Shallow-water Counterparts Research Articles

Quantitative analysis of body colouration in Sebastes rockfishes

Body colouration, a trait under strong selection, is influenced by the visual background of the environment. The stable influence of depth on visual background dynamics is due to light attenuation along the water column. Depth is also a key factor driving diversification in Sebastes rockfishes, influencing variations in several biological traits. Comparisons between closely related species suggest that brightly coloured species (red, orange, or yellow) tend to inhabit deeper waters and have slower growth rates compared to their shallow-water counterparts with dominance of dark colours (black, brown, or grey). Here, we used 377 photos from 100 Sebastes species, along with recently developed methods of colour quantification and phylogenetic comparative analyses, to assess this trend. Our analyses confirmed the separation of body colouration regarding depth, which was accompanied by differences in growth rates and morphological traits. This indicated that variations in body colourations are included in the ongoing correlational selection process dictated by depth. Analyses of closely related species indicated that depth is an initial driver of colour differentiation and that colour differences do not progressively increase with genetic divergences. We hypothesized that the bright-coloured rockfishes are found in deeper waters because in shallow environments they are more vulnerable to the potential negative effects of UV radiation and higher predation risk, while their predation successes are lessened, in comparison to dark-coloured rockfishes. Overall, this study emphasizes the intricate relationship between genetics, environment, adaptation, and the striking diversity of body colourations observed in Sebastes rockfishes.

Intrasediment gypsum in subtidal offshore carbonates as a witness of basin-wide evaporitic precipitation. The case of the southern Variscan foreland basin (mid-Carboniferous)

Widespread calcite and quartz pseudomorphs, interpreted as originally gypsum crystals, occur within hemipelagic calci-mudstone accumulated in subtidal offshore environments in the broad marine foreland basin developed on the southern flank of the Ouachita–Alleghanian–Variscan Orogen during mid-Carboniferous times, which acted as a marine corridor connecting the Panthalassa and Palaeo-Tethys Oceans during the Mississippian, and progressively narrowed during the assembly of Pangea. In this study, 67 outcrops of radiolaria-bearing calci-mudstone deposits that contain calcite and quartz pseudomorphs located in northern Spain and southern France were studied to constrain the gypsum spatial distribution and sedimentological features. The recognised microfacies indicate intrasediment gypsum precipitation, accompanied by less abundant bottom-grown precipitates and gypsum cumulates, in extensive offshore, probably several ten to a few hundred metres deep, basinal environments. Gypsum precipitation took place during a short-lived temporal episode during the early Bashkirian time (Voznesenkian), which can be correlated, on the basis of benthic foraminifera, with the coastal (inter- to supratidal) gypsum evaporites identified in NW Africa (Tindouf and Reggan successions in Morocco and Algeria) that would represent the shallow-water counterparts. The occurrence of gypsum precipitates both in offshore hemipelagic calci-mudstones of the Variscan foreland basin and in inter- to supratidal environments of the epeiric Sahara Platform indicates that hypersaline conditions affected vast marine areas, roughly coinciding with the estimated age of closure of the Panthalassa and Palaeo-Tethys marine connection. Therefore, the studied succession represents the trace of a basin-wide evaporitic episode extending for hundreds of kilometres driven by foreland basin restriction, mid-Carboniferous sea-level fall and arid climate. This study provides new insights for the interpretation of gypsum precipitates in offshore marine environments encountered in the Phanerozoic and whose genesis is poorly understood.

High compositional and functional similarity in the microbiome of deep-sea sponges.

Sponges largely depend on their symbiotic microbes for their nutrition, health, and survival. This is especially true in high microbial abundance (HMA) sponges, where filtration is usually deprecated in favor of a larger association with prokaryotic symbionts. Sponge-microbiome association is substantially less understood for deep-sea sponges than for shallow water species. This is most unfortunate, since HMA sponges can form massive sponge grounds in the deep sea, where they dominate the ecosystems, driving their biogeochemical cycles. Here, we assess the microbial transcriptional profile of three different deep-sea HMA sponges in four locations of the Cantabrian Sea and compared them to shallow water HMA and LMA (low microbial abundance) sponge species. Our results reveal that the sponge microbiome has converged in a fundamental metabolic role for deep-sea sponges, independent of taxonomic relationships or geographic location, which is shared in broad terms with shallow HMA species. We also observed a large number of redundant microbial members performing the same functions, likely providing stability to the sponge inner ecosystem. A comparison between the community composition of our deep-sea sponges and another 39 species of HMA sponges from deep-sea and shallow habitats, belonging to the same taxonomic orders, suggested strong homogeneity in microbial composition (i.e. weak species-specificity) in deep sea species, which contrasts with that observed in shallow water counterparts. This convergence in microbiome composition and functionality underscores the adaptation to an extremely restrictive environment with the aim of exploiting the available resources.

How long have we been mistaken? Multi-tools shedding light into the systematics of the widespread deep-water genus Madrepora Linnaeus, 1758 (Scleractinia)

Deep-water coral reefs are found worldwide and harbor biodiversity levels that are comparable to their shallow-water counterparts. However, the genetic diversity and population structure of deep-water species remain poorly explored, and historical taxonomical issues still need to be resolved. Here we used microsatellite markers as well as ultraconserved elements (UCE) and exons to shed light on the population structure, genetic diversity, and phylogenetic position of the genus Madrepora, which contains M. oculata, one of the most widespread scleractinian species. Population structure of 107 samples from three Southwestern Atlantic sedimentary basins revealed the occurrence of a cryptic species, herein named M. piresae sp. nov. (authored by Kitahara, Capel and Zilberberg), which can be found in sympatry with M. oculata. Phylogeny reconstructions based on 134 UCEs and exon regions corroborated the population genetic data, with the recovery of two well-supported groups, and reinforced the polyphyly of the family Oculinidae. In order to better accommodate the genus Madrepora, while reducing taxonomical confusion associated with the name Madreporidae, we propose the monogeneric family Bathyporidae fam. nov. (authored by Kitahara, Capel, Zilberberg and Cairns). Our findings advance the knowledge on the widespread deep-water genus Madrepora, resolve a long-standing question regarding the phylogenetic position of the genus, and highlight the need of a worldwide review of the genus.

Adaptive mechanisms of the deep-sea coral Polymyces wellsi (Flabellidae, Scleractinia) illuminate strategies for global climate change

An oxygen minimum zone (OMZ) typically occurs in the tropical western Pacific and is characterized by an unfavorably low pH, a rather low oxygen content and extreme food limitation. Understanding how deep-sea corals survive in these challenging conditions, especially how calcification occurs at depths near the aragonite saturation horizon, is anticipated to provide a strategy for stony corals to address global climate change. In this study, we collected the deep-sea solitary coral Polymyces wellsi living in the OMZ of the Caroline Ridge and analyzed its mitochondrial genome and transcriptome. Phylogenetic analysis based on mitochondrial genomes suggested that the solitary character and the deep-sea adaptations evolved at least three times in Scleractinia. In comparison to the transcriptomes of shallow-water counterparts, the genetic elements related to biomineralization, mitochondrial components, and ciliary motion underwent positive selection and expansion in P. wellsi, which suggested their significance in facilitating the adaptations to the stressors of low pH, insufficient oxygen content, scarce food resources, or the combined effects of these stressors within the OMZ. An interesting finding of this study was that the positively selected amino acids in P. wellsi increased the isoelectric points of its skeleton organic matrix proteins, which suggested a novel bio-indicator that may reflect the adaptive capacity to the external acidified seawater. Overall, this study not only provides insights into the adaptive mechanisms of deep-sea solitary corals but also illuminates strategies for global climate change.

Species-specific relationships between deep sea sponges and their symbiotic Nitrosopumilaceae.

Sponges thrive in the deep, dark and nutrient-depleted ocean and may rely on microbial symbionts for carbon acquisition and energy generation. However, these symbiotic relationships remain largely unexplored. In this study, we analyze the microbiome of deep-sea sponges and show that ammonia-oxidizing archaea (AOA) of the family Nitrosopumilaceae make up at least 75% of the microbial communities of the sponges Aphrocallistes sp., Farrea sp. and Paratimea sp.. Given the known autotrophic metabolism of AOAs, this implies that these sponge holobionts can have the capacity for primary production in the deep-sea. We also show that specific AOA lineages are highly specific towards their hosts, hinting towards an unprecedent vertical transmission of these symbionts in deep-sea sponges. Our results show that the ecology and evolution of symbiotic relationships in deep-sea sponge is distinct from that of their shallow-water counterparts.

Genome assembly of the deep-sea coral Lophelia pertusa.

Like their shallow-water counterparts, cold-water corals create reefs that support highly diverse communities, and these structures are subject to numerous anthropogenic threats. Here, we present the genome assembly of Lophelia pertusa from the southeastern coast of the USA, the first one for a deep-sea scleractinian coral species. We generated PacBio continuous long reads data for an initial assembly and proximity ligation data for scaffolding. The assembly was annotated using evidence from transcripts, proteins, and ab initio gene model predictions. This assembly is comparable to high-quality reference genomes from shallow-water scleractinian corals. The assembly comprises 2,858 scaffolds (N50 1.6Mbp) and has a size of 556.9Mbp. Approximately 57% of the genome comprises repetitive elements and 34% of coding DNA. We predicted 41,089 genes, including 91.1% of complete metazoan orthologs. This assembly will facilitate investigations into the ecology of this species and the evolution of deep-sea corals.

Biodiversity, environmental drivers, and sustainability of the global deep-sea sponge microbiome

In the deep ocean symbioses between microbes and invertebrates are emerging as key drivers of ecosystem health and services. We present a large-scale analysis of microbial diversity in deep-sea sponges (Porifera) from scales of sponge individuals to ocean basins, covering 52 locations, 1077 host individuals translating into 169 sponge species (including understudied glass sponges), and 469 reference samples, collected anew during 21 ship-based expeditions. We demonstrate the impacts of the sponge microbial abundance status, geographic distance, sponge phylogeny, and the physical-biogeochemical environment as drivers of microbiome composition, in descending order of relevance. Our study further discloses that fundamental concepts of sponge microbiology apply robustly to sponges from the deep-sea across distances of >10,000 km. Deep-sea sponge microbiomes are less complex, yet more heterogeneous, than their shallow-water counterparts. Our analysis underscores the uniqueness of each deep-sea sponge ground based on which we provide critical knowledge for conservation of these vulnerable ecosystems.

Mesophotic Gorgonian Corals Evolved Multiple Times and Faster Than Deep and Shallow Lineages

Mesophotic Coral Ecosystems (MCEs) develop on a unique environment, where abrupt environmental changes take place. Using a time-calibrated molecular phylogeny (mtDNA: mtMutS), we examined the lineage membership of mesophotic gorgonian corals (Octocorallia: Cnidaria) in comparison to shallow and deep-sea lineages of the wider Caribbean-Gulf of Mexico and the Tropical Eastern Pacific. Our results show mesophotic gorgonians originating multiple times from old deep-sea octocoral lineages, whereas shallow-water species comprise younger lineages. The mesophotic gorgonian fauna in the studied areas is related to their zooxanthellate shallow-water counterparts in only two clades (Gorgoniidae and Plexauridae), where the bathymetrical gradient could serve as a driver of diversification. Interestingly, mesophotic clades have diversified faster than either shallow or deep clades. One of this groups with fast diversification is the family Ellisellidae, a major component of the mesophotic gorgonian coral assemblage worldwide.

Red and white muscle proportions and enzyme activities in mesopelagic sharks.

In the last decade, there has been an increase in the study of the ecology of deep-sea organisms. One way to understand an organism's ecology is the study of its metabolism. According to literature, deep-sea sharks possess a lower anaerobic enzyme activity than their shallow-water counterparts, but no difference has been observed regarding their aerobic enzyme activities. These studies have suggested deep-sea sharks should be slow and listless swimmers. However, other studies based on video observations have revealed differences in cruise swimming speed between different species. The present study examined muscles of squaliform sharks, including both luminous and non-luminous species. We combined measurements of the relative amounts of red and white muscle with assays of enzymes that are used as markers for aerobic (citrate synthase, malate dehydrogenase) and anaerobic (lactate dehydrogenase) metabolism, searching for a relationship with cruising speeds. Non-luminous deep-sea species displayed lower aerobic enzyme activities but similar anaerobic enzyme activities than the benthic shallow-water counterpart (Squalus acanthias). Conversely, luminous Etmopteridae species were found to have similar aerobic enzyme activities to S. acanthias but displayed lower anaerobic enzyme activities. Analyses revealed that red muscle proportion and aerobic enzyme activities were positively related to the cruise swimming speed. In contrast, Dalatias licha, which swims at the slowest cruise swimming speed ever recorded, presented a very low aerobic metabolic phenotype (lower aerobic marker enzymes and less red muscle). Finally, the values obtained for white muscle proportion and anaerobic metabolic phenotype suggested a high burst capacity for D. licha and non-luminous sharks.

Population structure, activity potential and ecotype partitioning of Pseudoalteromonas along the vertical water column of the New Britain Trench.

Microbial degradation of organic matter along the vertical profile of the water column is a major process driving the carbon cycle in the ocean. Pseudoalteromonas has been identified as a dominant genus in pelagic marine environments worldwide, playing important roles in the remineralization of organic carbon. However, the current understanding of Pseudoalteromonas was mainly based on shallow water populations or cultivated species. This study analyzed for the first time the structure, activity potential and ecotypes differentiation of Pseudoalteromonas in the water column of the New Britain Trench (NBT) down to 6000m. Analysis on diversities of the 16S rRNA gene and their transcripts showed that Pseudoalteromonas was greatly enriched in deep-sea waters and showed high activity potentials. The deep-sea Pseudoalteromonas were significantly different from their shallow-water counterparts, suggesting an obvious ecotype division along with the vertical profile. Phylogenetic analysis on the 16S rRNA gene and hsp60 gene of 219 Pseudoalteromonas strains isolated from different depths further showed that the vertical ecotype division could even occur at the strain level, which might be a result of long-term adaptation to environmental conditions at different depths. The discovered depth-specific strains provide valuable models for further studies on adaptation, evolution and functions of the deep-sea Pseudoalteromonas.

A Deep-Sea Sponge Loop? Sponges Transfer Dissolved and Particulate Organic Carbon and Nitrogen to Associated Fauna

Cold-water coral reefs and sponge grounds are deep-sea biological hotspots, equivalent to shallow-water tropical coral reefs. In tropical ecosystems, biodiversity and productivity are maintained through efficient recycling pathways, such as the sponge loop. In this pathway, encrusting sponges recycle dissolved organic matter (DOM) into particulate detritus. Subsequently, the sponge-produced detritus serves as a food source for other organisms on the reef. Alternatively, the DOM stored in massive sponges was recently hypothesized to be transferred to higher trophic levels through predation of these sponges, instead of detritus production. However, for deep-sea sponges, the existence of all prerequisite, consecutive steps of the sponge loop have not yet been established. Here, we tested whether cold-water deep-sea sponges, similar to their tropical shallow-water counterparts, take up DOM and transfer assimilated DOM to associated fauna via either detritus production or predation. We traced the fate of 13carbon (C)- and 15nitrogen (N)-enriched DOM and particulate organic matter (POM) in time using a pulse-chase approach. During the 24-h pulse, the uptake of 13C/15N-enriched DOM and POM by two deep-sea sponge species, the massive species Geodia barretti and the encrusting species Hymedesmia sp., was assessed. During the subsequent 9-day chase in label-free seawater, we investigated the transfer of the consumed food by sponges into brittle stars via two possible scenarios: (1) the production and subsequent consumption of detrital waste or (2) direct feeding on sponge tissue. We found that particulate detritus released by both sponge species contained C from the previously consumed tracer DOM and POM, and, after 9-day exposure to the labeled sponges and detritus, enrichment of 13C and 15N was also detected in the tissue of the brittle stars. These results therefore provide the first evidence of all consecutive steps of a sponge loop pathway via deep-sea sponges. We cannot distinguish at present whether the deep-sea sponge loop is acting through a detrital or predatory pathway, but conclude that both scenarios are feasible. We conclude that sponges could play an important role in the recycling of DOM in the many deep-sea ecosystems where they are abundant, although in situ measurements are needed to confirm this hypothesis.

Growth and survival dynamics of mesophotic coral juveniles in shallow reefs

As many degrading shallow reefs suffer from recruitment failure, mesophotic coral ecosystems have been suggested as a potential source of coral propagules promoting the recolonization of these reefs. However, whether mesophotic coral populations can repopulate shallower reefs is currently debatable. Here, we compared the response of corals settled on mesophotic (50 m) tiles and transplanted to the shallow reef (10 m), at a Nature Reserve and an unprotected site in the northern Gulf of Eilat/Aqaba, Red Sea. Mortality was substantially higher for the transplanted corals, as compared with their shallow counterparts living on the tiles, with over half of the transplanted juveniles dying 1 yr post-transplantation. Transplanted corals exhibited a 24% higher survivorship at the MPA site. We further explored the survival and growth rates of the 4 most common transplanted coral genera (Cyphastrea, Porites, Psammocora, and Stylophora). An inverse relationship between survival and growth was evident among the transplanted coral genera; Porites and Stylophora demonstrated similar net growth rates to those of their shallow-water counterparts but lower post-transplantation survivorship, while Cyphastrea and Psammocora showed the opposite trend. Although this study demonstrates the reduced plasticity of mesophotic coral juveniles to cope with shallow-water conditions, it nonetheless offers some potential to facilitate shallow-reef recovery.

Characterization of deep-sea benthic invertebrate megafauna of the Galapagos Islands

The deep sea represents the largest and least explored biome on the planet. Despite the iconic status of the Galapagos Islands and being considered one of the most pristine locations on earth, the deep-sea benthic ecosystems of the archipelago are virtually unexplored in comparison to their shallow-water counterparts. In 2015, we embarked on a multi-disciplinary scientific expedition to conduct the first systematic characterization of deep-sea benthic invertebrate communities of the Galapagos, across a range of habitats. We explored seven sites to depths of over 3,300 m using a two-part Remotely Operated Vehicle (ROV) system aboard the E/V Nautilus, and collected 90 biological specimens that were preserved and sent to experts around the world for analysis. Of those, 30 taxa were determined to be undescribed and new to science, including members of five new genera (2 sponges and 3 cnidarians). We also systematically analysed image frame grabs from over 85 h of ROV footage to investigate patterns of species diversity and document the presence of a range of underwater communities between depths of 290 and 3,373 m, including cold-water coral communities, extensive glass sponge and octocoral gardens, and soft-sediment faunal communities. This characterization of Galapagos deep-sea benthic invertebrate megafauna across a range of ecosystems represents a first step to study future changes that may result from anthropogenic impacts to the planet’s climate and oceans, and informed the creation of fully protected deep-water areas in the Galapagos Marine Reserve that may help preserve these unique communities in our changing planet.

Predictive modeling of suitable habitat for deep-sea corals offshore the Northeast United States

Deep-sea corals (DSCs) are important living marine resources, forming both oases of biodiversity and three-dimensional habitat structure for fishes and invertebrates. However, because of logistical difficulties and expense of deep-sea exploration, much less is known about the distribution of DSCs than is known for their shallow-water counterparts. Predictive modeling, therefore, is essential for estimating the extent of DSC habitat in areas that are unexplored in order to support conservation efforts, to provide information for effective management of offshore activities affecting the seafloor, and for future exploration and research. In support of research and management efforts in the U.S. Northeast (Cape Hatteras, NC north to the Canadian border), we developed a comprehensive set of habitat suitability models covering this entire geographic region for nine taxonomic groups of DSCs (Alcyonacea, gorgonian corals, non-gorgonian corals, Scleractinia, Caryophylliidae, Flabellidae, Pennatulacea, Sessiliflorae, and Subselliflorae). Maximum entropy (MaxEnt) models were fit to DSC presence records and spatially-explicit environmental predictors depicting depth and seafloor topography, surficial sediment characteristics, and oceanography. A stepwise model selection procedure was then implemented to identify the set of predictor variables that maximized predictive performance for each taxonomic group. To allow for comparisons across taxonomic groups, the standard MaxEnt logistic predictions were converted into calibrated classes of habitat suitability. Overall, model performance was high for all taxonomic groups. Model fit was best for Caryophylliidae, Sessiliflorae, and Flabellidae, whereas model stability was greatest for the three taxonomic groups of Alcyonacea. Model results reported here corroborate known distributions of corals in the region. For example, large structure-forming taxa are predicted to occur mainly in canyon environments, particularly in areas of steep slope (>30°); sea pens in softer sediments of the continental shelf and slope. Additionally, the models successfully predicted DSC locations during field testing. Despite the limitations of presence-only data, several novel extensions to the traditional MaxEnt analysis workflow improved model selection, accuracy assessment, and comparability of results across taxonomic groups. This approach, when integrated with management processes, could be a powerful tool for science-based conservation, management, and spatial planning for these marine resources.

Ecological features and swimming capabilities of deep-sea sharks from New Zealand

Currently the ecology of deep-water sharks is poorly documented, especially in situ information for these elusive species are lacking. In this study, stereo-Baited Remote Underwater Videos (stereo-BRUVs) were deployed to collect ecological data from New Zealand deep-sea sharks. The results showed differences in abundance between species, with Etmopterus granulosus (Etmopteridae) found in greatest numbers. Moreover, the known depth range increased for Scymnodon macracanthus (Centrophiridae). Deep-sea shark species were generally found to swim at slower cruise speeds (0.36 ± 0.04 m s−1) than their shallow-water counterparts (0.63 ± 0.05 m s−1). However, the swimming speed of deep-sea sharks was clearly not uniform, with some species displaying higher cruise swimming speeds than others. The fastest sharks (Centrophorus harrissoni, Etmopterus granulosus and Etmopterus molleri) had swimming abilities comparable to benthic shallow water sharks (0.48 ± 0.02 m s−1). The higher cruise swimming speed in the family Etmopteridae could be an advantage for these luminous sharks if they follow isolumes to match their ventral light intensity with the down-welling light of their environment. This study revealed that alternative non-destructive methods can be effective for ecological studies of deep-sea marine fauna.

Deep-sea sharks: Relation between the liver's buoyancy and red aerobic muscle volumes, a new approach

Shark's buoyancy depends on two types of force: (i) the hydrostatic force which is mainly provided by their liver filled with low density lipids and (ii) the hydrodynamic force which is provided by the morphology of their body and fins. Shallow-water shark species are usually negatively buoyant, whereas deep-sea shark species have been suggested to display neutral buoyancy. It has been suggested that species that are close to the neutrality would have less red aerobic muscle fibers. Here, we investigated several liver features (the hepatosomatic index, the oil content and the lipid composition) playing a major role regarding the buoyancy of three deep-sea shark species (Etmopterus molleri, Etmopterus spinax and Isistius brasiliensis) and one shallow-water counterpart (Galeus melastomus). We used FT-Raman and FT-MIR spectroscopy to qualify/quantify the lipid composition of their liver. Our results showed that most deep-sea shark species studied have liver features providing more buoyancy than their shallow-water counterparts, appart from E. molleri which shows liver's features that resemble more shallow-water shark species (e.g. G. melastomus). Finally, data regarding liver features of several deep-sea shark species from the literature were added and the red aerobic muscle distribution/proportion of nine species was measured, to reveal how these parameters might be related. Our results showed that sharks characterized by a liver providing more hydrostatic force possess proportionally less red aerobic muscles than sharks having a liver that contributes less to their buoyancy. Therefore, our results i.e. deep-sea shark displaying less red aerobic muscle with a liver providing more buoyancy, support low metabolic rates hence slow swimming speed.

Using 3D photogrammetry from ROV video to quantify cold-water coral reef structural complexity and investigate its influence on biodiversity and community assemblage

Fine-scale structural complexity created by reef-building coral in shallow-water environments is influential on biodiversity, species assemblage and functional trait expression. Cold-water coral reefs are also hotspots of biodiversity, often attributed to the hard surface and structural complexity provided by the coral. However, that complexity has seldom been quantified on a centimetric scale in cold-water coral reefs, unlike their shallow-water counterparts, and has therefore never been linked in a similar way to the reef inhabitant community. Structure from motion techniques which create high-resolution 3D models of habitats from sequences of photographs is being increasingly utilised, in tandem with 3D spatial analysis to create useful 3D metrics, such as rugosity. Here, we demonstrate the use of ROV video transect data for 3D reconstructions of cold-water coral reefs at depths of nearly 1000 m in the Explorer Canyon, a tributary of Whittard Canyon, NE Atlantic. We constructed 40 3D models of approximately 25-m-length video transects using Agisoft Photoscan software, resulting in sub-centimetre resolution reconstructions. Digital elevation models were utilised to derive rugosity metrics, and orthomosaics were used for coral coverage assessment. We found rugosity values comparable to shallow-water tropical coral reef rugosity. Reef and nearby non-reef communities differed in assemblage composition, which was driven by depth and rugosity. Species richness, epifauna abundance and fish abundance increased with structural complexity, being attributed to an increase in niches, food, shelter and alteration of physical water movement. Biodiversity plateaued at higher rugosity, illustrating the establishment of a specific reef community supported by more than 30% coral cover. The proportion of dead coral to live coral had limited influence on the community structure; instead, within-reef patterns were explained by depth and rugosity, though our results were confounded to a certain extent by multi-collinearity. Fine-scale structural complexity appeared to be integral to local-scale ecological patterns in cold-water coral reef communities.

Marginal sinks or potential refuges? Costs and benefits for coral-obligate reef fishes at deep range margins.

Escalating climate-related disturbances and asymmetric habitat losses will increasingly result in species living in more marginal habitats. Marginal habitats may represent important refuges if individuals can acquire adequate resources to survive and reproduce. However, resources at range margins are often distributed more sparsely; therefore, increased effort to acquire resources can result in suboptimal performance and lead to marginal populations becoming non-self-sustaining sink-populations. Shifting resource availability is likely to be particularly problematic for dietary specialists. Here, we use extensive in situ behavioural observations and physiological condition measurements to examine the costs and benefits of resource-acquisition along a depth gradient in two obligate corallivore reef fishes with contrasting levels of dietary specialization. As expected, the space used to secure coral resources increased towards the lower depth margin. However, increased territory sizes resulted in equal or greater availability of resources within deeper territories. In addition, we observed decreased competition and no differences in foraging distance, pairing behaviour, body condition or fecundity at greater depths. Contrary to expectation, our results demonstrate that coral-obligate fishes can select high-quality coral patches on the deeper-reef to access equal or greater resources than their shallow-water counterparts, with no extra costs. This suggests depth offers a viable potential refuge for some at-risk coral-specialist fishes.

Biodiversity and Functional Ecology of Mesophotic Coral Reefs

Mesophotic coral reefs, currently defined as deep reefs between 30 and 150 m, are linked physically and biologically to their shallow water counterparts, have the potential to be refuges for shallow coral reef taxa such as coral and sponges, and might be a source of larvae that could contribute to the resiliency of shallow water reefs. Mesophotic coral reefs are found worldwide, but most are undescribed and understudied. Here, we review our current knowledge of mesophotic coral reefs and their functional ecology as it relates to their geomorphology, changes in the abiotic environment along depth gradients, trophic ecology, their reproduction, and their connectivity to shallow depths. Understanding the ecology of mesophotic coral reefs, and the connectivity between them and their shallow water counterparts, is now a primary focus for many reef studies as the worldwide degradation of shallow coral reefs, and the ecosystem services they provide, continues unabated.