Variability in the functional composition of coral reef fish communities on submerged and emergent reefs in the central Great Barrier Reef, Australia

On coral reefs, depth and gradients related to depth (e.g. light and wave exposure) influence the composition of fish communities. However, most studies focus only on emergent reefs that break the sea surface in shallow waters (<10 m). On the Great Barrier Reef (GBR), submerged reefs (reefs that do not break the sea surface) occupy an area equivalent to all emergent reefs. However, submerged reefs have received comparatively little research attention, and fish communities associated with submerged reefs remain poorly quantified. Here, we quantify fish assemblages at each of three depths (10, 20 and 30 m) on eight submerged reefs (four mid-shelf and four outer-shelf) and two nearby emergent reefs in the central GBR where reef habitat extends from 0-~25 m depth. We examine how total fish abundance, the abundance of 13 functional groups, and the functional composition of fish communities varies among depths, reef types (submerged versus emergent reefs), and shelf position (mid-shelf versus outer-shelf). Overall fish abundance decreased sevenfold with depth, but declined less steeply (twofold) on outer-shelf submerged reefs than on both mid-shelf submerged reefs and emergent reefs. The functional composition of the fish assemblage also varied significantly among depths and reef types. Turnover in the functional composition of the fish community was also steeper on the mid-shelf, suggesting that shallow-affiliated groups extend further in deeper water on the outer-shelf. Ten of the 13 functional groups were more strongly associated with the shallowest depths (the upper reef slope of emergent reefs or the 'crests' of submerged reefs), two groups (soft coral/sponge feeders and mesopredators) were more abundant at the deepest sites. Our results confirm that submerged reefs in the central GBR support a wide range of coral reef fishes, and are an important component of the GBR ecosystem

Acropora - The most-studied coral genus

Over the last 2 million years, Acropora (the staghorn or elkhorn corals) has become the dominant genus of reef-building corals throughout the Indo-Pacific. Like other coral genera, Acropora taxonomy is presently undergoing comprehensive revision as a consequence of large-scale sequencing projects, which will substantially alter our understanding of the diversity and biogeography of the group. Acropora spp. are among the most stress sensitive of corals, and as a consequence, population declines are occurring worldwide, most significantly due to global warming. Despite diverse adult colony morphology, all members of the genus are similar at the tissue and polyp level and follow common reproductive and developmental programs. Some progress has recently been made in development of gene knockdown/knockout methods despite the long generation times of corals and need to rely on natural spawning events. Since 2018, the number of Acropora genome sequences available has increased dramatically, although taxonomic uncertainties complicate their evolutionary interpretation. Areas of particularly active research include settlement and the control of metamorphosis; metabolic interactions with both bacterial and eukaryotic symbionts, particularly Symbiodinaceae, and their gain and loss; and how best to restore reefs destroyed by storms and coral bleaching

Spatial patterns in the distribution of benthic assemblages across a large depth gradient in the Coral Sea, Australia

The Queensland Plateau in the Coral Sea off north-eastern Australia supports numerous submerged and emergent reefs. Osprey Reef is an emergent reef at the northern tip of the plateau ~1500 m in elevation. Over such a large depth gradient, a wide range of abiotic factors (e.g. light, temperature, substratum etc.) are likely to influence benthic zonation. Despite the importance of understanding the biodiversity of Australia's Coral Sea, there is a lack of biological information on deep-water habitats below diving depths. Here we used a deep-water ROV transect to capture video, still photos and live samples over a depth range spanning 92 to 787 m at North Horn on Osprey Reef. Video analysis, combined with bathymetry data, was used to identify the zones of geomorphology and the benthic assemblages along the depth gradient. There were marked changes in the geomorphology and the substrate along this depth gradient which likely influence the associated benthos. Cluster analysis indicated five benthic assemblage groups, which showed clear zonation patterns and were generally predictable based on the depth and sedimentary environment. These results are the first quantitative observations to such depths and confirm that the waters of the Coral Sea support diverse benthic assemblages, ranging from shallow-water coral reefs to mesophotic coral ecosystems, to deep-water azooxanthellate corals and sponge gardens. The knowledge provided by our study can inform management plans for the Coral Sea Commonwealth Marine Reserve that incorporate the deeper reef habitats and help to minimise future damage to these marine ecosystems

Substratum selection in coral reef sponges and their interactions with other benthic organisms

Substratum preferences and contact interactions among sessile organisms can be a major determinant of biotic gradients in the structure of benthic communities on coral reefs. Sponges are a substantial component of these communities, but their substratum requirements and interactions with other benthic taxa are poorly understood. Here, we quantified sponge substratum preferences and interactions from 838 randomly selected photo-quadrats across different depths (5, 10 and 15 m), exposure (sheltered and exposed), and substratum topography (horizontal, inclined and vertical surfaces) on coastal coral reefs in Kimbe Bay. A high proportion (55%) of sponge colonies were associated with dead coral, unconsolidated coral rubble (7%) and calcium carbonate rock (CaCO3 rock) (7%), even though they represented only 10%, 4% and 1% of the available substratum, respectively. Sponges interacted most frequently with algae (~ 34%), corals (~ 30%) and crustose coralline algae (CCA ~ 19%) that represented ~ 46%, ~ 18% and ~ 14% of the substratum cover, respectively. The microhabitat preferences of sponges and frequency of interactions with other taxa were mostly consistent across various exposure, depth and substratum topography conditions. Most interactions appeared to be “stand-offs” (71%) which are interactions with no clear winner or loser. However, when overgrowth occurred, sponges were usually winners, overgrowing corals (92%), CCA (81%) and macroalgae (65%). Three sponge species Dysidea sp1, Lamellodysidea cf. chlorea and Lamellodysidea chlorea accounted for 51% to 96% of the overgrowth of sponges over algae, corals and CCA, but there was no one species found to always win or lose. Our results suggest that sponges avoid other biological substrata by preferentially settling on dead coral, coral rubble and CaCO3 rock, but when they do come into contact with algae and corals, they frequently overgrow their spacial competitors

Predicting impact to assess the efficacy of community-based marine reserve design

During the planning phase the efficacy of different strategies to manage marine resources should ultimately be assessed by their potential impact, or ability to make a difference to ecological and social outcomes. While community-based and systematic approaches to establishing marine protected areas have their strengths and weaknesses, comparisons of their effectiveness often fail to explicitly address potential impact. Here, we predict conservation impact to compare recently implemented community-based marine reserves in Tonga to a systematic configuration specifically aimed at maximizing impact. Boosted regression tree outputs indicated that fishing pressure accounted for ∼24% of variation in target species biomass. We estimate that the community-based approach provides 84% of the recovery potential of the configuration with the greatest potential impact. This high potential impact results from community-based reserves being located close to villages, where fishing pressure is greatest. These results provide strong support for community-based marine management, with short-term benefits likely to accrue even where there is little scope for systematic reserve design

Functional consequences of Palaeozoic reef collapse

Biogenic reefs have been hotspots of biodiversity and evolutionary novelty throughout the Phanerozoic. The largest reef systems in Earth’s history occurred in the Devonian period, but collapsed during the Late Devonian Mass Extinction. However, the consequences for the functional diversity of Palaeozoic reefs have received little attention. Here, we examine changes in the functional diversity of tabulate coral assemblages over a 35 million year period from the middle Devonian to the Carboniferous, straddling the multiphase extinction event to identify the causes and ecological consequences of the extinction for tabulate corals. By examining five key morphological traits, we show a divergent response of taxonomic and functional diversity to the mass extinction: taxonomic richness peaked during the Givetian (~ 388–383 Ma) and coincided with peak reef building, but functional diversity was only moderate because many species had very similar trait combinations. The collapse of taxonomic diversity and reef building in the late Devonian had minimal impact on functional richness of coral assemblages. However, non-random shifts towards species with larger corallites and lower colony integration suggest a shift from photosymbiotic to asymbiotic taxa associated over the study period. Our results suggest that the collapse of the huge Devonian reef systems was correlated with a breakdown of photosymbiosis and extinction of photosymbiotic tabulate coral taxa. Despite the appearance of new tabulate coral species over the next 35 million years, the extinction of taxa with photosymbiotic traits had long-lasting consequences for reef building and, by extension, shallow marine ecosystems in the Palaeozoic

Benthic communities of the lower mesophotic zone on One Tree shelf edge, southern Great Barrier Reef, Australia

Context: Increasing interest in mesophotic coral ecosystems has shown that reefs in deep water show considerable geomorphic and ecological variability among geographic regions. Aims: We provide the first investigation of mesophotic reefs at the southern extremity of the Great Barrier Reef (GBR) to understand the biotic gradients and habitat niches in the lower mesophotic zone. Methods: Multibeam data were used to target five benthic imagery transects collected in the lower mesophotic (80–130 m) zone from the shelf edge near One Tree Island (23°S, 152°E) by using a single HD-SDI subsea camera. Key results: Transects supported similar benthic communities in depths of 80–110 m, with the abundance of sessile benthos declining below ~110 m where the shelf break grades into the upper continental slope. Conclusions: The effect of the Capricorn Eddy may be promoting homogeneity of benthic assemblages, because it provides similar environmental conditions and potential for connectivity. Variation in benthic communities between hard and soft substrate and differing topographic relief within the study site are likely to be influenced by variation in sedimentation, including sensitivity to suspended particles. Implications: This study highlighted that the lower mesophotic region on the One Tree shelf edge supports mesophotic coral ecosystems that vary depending on depth and substrate

Incentivizing co-management for impact: mechanisms driving the successful national expansion of Tonga's Special Management Area program

The expansion of coastal marine protected areas can suffer from two key drawbacks: (a) the difficulty of incentivizing local communities to manage areas for conservation when their livelihoods also depend on resource use; and (b) that many protected areas get situated residually, or in locations with limited value for either biodiversity conservation or livelihoods. Here, we discuss and analyze key characteristics of Tonga's Special Management Area (SMA) program, including both the mechanisms that have motivated its successful national expansion and its ability to configure no-take reserves in areas that are considered to have high value to resource users. Granting communities exclusive access zones in exchange for implementing no-take reserves has encouraged conservation actions while fostering long-term relationships with resources. Ensuring no-take reserves occurred within the boundaries of exclusive access zones enabled communities to protect areas of greater extractive values than they would have otherwise. We conclude that the success of this program offers a way forward for achieving targets in the global expansion marine protected areas

A tenuis relationship: traditional taxonomy obscures systematics and biogeography of the ‘Acropora tenuis’ (Scleractinia: Acroporidae) species complex

Molecular phylogenetics has fundamentally altered our understanding of the taxonomy, systematics and biogeography of corals. Recently developed phylogenomic techniques have started to resolve species-level relationships in the diverse and ecologically important genus Acropora, providing a path to resolve the taxonomy of this notoriously problematic group. We used a targeted capture dataset (2032 loci) to investigate systematic relationships within an Acropora clade containing the putatively widespread species Acropora tenuis and its relatives. Using maximum likelihood phylogenies and genetic clustering of single nucleotide polymorphisms from specimens, including topotypes, collected across the Indo-Pacific, we show ≥ 11 distinct lineages in the clade, only four of which correspond to currently accepted species. Based on molecular, morphological and geographical evidence, we describe two new species; Acropora rongoi n. sp. and Acropora tenuissima n. sp. and remove five additional nominal species from synonymy. Systematic relationships revealed by our molecular phylogeny are incongruent with traditional morphological taxonomy and demonstrate that characters traditionally used to delineate species boundaries and infer evolutionary history are homoplasies. Furthermore, we show that species within this clade have much smaller geographical ranges and, consequently, population sizes than currently thought, a finding with profound implications for conservation and management of reef corals

Contrasting patterns of changes in abundance following a bleaching event between juvenile and adult scleractinian corals

Funding was provided by the Australian Research Council Centre of Excellence for Coral Reef Studies (CE140100020) and the Templeton Foundation (Grant #60501, ‘Putting the Extended Evolutionary Synthesis to the Test’). MD is grateful to the Scottish Funding Council (MASTS, grant reference HR09011) and the European Research Council (grant BioTIME). The study was partially supported by Australian Research Council grants DP1093448 and FT110100609.Coral bleaching events have caused extensive mortality on reefs around the world. Juvenile corals are generally less affected by bleaching than their conspecific adults and therefore have the potential to buffer population declines and seed recovery. Here, we use juvenile and adult abundance data at 20 sites encircling Lizard Island, Great Barrier Reef, before and after the 2016 bleaching event to quantify: (1) correlates of changes in juvenile abundance following a bleaching event; (2) differences in susceptibility to extreme thermal stress between juveniles and adults. Declines in juvenile abundance were lower at sites closer to the 20-m-depth contour and higher for Acropora and Pocillopora juveniles than for other taxa. Juveniles of Acropora and Goniastrea were less susceptible to bleaching than adults, but the opposite was true for Pocillopora spp. and taxa in the family Merulinidae. Our results indicate that the potential of the juvenile life stage to act as a buffer during bleaching events is taxon-dependent.PostprintPeer reviewe