a positive predictor of productivity for all diet groups. These findings indicate that post-bleaching reef fish productivity is strongly influenced by benthic recovery trajectories, and demonstrates the importance of herbivore and invertivore species in sustaining small-scale inshore fisheries following climatic disturbances.
  PubDate: 2022-05-13
Differential responses of bacterial communities in coral tissue and mucus
to bleaching
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  Abstract: Abstract Investigation of the association between microbial communities in different compartments and the health status of coral hosts will support an in-depth understanding of the relationship between the microbial symbionts and coral health. In this study, next-generation sequencing of 16S ribosomal RNA genes (rDNA) and ribosomal RNA (rRNA) was used to characterize bacterial communities from the mucus and tissues of healthy and bleached Pocillopora damicornis. The cDNA (active bacterial members) and DNA (the total bacterial community) libraries of the same coral sample were similar, suggesting the fact that the bacteria inhabiting P. damicornis were active. Upon coral bleaching, the bacterial communities changed considerably in the tissue, but remained stable in the coral mucus layer. The relative abundance of Endozoicomonas decreased sharply in the bleached coral tissue. Presumably, microbial function based on the taxonomic compositions was accordingly changed in coral tissue, but not in the mucus layer, when the coral bleached. This study suggests that both rRNA- and rDNA-based methods for bacterial community analysis are fit for evaluating P. damicornis health implications. Furthermore, the results of this study demonstrate the differential responses of mucus- and tissue-associated bacterial communities to coral bleaching.
  PubDate: 2022-05-12
Correction to: Living on the edge: environmental variability of a shallow
late Holocene cold-water coral mound
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  PubDate: 2022-05-11
Biogeography of endosymbionts (Symbiodiniaceae) associated with
zoantharian species (Hexacorallia: Anthozoa) from the Macaronesia and Cape
Verde ecoregions
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  Abstract: Abstract Populations of some zoantharian species (Cnidaria: Anthozoa) have been able to proliferate in locations where abiotic conditions hinder scleractinian corals’ survival. One of the contributing factors might be the advantageous host-symbiont associations that could lead holobionts to be more resilient to environmental variation, even in marginal-environments. However, few studies have addressed these Zoantharia–Symbiodiniaceae associations, especially little is known about their nature and distribution in the Atlantic Ocean. In this study, we use the large ribosomal subunit (LSU-rDNA) and Internal Transcribed Spacer 2 (ITS2) to examine the biodiversity and distribution of Symbiodiniaceae species within zoantharians that inhabit along the latitudinal gradient Madeira (40ºN)—Cape Verde (16ºN). Moreover, we determine the number of different endosymbionts genera inhabiting the same specimen, in order to estimate their ability to face alterations in the environment. The results showed that Symbiodiniaceae diversity increases towards the tropics, with a total of six ITS2 types belonging to Symbiodinium and Cladocopium, the latter being the most frequent genus. Furthermore, we have found a possible undescribed species inhabiting P. aff. clavata collected at Madeira Island, the northernmost limit of brachycnemic zoantharian distributions in the East Atlantic. These results, combined with the literature reviewed, constitute the first mention of the genus Symbiodinium in a species host that is not Zoanthus spp and for the archipelago of the Canary Islands. An appendix summarizing Zoantharia–Symbiodiniaceae distributions around the Atlantic is included to facilitate future research on these holobiont associations.
  PubDate: 2022-05-10
Conservation of coral reef fishes: a field-hardy method to cryopreserve
spermatogonial cells
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  Abstract: Abstract The biodiversity of marine fishes is threatened globally by climate change and other anthropogenic activities, particularly in coral reef ecosystems. We present a simple, field-hardy method to cryopreserve marine fish gonads, targeting spermatogonial cells (undifferentiated diploid germ cells) with the ultimate goal of permitting recovery of threatened species and populations via gonadal diploid germ cell transplantation technologies. The use of a simplified cryopreservation extender based on L-15 medium resulted in minimal decline in spermatogonial cell viability post-thaw. Moreover, we compared post-cryopreservation viability of sperm and spermatogonial cells from gonads cryopreserved with freshly prepared cryoprotectant and with cryoprotectant prepared in advance and stored at −20 °C. We found no significant difference, suggesting that these solutions may be prepared in advance and frozen, ready for later use. We urge conservation, academic, and regulatory agencies to cryobank fish gonads as part of their sample collection processes to support the biodiversity and security of valuable marine fish resources, alongside other restoration efforts.
  PubDate: 2022-05-10
A stratified transect approach captures reef complexity with
canopy-forming organisms
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  Abstract: Abstract On the Great Barrier Reef (GBR), persistent changes to reef communities have begun to be documented, and on inshore reefs these shifts may favour the proliferation of macroalgae. Critical to understanding changes to reef community structure in response to anthropogenic impacts is developing effective methods to accurately document the abundance of different reef organisms. Effective monitoring must be time and cost efficient, replicable, and able to sufficiently and accurately detect disturbances to allow development of strategies to mitigate their impacts. Traditional techniques to document coral reef communities (i.e. photo-quadrats, benthic intercept transects) rely on planar views, which tend to either over- or under-represent canopy-forming organisms. As canopy-forming organisms are likely to be affected by anthropogenic influences (corals negatively, algae positively), it is essential for monitoring programs to implement methods sufficient to document changes to the vertical dimension of coral reefs. Here we build on previous work to document the canopy effect in coral-dominated ecosystems and propose a new survey approach suitable for implementation in algal-dominated systems. A vertically stratified transect, modified from a traditional point intercept transect, captures benthic and canopy-forming members of reef communities and provides information on three-dimensional complexity. To test the capability of the new method to detect changes in vertical reef structure, seaweed was removed from experimental quadrats and monitoring techniques were applied before and after four months of regrowth. A stratified method more accurately captured the three-dimensional change resulting from algal canopy growth, while resolving the over- and under-representation of algal biomass in two traditional techniques. We propose that a stratified transect method improves abundance estimates of canopy-forming organisms whilst maintaining data compatibility with traditional methods.
  PubDate: 2022-05-09
Highly diverse and geographically differentiated Symbiodiniaceae
communities associated with the hydrocoral Millepora alcicornis in the
Atlantic Ocean
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  Abstract: Abstract The symbiotic relationship between corals and symbiodiniaceans can favor reef formation, but is easily rupted when these organisms are exposed to thermal anomalies. Here, we assessed the ITS2 rDNA phylotype diversity of dominant Symbiodiniaceae lineages associated with the hydrocoral Millepora alcicornis and investigated host–symbiont distribution patterns in the Atlantic Ocean. This is the first effort to assess the symbiont community of this hydrocoral over nearly its entire distributional range. Millepora alcicornis is highly generalist in the composition of its photosymbiont community. We found 16 ITS2 phylotypes, mainly of the genus Breviolum but also of the genera Symbiodinium and Cladocopium; nine of them are new lineages. The distribution patterns of the M. alcicornis–Symbiodiniaceae associations were explained by differences in primary productivity, photosynthetically active radiation, water turbidity, and temperature. Six geographic sections were identified. Colonies from the Brazilian Northeastern Region showed the most stable associations, with two Breviolum phylotypes, while those from the Brazilian Eastern Region showed the most diverse symbiont community, composed of three genera of Symbiodiniaceae. A new and dominant phylotype of Breviolum was identified in the Brazilian Southern Region. Our results suggest a radiation of Breviolum lineages associated with M. alcicornis through the Atlantic Ocean. The impressive diversity of symbiotic associations observed characterizes an adaptable host–symbiont relationship, which can be key for colonization of new habitats and the resilience of milleporids to environmental changes.
  PubDate: 2022-05-07
Species-specific elementomes for scleractinian coral hosts and their
associated Symbiodiniaceae
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  Abstract: Abstract Increasing anthropogenic pressure on coral reefs is creating an urgent need to understand how and where corals can proliferate both now and under future scenarios. Resolving environmental limits of corals has progressed through the accurate identification of corals’ ‘realised ecological niche’. Here we expand the ecological niche concept to account for corals’ ‘biogeochemical niche’ (BN), defined as the chemical space in which a coral is adapted to survive, and which is identifiable by a unique quantity and proportion of elements (termed “elementome”). BN theory has been commonly applied to other taxa, successfully predicting species distributions and stress responses by their elementomes. Here, we apply the BN theory to corals for the first time, by using dry combustion and inductively coupled plasma–mass spectrometry (ICP-MS) to determine five key macronutrients and thirteen trace elements of four diverse scleractinian coral species from the Great Barrier Reef (GBR): Acropora hyacinthus; Echinopora lamellosa; Pocillopora cf. meandrina; and Pocillopora cf. verrucosa. The elementomes were investigated in both host and Symbiodiniaceae, and the latter had the highest elemental concentrations (except molybdenum). Each coral species associated with distinct members of the genus Cladocopium (determined by ITS2 analysis) with photo-physiological data suggesting specialisation of Cladocopium functional biology. Distinct endosymbiont community structure and functioning between corals with different elementomes confirms that BN theory holds as metabolic compatibility alters across host–symbiont associations. Additional work is needed to understand the plasticity of coral elementomes, and in turn BN, over space and time to aid predictions on coral distribution and survival with environmental change.
  PubDate: 2022-05-04
Genomic assessment of an endemic Hawaiian surgeonfish, Acanthurus
triostegus sandvicensis, reveals high levels of connectivity and
fine-scale population structure
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  Abstract: Abstract The Hawaiian Archipelago has served as a natural laboratory to assess genetic connectivity patterns across a broad spectrum of taxonomic and ecological diversity. Almost all these studies were based on a few targeted loci, but technologies now allow us to assess population structure with genomic coverage and greater resolution. Here, we provide a SNP-based analysis for an endemic surgeonfish, Acanthurus triostegus sandvicensis (manini) across the Hawaiian Archipelago and adjacent Johnston Atoll (N = 461). Based on 3649 SNPs, manini showed population structure in the main Hawaiian Islands, but genetic homogeneity across most of the northwestern extent of the archipelago (overall FST = 0.033, P < 0.001). Net migration occurred from Johnston Atoll into Hawai‘i, providing further support for Johnston Atoll being a pathway for dispersal (or colonization) into Hawai′i. These results highlight the higher efficacy of genomic sequencing to characterize fine-scale patterns of connectivity relative to a targeted loci approach and, moving forward, may invoke a reassessment of past connectivity studies in a genomics framework.
  PubDate: 2022-05-02
Highly repetitive space-use dynamics in parrotfishes
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  Abstract: Abstract Understanding space-use patterns in parrotfish (Scarini) is especially important for coral reefs, where herbivores play a key role in maintaining reef health. However, we lack the long-term high-resolution data needed to understand parrotfish space-use patterns over time. We examine long-term space-use dynamics in parrotfish (median duration: 359 days) by tracking 23 terminal-males of three species along a coral reef in the Northern Red Sea Gulf of Aqaba. We used acoustic telemetry to track horizontal movement along the reef, depth, and activity (through tri-axial accelerometers), and quantify the effect of environmental factors such as water temperature and algal abundance on long-term patterns. We found that in the species examined, nearly all individuals maintained a highly consistent spatial pattern throughout the study duration, in which they repeatedly moved along a distinct route between nighttime sleeping areas and spatially constrained daytime areas. Individual patterns were surprisingly conserved over time, despite seasonal changes and variation in resource abundance. Individuals differed considerably in their daily travel distance, with some traveling up to 2000 m along the reef, while others moved only between deep and shallow areas. Activity levels consistently peaked around mid-day, regardless of the location of their daytime areas, which behavioral surveys confirm are associated with feeding activities. We also detected peaks in activity during early mornings when surveys detect that parrotfish interacted in large aggregates (typically 30–60 individuals). Our findings indicate that space-use patterns vary among individuals but are extremely conservative within individuals over long time periods. This in turn may have important consequences to the health of reefs, as individual parrotfish may be less likely to alter foraging patterns as conditions change. Furthermore, the high fidelity to limited sleeping and daytime areas may be a source of concern, as local small-scale disturbances may strongly disrupt individual space-use patterns.
  PubDate: 2022-04-27
Significance of fish–sponge interactions in coral reef ecosystems
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  Abstract: Abstract Sponges (Porifera) are a key component of many coral reef ecosystems. In some biogeographic regions, they are considered the dominant benthic fauna and they have the capacity to fulfil many similar roles to reef-building scleractinians. Certainly, sponges predominate at depth, below the critical thresholds of most coral species. The biological and physical attributes of these biogenic communities contribute essential resources for many reef-associated fishes. However, while fish–sponge interactions have been widely documented, there is no global synthesis of the literature on these interrelationships from the perspective of fish ecology. Here we evaluate coral reef fish–sponge relationships, including the role of sponges in providing food and shelter for fishes, the influence fishes have on sponge distribution and abundance and possible outcomes of climate change on fish–sponge interactions. To date, 16 fish families have been shown to associate with 56 different sponge genera, using them as either a source of shelter (n = 17) or a food source (n = 50), although methodologies for the latter currently lack consistency. We demonstrate that a more comprehensive understanding of fish–sponge interactions has been garnered from tropical Atlantic coral reefs, which has resulted in a strong biogeographic bias. While it is evident that in some areas of the Caribbean fish are key in shaping the distribution and abundance of sponges, it is not yet known whether this conclusion applies to the Indo-Pacific. With increasing stresses such as bleaching events impacting coral reef ecosystems, further work is needed to evaluate whether sponges can fulfil similar functional roles to those previously provided by reef-building scleractinians. Similarly, determining whether sponge expansion will compensate for the negative effects of reef degradation, or contribute to their decline, is vital.
  PubDate: 2022-04-24
Transient demographic approaches can drastically expand the toolbox of
coral reef science
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  Abstract: Abstract Coral communities are threatened by an increasing plethora of abiotic and biotic disturbances. Preventing the ensuing loss of coral coverage and diversity calls for a mechanistic understanding of resilience across coral species and populations that is currently lacking in coral reef science. Assessments into the dynamics of coral populations typically focus on their long-term (i.e. asymptotic) characteristics, tacitly assuming stable environments in which populations can attain their long-term characteristics. Instead, we argue that greater focus is needed on investigating the transient (i.e. short-term) dynamics of coral populations to describe and predict their characteristics and trajectories within unstable environments. Applying transient demographic approaches to evaluating and forecasting the responses of coral populations to disturbance holds promise for expediting our capacity to predict and manage the resilience of coral populations, species, and communities.
  PubDate: 2022-04-23
Large-scale biogeographic patterns are reflected in the genetic structure
of a broadcast spawning stony coral
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  Abstract: Abstract Countries in the Western Indian Ocean (WIO) and along the Red Sea are particularly vulnerable to coral reef degradation, and understanding the degree of connectivity among coral reefs is a first step toward efficient conservation. The aim of this study is to investigate the genetic diversity, population structure and connectivity patterns of the broadcast spawning coral Acropora tenuis, first at a large scale comparing the Red Sea and the WIO, and second at a smaller scale comparing sites within the WIO. In total 689 individual A. tenuis colonies were sampled on 28 locations in Saudi Arabia, Kenya, Tanzania, Mozambique and Madagascar and analysed with seven microsatellite markers. The sample site in the Red Sea was found to be differentiated from all other sites in the WIO, which confirms the hypothesised genetic break. High differentiation was found between the African mainland and Madagascar and within Madagascar. However, there is evidence for long-distance larval dispersal for A. tenuis in the North Mozambique Channel region, with exchange between northern Mozambique and northern Madagascar. The sites in the southwest of Madagascar show mixing with sites in northern Madagascar, as well as exchange with sites in northern Mozambique and Tanzania. Southern Mozambique forms a separate group in Bayesian clustering. High genetic connectivity was found for most sites along the East African mainland coast, with no indication for strict genetic barriers. These results support biophysical modelling studies, which propose Tanzania as a seeding source of larvae for downstream Kenya. These patterns of high genetic connectivity combined with contemporary dispersal barriers can be explained by the long larval duration of A. tenuis and the prevailing northbound East African Coast Current that facilitates higher genetic connectivity along the northern East African Coast, while eddies in the Mozambique Channel are causing larval retention in southern Mozambique and Madagascar.
  PubDate: 2022-04-22
Juvenile age and available coral species modulate transition probability
from herbivory to corallivory in Acanthaster cf. solaris (Crown-of-Thorns
Seastar)
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  Abstract: Abstract Prior to transitioning to a coral diet, juvenile Acanthaster cf. solaris, the Crown-of-Thorns Seastar (COTS), feed on crustose coralline algae. Although a detailed understanding of juvenile ecology is crucial to predict and prevent outbreaks, the exact timing of the transition is unresolved. Two experiments were conducted to measure time and size of COTS at the transition, and investigate potential modulating effects of different coral species. COTS began early transitions at similar sizes (7.5–8.5 mm), and these first transitions were observed around 136–145 d. Between 175 and 191 d, a 50% transition in the presence of Acropora tenuis was measured. After 175d, the percentage of COTS cohorts that had transitioned was significantly lower in A. millepora (38%) and Stylophora pistillata (7%) compared to A. tenuis (51%). These data fill important knowledge gaps in juvenile ecology, and the influence of coral species on transition suggests an undescribed feedback mechanism between prey and predator.
  PubDate: 2022-04-20
Impact of nitrogen (N) and phosphorus (P) enrichment and skewed N:P
stoichiometry on the skeletal formation and microstructure of symbiotic
reef corals
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  Abstract: Abstract Reported divergent responses of coral growth and skeletal microstructure to the nutrient environment complicate knowledge-based management of water quality in coral reefs. By re-evaluating published results considering the taxonomy of the studied corals and the N:P stoichiometry of their nutrient environment, we could resolve some of the major apparent contradictions. Our analysis suggests that Acroporids behave differently to several other common genera and show distinct responses to specific nutrient treatments. We hypothesised that both the concentrations of dissolved inorganic N and P in the water and their stoichiometry shape skeletal growth and microstructure. We tested this hypothesis by exposing Acropora polystoma fragments to four nutrient treatments for > 10 weeks: high nitrate/high phosphate (HNHP), high nitrate/low phosphate (HNLP), low nitrate/high phosphate (LNHP) and low nitrate/low phosphate (LNLP). HNHP corals retained high zooxanthellae densities and their linear extension and calcification rates were up to ten times higher than in the other treatments. HNLP and LNLP corals bleached through loss of symbionts. The photochemical efficiency (Fv/Fm) of residual symbionts in HNLP corals was significantly reduced, indicating P-starvation. Micro-computed tomography (µCT) of the skeletal microstructure revealed that reduced linear extension in nutrient limited or nutrient starved conditions (HNLP, LNHP, LNLP) was associated with significant thickening of skeletal elements and reduced porosity. These changes can be explained by the strongly reduced linear extension rate in combination with a smaller reduction in the calcification rate. Studies using increased skeletal density as a proxy for past thermal bleaching events should consider that such an increase in density may also be associated with temperature-independent response to the nutrient environment. Furthermore, the taxonomy of corals and seawater N:P stoichiometry should be considered when analysing and managing the impacts of nutrient pollution.
  PubDate: 2022-04-20
Early successional trajectory of benthic community in an uninhabited reef
system three years after mass coral bleaching
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  Abstract: Abstract Severe thermal stress events occurring on the backdrop of globally warming oceans can result in mass coral mortality. Tracking the ability of a reef community to return to pre-disturbance composition is important to inform the likelihood of recovery or the need for active management to conserve these ecosystems. Here, we quantified annual, temporal changes in the benthic communities for the three years following mass coral mortality at Jarvis Island—an uninhabited island in the Pacific Remote Islands Marine National Monument. While Jarvis experienced catastrophic coral mortality in 2015 due to heat stress resulting from the 2015/16 El Niño, significant annual shifts were documented in the benthic community in the three years post-disturbance. Macroalgal and turf dominance of the benthos was temporary—likely reflecting the high biomass of herbivorous reef fishes post-bleaching—giving way to calcifiers such as crustose coralline algae and Halimeda, which may facilitate rather than impede coral recovery. By 2018, indications of recovery were detectable in the coral community itself as juvenile densities increased and stress-tolerant genera, such as Pavona, exceeded their pre-disturbance densities. However, densities of Montipora and Pocillopora remain low, suggesting recovery will be slow for these formerly dominant taxa. Collectively, the assemblage and taxon-specific shifts observed in the benthic and coral community support cautious optimism for the potential recovery of Jarvis Island’s coral reefs to their pre-disturbance state. Continued monitoring will be essential to assess whether reassembly is achieved before further climate-related disturbance events affect this reef system.
  PubDate: 2022-04-19
Coral extension rate analysis using computed axial tomography
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  Abstract: Abstract Biology, geology, and paleoclimatology studies of coral reefs often rely on annual rates of corallite extension determined from cores of skeletal material recovered from scleractinian corals. To examine how corallite orientation within such cores may affect measured extension rates, we used high-resolution X-ray computed axial tomography (CT) to image a suite of short Orbicella faveolata cores from corals living in the northwest Gulf of Mexico. This nondestructive method enabled skeletal extension rates to be determined several ways: (1) along transects that cross individual slice/slab images, (2) along transects that cross both a 0.6-mm-thick CT slice and the corresponding conventional X-ray image from a 8.4-mm-thick slab of skeletal material, and (3) along transects that cross slices/slab images oriented at varied angles relative to a coral core’s longitudinal axis. Additionally, the true three-dimensional (3D) extension rates of individual corallites were determined and compared to the extension rates apparent on a two-dimensional (2D) slice/slab. Results suggest that differences between all methods are not statistically significant, confirming that the conventional extension rate methodology is suitable for studies if the core’s longitudinal axis is appropriately aligned with the growth axis of the corallites. Effects of core misalignment on extension rates measurement and the appearance of skeletal structures in 2D images are described, and the applicability of 3D imagery for investigations of coral polyp growth patterns and coral colony development is demonstrated.
  PubDate: 2022-04-16