Genetic structure in the coral, Montastraea cavernosa: assessing genetic differentiation among and within Mesophotic reefs

Function and stability of mesophotic coral reefs

The function and stability of mesophotic coral ecosystems (MCEs) have been extensively studied in recent years. These deep reefs are characterized by local physical processes, particularly the steep gradient in irradiance with increasing depth, and their impact on trophic resources. Mesophotic reefs exhibit distinct zonation patterns that segregate shallow reef biodiversity from ecologically unique deeper communities of endemic species. While mesophotic reefs are hypothesized as relatively stable refuges from anthropogenic stressors and a potential seed bank for degraded shallow reefs, these are site-specific features, if they occur at all. Mesophotic reefs are now known to be susceptible to many of the same stressors that are degrading shallow reefs, suggesting that they require their own specific conservation and management strategies.

Phylosymbiosis and metabolomics resolve phenotypically plastic and cryptic sponge species in the genus Agelas across the Caribbean basin

Fundamental to holobiont biology is recognising how variation in microbial composition and function relates to host phenotypic variation. Sponges often exhibit considerable phenotypic plasticity and also harbour dense microbial communities that function to protect and nourish hosts. One of the most prominent sponge genera on Caribbean coral reefs is Agelas. Using a comprehensive set of morphological (growth form, spicule), chemical and molecular data on 13 recognised species of Agelas in the Caribbean basin, we were able to define only five species (=clades) and found that many morphospecies designations were incongruent with phylogenomic and population genetic analyses. Microbial communities were also strongly differentiated between phylogenetic species, showing little evidence of cryptic divergence and relatively low correlation with morphospecies assignment. Metagenomic analyses also showed strong correspondence to phylogenetic species, and to a lesser extent, geographical and morphological characters. Surprisingly, the variation in secondary metabolites produced by sponge holobionts was explained by geography and morphospecies assignment, in addition to phylogenetic species, and covaried significantly with a subset of microbial symbionts. Spicule characteristics were highly plastic, under greater impact from geographical location than phylogeny. Our results suggest that while phenotypic plasticity is rampant in Agelas, morphological differences within phylogenetic species affect functionally important ecological traits, including the composition of the symbiotic microbial communities and metabolomic profiles.

Depth variation in benthic community response to repeated marine heatwaves on remote Central Indian Ocean reefs

Coral reefs are increasingly impacted by climate-induced warming events. However, there is limited empirical evidence on the variation in the response of shallow coral reef communities to thermal stress across depths. Here, we assess depth-dependent changes in coral reef benthic communities following successive marine heatwaves from 2015 to 2017 across a 5-25 m depth gradient in the remote Chagos Archipelago, Central Indian Ocean. Our analyses show an overall decline in hard and soft coral cover and an increase in crustose coralline algae, sponge and reef pavement following successive marine heatwaves on the remote reef system. Our findings indicate that the changes in benthic communities in response to elevated seawater temperatures varied across depths. We found greater changes in benthic group cover at shallow depths (5-15 m) compared with deeper zones (15-25 m). The loss of hard coral cover was better predicted by initial thermal stress, while the loss of soft coral was associated with repeated thermal stress following successive warming events. Our study shows that benthic communities extending to 25 m depth were impacted by successive marine heatwaves, supporting concerns about the resilience of shallow coral reef communities to increasingly severe climate-driven warming events.

Morphological and genetic mechanisms underlying the plasticity of the coral Porites astreoides across depths in Bermuda

The widespread decline of shallow-water coral reefs has fueled interest in assessing whether mesophotic reefs can act as refugia replenishing deteriorated shallower reefs through larval exchange. Here we explore the morphological and molecular basis facilitating survival of planulae and adults of the coral Porites astreoides (Lamarck, 1816; Hexacorallia: Poritidae) along the vertical depth gradient in Bermuda. We found differences in micro-skeletal features such as bigger calyxes and coarser surface of the skeletal spines in shallow corals. Yet, tomographic reconstructions reveal an analogous mineral distribution between shallow and mesophotic adults, pointing to similar skeleton growth dynamics. Our study reveals patterns of host genetic connectivity and minimal symbiont depth-zonation across a broader depth range than previously known for this species in Bermuda. Transcriptional variations across life stages showed different regulation of metabolism and stress response functions, unraveling molecular responses to environmental conditions at different depths. Overall, these findings increase our understanding of coral acclimatory capability across broad vertical gradients, ultimately allowing better evaluation of the refugia potential of mesophotic reefs.

Does depth divide? Variable genetic connectivity patterns among shallow and mesophotic Montastraea cavernosa coral populations across the Gulf of Mexico and western Caribbean

Despite general declines in coral reef ecosystems in the tropical western Atlantic, some reefs, including mesophotic reefs (30-150 m), are hypothesized to function as coral refugia due to their relative isolation from anthropogenic stressors. Understanding the connectivity dynamics among these putative refugia and more degraded reefs is critical to develop effective management strategies that promote coral metapopulation persistence and recovery. This study presents a geographically broad assessment of shallow (<30 m) and mesophotic (>30 m) connectivity dynamics of the depth-generalist coral species Montastraea cavernosa. Over 750 coral genets were collected across the Northwest and Southern Gulf of Mexico, Florida, Cuba, and Belize, and ~5000 SNP loci were generated to quantify high-resolution genetic structure and connectivity among these populations. Generally, shallow and mesophotic populations demonstrated higher connectivity to distant populations within the same depth zone than to adjacent populations across depth zones. However, exceptions to this pattern include the Northwest Gulf of Mexico and the Florida Keys which exhibited relatively high vertical genetic connectivity. Furthermore, estimates of recent gene flow emphasize that mesophotic M. cavernosa populations are not significant sources for their local shallow counterparts, except for the Northwest Gulf of Mexico populations. Location-based differences in vertical connectivity are likely a result of diverse oceanographic and environmental conditions that may drive variation in gene flow and depth-dependent selection. These results highlight the need to evaluate connectivity dynamics and refugia potential of mesophotic coral species on a population-by-population basis and to identify stepping-stone populations that warrant incorporation in future international management approaches.

Physiological and morphological plasticity in Stylophora pistillata larvae from Eilat, Israel, to shallow and mesophotic light conditions

Mesophotic reefs have been proposed as climate change refugia but are not synonymous ecosystems with shallow reefs and remain exposed to anthropogenic impacts. Planulae from the reef-building coral Stylophora pistillata, Gulf of Aqaba, from 5- and 45-m depth were tested ex situ for capacity to settle, grow, and acclimate to reciprocal light conditions. Skeletons were scanned by phase contrast-enhanced micro-CT to study morphology. Deep planulae had reduced volume, smaller diameter on settlement, and greater algal symbiont density. Light conditions did not have significant impact on settlement or mortality rates. Photosynthetic acclimation of algal symbionts was evident within 21-35 days after settlement but growth rate and polyp development were slower for individuals translocated away from their parental origin compared to controls. Though our data reveal rapid symbiont acclimation, reduced growth rates and limited capacity for skeletal modification likely limit the potential for mesophotic larvae to settle on shallow reefs.

Sponge holobionts shift their prokaryotic communities and antimicrobial activity from shallow to lower mesophotic depths

In this study, we used 16S rRNA gene amplicon sequencing to investigate prokaryotic community composition of the Caribbean sponges Xestospongia muta and Agelas sventres from three depth ranges: < 30 m (shallow), 30–60 m (upper mesophotic), and 60–90 m (lower mesophotic). The prokaryotic community in shallow samples of X. muta was enriched in Cyanobacteria, Chloroflexota, and Crenarchaeota compared to samples from mesophotic depths, while mesophotic samples of X. muta were enriched in Acidobacteriota. For A. sventres, relative abundance of Acidobacteriota, Chloroflexota, and Gammaproteobacteria was higher in shallow samples, while Proteobacteria and Crenarchaeota were enriched in mesophotic A. sventres samples. Antimicrobial activity was evaluated by screening crude extracts of sponges against a set of Gram-positive and Gram-negative bacteria, a yeast, and an oomycete. Antibacterial activities from crude extracts of shallow sponge individuals were generally higher than observed from mesophotic individuals, that showed limited or no antibacterial activities. Conversely, the highest anti-oomycete activity was found from crude extracts of X. muta individuals from lower mesophotic depth, but without a clear pattern across the depth gradient. These results indicate that sponge-associated prokaryotic communities and the antimicrobial activity of sponges change within species across a depth gradient from shallow to mesophotic depth.

Green fluorescent protein-like pigments optimize the internal light environment in symbiotic reef building corals

Pigments homologous to the green fluorescent protein (GFP) have been proposed to fine-tune the internal light microclimate of corals, facilitating photoacclimation of photosynthetic coral symbionts (Symbiodiniaceae) to life in different reef habitats and environmental conditions. However, direct measurements of the in vivo light conditions inside the coral tissue supporting this conclusion are lacking. Here, we quantified the intra-tissue spectral light environment of corals expressing GFP-like proteins from widely different light regimes. We focus on: (1) photoconvertible red fluorescent proteins (pcRFPs), thought to enhance photosynthesis in mesophotic habitats via wavelength conversion, and (2) chromoproteins (CPs), which provide photoprotection to the symbionts in shallow water via light absorption. Optical microsensor measurements indicated that both pigment groups strongly alter the coral intra-tissue light environment. Estimates derived from light spectra measured in pcRFP-containing corals showed that fluorescence emission can contribute to >50% of orange-red light available to the photosynthetic symbionts at mesophotic depths. We further show that upregulation of pink CPs in shallow-water corals during bleaching leads to a reduction of orange light by 10–20% compared to low-CP tissue. Thus, screening by CPs has an important role in mitigating the light-enhancing effect of coral tissue scattering and skeletal reflection during bleaching. Our results provide the first experimental quantification of the importance of GFP-like proteins in fine-tuning the light microclimate of corals during photoacclimation.

Deep connections: divergence histories with gene flow in mesophotic Agaricia corals

Largely understudied, mesophotic coral ecosystems lie below shallow reefs (at >30 m depth) and comprise ecologically distinct communities. Brooding reproductive modes appear to predominate among mesophotic‐specialist corals and may limit genetic connectivity among populations. Using reduced representation genomic sequencing, we assessed spatial population genetic structure at 50 m depth in an ecologically important mesophotic‐specialist species Agaricia grahamae, among locations in the Southern Caribbean. We also tested for hybridisation with the closely related (but depth‐generalist) species Agaricia lamarcki, within their sympatric depth zone (50 m). In contrast to our expectations, no spatial genetic structure was detected between the reefs of Curaçao and Bonaire (~40 km apart) within A. grahamae. However, cryptic taxa were discovered within both taxonomic species, with those in A. lamarcki (incompletely) partitioned by depth and those in A. grahamae occurring sympatrically (at the same depth). Hybrid analyses and demographic modelling identified contemporary and historical gene flow among cryptic taxa, both within and between A. grahamae and A. lamarcki. These results (1) indicate that spatial connectivity and subsequent replenishment may be possible between islands of moderate geographic distances for A. grahamae, an ecologically important mesophotic species, (2) that cryptic taxa occur in the mesophotic zone and environmental selection along shallow to mesophotic depth gradients may drive divergence in depth‐generalists such as A. lamarcki, and (3) highlight that gene flow links taxa within this relativity diverse Caribbean genus.

A molecular census of early-life stage scleractinian corals in shallow and mesophotic zones

The decline of coral reefs has fueled interest in determining whether mesophotic reefs can shield against disturbances and help replenish deteriorated shallower reefs. In this study, we characterized spatial (horizontal and vertical) and seasonal patterns of diversity in coral recruits from Dabaisha and Guiwan reefs at Ludao, Taiwan. Concrete blocks supporting terra-cotta tiles were placed at shallow (15m) and mesophotic (40m) depths, during 2016-2018. Half of the tiles were retrieved and replaced biannually over three 6-month surveys (short-term); the remainder retrieved at the end of the 18-month (long-term) survey. 451 recruits were located using fluorescent censusing and identified by DNA barcoding. Barcoding the mitochondrial cytochrome oxidase I (COI) gene resulted in 17 molecular operational taxonomic units (MOTUs). To obtain taxonomic resolution to the generic level, Pocillopora were phylotyped using the mitochondrial open reading frame (ORF), resolving eight MOTUs. Acropora, Isopora, and Montipora recruits were identified by the nuclear PaxC intron, yielding ten MOTUs. Overall, 35 MOTUs were generated and were comprised primarily of Pocillopora and, in fewer numbers, Acropora, Isopora, Pavona, Montipora, Stylophora, among others. 40% of MOTUs recruited solely within mesophotic reefs while 20% were shared by both depth zones. MOTUs recruiting across a broad depth distribution appear consistent with the hypothesis of mesophotic reefs acting as a refuge for shallow-water coral reefs. In contrast, Acropora and Isopora MOTUs were structured across depth zones representing an exception to this hypothesis. This research provides an imperative assessment of coral recruitment in understudied mesophotic reefs and imparts insight into the refuge hypothesis.

Strong horizontal and vertical connectivity in the coral Pocillopora verrucosa from Ludao, Taiwan, a small oceanic island

Mesophotic habitats could be sheltered from natural and anthropogenic disturbances and act as reproductive refuges, providing propagules to replenish shallower populations. Molecular markers can be used as proxies evaluating the connectivity and inferring population structure and larval dispersal. This study characterizes population structure as well as horizontal and vertical genetic connectivity of the broadcasting coral Pocillopora verrucosa from Ludao, a small oceanic island off the eastern coast of Taiwan. We genotyped 75 P. verrucosa specimens from three sites (Gongguan, Dabaisha, and Guiwan) at three depth ranges (Shallow: 7-15 m, Mid-depth: 23-30 m, and Deep: 38-45 m), spanning shallow to upper mesophotic coral reefs, with eight microsatellite markers. F-statistics showed a moderate differentiation (FST = 0.106, p<0.05) between two adjacent locations (Dabaisha 23-30 and Dabaisha 38-45 m), but no differentiation elsewhere, suggesting high levels of connectivity among sites and depths. STRUCTURE analysis showed no genetic clustering among sites or depths, indicating that all Pocillopora individuals could be drawn from a single panmictic population. Simulations of recent migration assigned 30 individuals (40%) to a different location from where they were collected. Among them, 1/3 were assigned to deeper locations, 1/3 to shallower populations and 1/3 were assigned to the right depth but a different site. These results suggest high levels of vertical and horizontal connectivity, which could enhance the recovery of P. verrucosa following disturbances around Ludao, a feature that agrees with demographic studies portraying this species as an opportunistic scleractinian.

Local Adaptation in Marine Foundation Species at Microgeographic Scales

AbstractNearshore foundation species in coastal and estuarine systems (e.g., salt marsh grasses, mangroves, seagrasses, corals) drive the ecological functions of ecosystems and entire biomes by creating physical structure that alters local abiotic conditions and influences species interactions and composition. The resilience of foundation species and the ecosystem functions they provide depends on their phenotypic and genetic responses to spatial and temporal shifts in environmental conditions. In this review, we explore what is known about the causes and consequences of adaptive genetic differentiation in marine foundation species over spatial scales shorter than dispersal capabilities (i.e., microgeographic scales). We describe the strength of coupling field and laboratory experiments with population genetic techniques to illuminate patterns of local adaptation, and we illustrate this approach by using several foundation species. Among the major themes that emerge from our review include (1) adaptive differentiation of marine foundation species repeatedly evolves along vertical (i.e., elevation or depth) gradients, and (2) mating system and phenology may facilitate this differentiation. Microgeographic adaptation is an understudied mechanism potentially underpinning the resilience of many sessile marine species, and this evolutionary mechanism likely has particularly important consequences for the ecosystem functions provided by foundation species.

Environmental specialization and cryptic genetic divergence in two massive coral species from the Florida Keys Reef Tract

Broadcast-spawning coral species have wide geographical ranges spanning strong environmental gradients, but it is unclear how much spatially varying selection these gradients actually impose. Strong divergent selection might present a considerable barrier for demographic exchange between disparate reef habitats. We investigated whether the cross-shelf gradient is associated with spatially varying selection in two common coral species, Montastraea cavernosa and Siderastrea siderea, in the Florida Keys. To this end, we generated a de novo genome assembly for M. cavernosa and used 2bRAD to genotype 20 juveniles and 20 adults of both species from each of the three reef zones to identify signatures of selection occurring within a single generation. Unexpectedly, each species was found to be composed of four genetically distinct lineages, with gene flow between them still ongoing but highly reduced in 13.0%-54.7% of the genome. Each species includes two sympatric lineages that are only found in the deep (20 m) habitat, while the other lineages are found almost exclusively on the shallower reefs (3-10 m). The two "shallow" lineages of M. cavernosa are also specialized for either nearshore or offshore: comparison between adult and juvenile cohorts indicates that cross-shelf migrants are more than twice as likely to die before reaching adulthood than local recruits. S. siderea and M. cavernosa are among the most ecologically successful species on the Florida Keys Reef Tract, and this work offers important insight into the genomic background of divergent selection and environmental specialization that may in part explain their resilience and broad environmental range.

Genetic structure in the endosymbiont Breviolum 'muscatinei' is correlated with geographical location, environment and host species

Corals and cnidarians form symbioses with dinoflagellates across a wide range of habitats from the tropics to temperate zones. Notably, these partnerships create the foundation of coral reef ecosystems and are at risk of breaking down due to climate change. This symbiosis couples the fitness of the partners, where adaptations in one species can benefit the holobiont. However, the scales over which each partner can match their current-and future-environment are largely unknown. We investigated population genetic patterns of temperate anemones (Anthopleura spp.) and their endosymbiont Breviolum 'muscatinei', across an extensive geographical range to identify the spatial scales over which local adaptation is possible. Similar to previously published results, two solitary host species exhibited isolation by distance across hundreds of kilometres. However, symbionts exhibited genetic structure across multiple spatial scales, from geographical location to depth in the intertidal zone, and host species, suggesting that symbiont populations are more likely than their hosts to adaptively mitigate the impact of increasing temperatures.

Depth-dependent parental effects create invisible barriers to coral dispersal

Historically, marine populations were considered to be interconnected across large geographic regions due to the lack of apparent physical barriers to dispersal, coupled with a potentially widely dispersive pelagic larval stage. Recent studies, however, are providing increasing evidence of small-scale genetic segregation of populations across habitats and depths, separated in some cases by only a few dozen meters. Here, we performed a series of ex-situ and in-situ experiments using coral larvae of three brooding species from contrasting shallow- and deep-water reef habitats, and show that their settlement success, habitat choices, and subsequent survival are substantially influenced by parental effects in a habitat-dependent manner. Generally, larvae originating from deep-water corals, which experience less variable conditions, expressed more specific responses than shallow-water larvae, with a higher settlement success in simulated parental-habitat conditions. Survival of juvenile corals experimentally translocated to the sea was significantly lower when not at parental depths. We conclude that local adaptations and parental effects alongside larval selectivity and phenotype-environment mismatches combine to create invisible semipermeable barriers to coral dispersal and connectivity, leading to habitat-dependent population segregation.

Tom Shlesinger and Yossi Loya use ex-situ and in-situ experiments with coral larvae of three brooding species from contrasting shallow- and deep-water habitats and show that larvae originating from deep-water corals have narrower tolerances and higher habitat-specificity in simulated parental-habitat conditions. They also show that survival of juvenile corals experimentally translocated to the sea was significantly lower when not at parental depths. Together these results demonstrate that local adaptations and parental effects interact with larval selectivity and phenotype-environment mismatches to create semipermeable barriers to coral dispersal and connectivity.

Population genetic structure of the great star coral, Montastraea cavernosa, across the Cuban archipelago with comparisons between microsatellite and SNP markers

Coral reef habitats surrounding Cuba include relatively healthy, well-developed shallow and mesophotic (30–150 m) scleractinian communities at the cross-currents of the Tropical Western Atlantic (TWA). However, Cuba’s coral communities are not immune to the declines observed throughout the TWA, and there is limited information available regarding genetic connectivity, diversity, and structure among these populations. This represents an immense gap in our understanding of coral ecology and population dynamics at both local and regional scales. To address this gap, we evaluated the population genetic structure of the coral Montastraea cavernosa across eight reef sites surrounding Cuba. Colonies were genotyped using nine microsatellite markers and > 9,000 single nucleotide polymorphism (SNP) markers generated using the 2bRAD approach to assess fine-scale genetic structure across these sites. Both the microsatellite and SNP analyses identified patterns of genetic differentiation among sample populations. While the microsatellite analyses did not identify significant genetic structure across the seven shallow M. cavernosa sampling sites, the SNP analyses revealed significant pairwise population differentiation, suggesting that differentiation is greater between eastern and western sites. This study provides insight into methodological differences between microsatellite and SNP markers including potential trade-offs between marker-specific biases, sample size, sequencing costs, and the ability to resolve subtle patterns of population genetic structure. Furthermore, this study suggests that locations in western Cuba may play important roles in this species’ regional metapopulation dynamics and therefore may merit incorporation into developing international management efforts in addition to the local management the sites receive.

Fine-scale structure among mesophotic populations of the great star coral Montastraea cavernosa revealed by SNP genotyping

Mesophotic reefs (30-150 m) have been proposed as potential refugia that facilitate the recovery of degraded shallow reefs following acute disturbances such as coral bleaching and disease. However, because of the technical difficulty of collecting samples, the connectivity of adjacent mesophotic reefs is relatively unknown compared with shallower counterparts. We used genotyping by sequencing to assess fine-scale genetic structure of Montastraea cavernosa at two sites at Pulley Ridge, a mesophotic coral reef ecosystem in the Gulf of Mexico, and downstream sites along the Florida Reef Tract. We found differentiation between reefs at Pulley Ridge (~68 m) and corals at downstream upper mesophotic depths in the Dry Tortugas (28-36 m) and shallow reefs in the northern Florida Keys (Key Biscayne, ~5 m). The spatial endpoints of our study were distinct, with the Dry Tortugas as a genetic intermediate. Most striking were differences in population structure among northern and southern sites at Pulley Ridge that were separated by just 12km. Unique patterns of clonality and outlier loci allele frequency support these sites as different populations and suggest that the long-distance horizontal connectivity typical of shallow-water corals may not be typical for mesophotic systems in Florida and the Gulf of Mexico. We hypothesize that this may be due to the spawning of buoyant gametes, which commits propagules to the surface, resulting in greater dispersal and lower connectivity than typically found between nearby shallow sites. Differences in population structure over small spatial scales suggest that demographic constraints and/or environmental disturbances may be more variable in space and time on mesophotic reefs compared with their shallow-water counterparts.

Depth Influences Symbiodiniaceae Associations Among Montastraea cavernosa Corals on the Belize Barrier Reef

In Belize, shallow populations (10 and 16 m) of the coral species Montastraea cavernosa from the back reef and reef crest are genetically differentiated from deeper populations on the fore reef and reef wall (25 and 35 m). Like many species of scleractinian corals, M. cavernosa has an obligate symbiosis with dinoflagellate microalgae from the family Symbiodiniaceae. Here, we describe the Symbiodiniaceae taxa found within previously sampled and genotyped M. cavernosa populations along a depth gradient on the Belize Barrier Reef by implementing high-throughput sequencing of the ITS2 region of Symbiodiniaceae ribosomal DNA and the SymPortal analysis framework. While Symbiodiniaceae ITS2 type profiles across all sampling depths were almost entirely (99.99%) from the genus Cladocopium (formerly Symbiodinium Clade C), shallow (10 and 16 m) populations had a greater diversity of ITS2 type profiles in comparison to deeper (25 and 35 m) populations. Permutational multivariate analysis of variance (PERMANOVA) confirmed significant differences in ITS2 type profiles between shallow and deep sample populations. Overall Symbiodiniaceae communities changed significantly with depth, following patterns similar to the coral host's population genetic structure. Though physiological differences among species in the cosmopolitan genus Cladocopium are not well-described, our results suggest that although some members of Cladocopium are depth-generalists, shallow M. cavernosa populations in Belize may harbor shallow-specialized Symbiodiniaceae not found in deeper populations.

Considerations for maximizing the adaptive potential of restored coral populations in the western Atlantic

Active coral restoration typically involves two interventions: crossing gametes to facilitate sexual larval propagation; and fragmenting, growing, and outplanting adult colonies to enhance asexual propagation. From an evolutionary perspective, the goal of these efforts is to establish self-sustaining, sexually reproducing coral populations that have sufficient genetic and phenotypic variation to adapt to changing environments. Here, we provide concrete guidelines to help restoration practitioners meet this goal for most Caribbean species of interest. To enable the persistence of coral populations exposed to severe selection pressure from many stressors, a mixed provenance strategy is suggested: genetically unique colonies (genets) should be sourced both locally as well as from more distant, environmentally distinct sites. Sourcing three to four genets per reef along environmental gradients should be sufficient to capture a majority of intraspecies genetic diversity. It is best for practitioners to propagate genets with one or more phenotypic traits that are predicted to be valuable in the future, such as low partial mortality, high wound healing rate, high skeletal growth rate, bleaching resilience, infectious disease resilience, and high sexual reproductive output. Some effort should also be reserved for underperforming genets because colonies that grow poorly in nurseries sometimes thrive once returned to the reef and may harbor genetic variants with as yet unrecognized value. Outplants should be clustered in groups of four to six genets to enable successful fertilization upon maturation. Current evidence indicates that translocating genets among distant reefs is unlikely to be problematic from a population genetic perspective but will likely provide substantial adaptive benefits. Similarly, inbreeding depression is not a concern given that current practices only raise first-generation offspring. Thus, proceeding with the proposed management strategies even in the absence of a detailed population genetic analysis of the focal species at sites targeted for restoration is the best course of action. These basic guidelines should help maximize the adaptive potential of reef-building corals facing a rapidly changing environment.

Populations of the coral species Montastraea cavernosa on the Belize Barrier Reef lack vertical connectivity

Larval connectivity among and within coral reefs is important for sustaining coral metapopulations, enhancing ecosystem resilience through species and genetic diversity, and maintaining reef ecosystems’ structure and functions. This study characterized genetic structure and assessed horizontal and vertical connectivity among populations of the ubiquitous gonochoric broadcast spawning coral Montastraea cavernosa in Belize. Using nine polymorphic microsatellite loci, we genotyped M. cavernosa colonies from four depth zones at four study sites within Belizean marine management zones. Study sites were selected within South Water Caye Marine Reserve (3 sites) and Glover’s Reef Marine Reserve (1 site). Strong contemporary genetic differentiation was observed between relatively shallow M. cavernosa populations (10 m, 16 m) and relatively deep (25 m, 35 m) populations, coinciding with a transition from reef crest to reef slope. These results were consistent across both marine reserves. Vertical and horizontal migration models suggest that all populations were historically panmictic, with little unidirectional migration. The relative local isolation of shallow and mesophotic M. cavernosa populations in Belize, coupled with the importance of Belize’s upper mesophotic populations as potential larval sources for other areas in the Tropical Western Atlantic, reinforces the need for management strategies that conserve coral populations across all depth zones.

Montastraea cavernosa corallite structure demonstrates distinct morphotypes across shallow and mesophotic depth zones in the Gulf of Mexico

This study assessed morphological variation of the depth-generalist coral Montastraea cavernosa across shallow and mesophotic coral ecosystems in the Gulf of Mexico (GOM) using thirteen corallite metrics. While corallite structure differed significantly across sites, we observed that mean corallite diameters were smaller and spacing was greater in mesophotic corals as compared to shallow corals. Additional corallite variation, including greater mean corallite height of mesophotic samples, are hypothesized to be photoadaptive responses to low light environments. Multivariate analyses also revealed two distinct morphotypes identified by significant variation in corallite spacing with >90% accuracy. A 'shallow' morphotype was characterized by larger, more closely-spaced corallites, while a 'depth-generalist' type exhibited smaller, further-spaced corallites. Variable presence of morphotypes within some sites suggests genotypic influence on corallite morphology as there was a slight, but significant, impact of morphotype on genetic structure within shallow zones in the Flower Garden Banks. Patterns of increased algal symbiont (Symbiodiniaceae) density and chlorophyll concentration were retained in the depth-generalist morphotype even in shallow zones, identifying multiple photoadaptive strategies between morphotypes. The results of this study suggest that morphological variation among M. cavernosa represents a combination of genotypic variation and phenotypic plasticity rather than responses to environmental stimuli alone.

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.

Can mesophotic reefs replenish shallow reefs? Reduced coral reproductive performance casts a doubt

Mesophotic coral ecosystems (i.e., deep coral reefs at 30-120 m depth) appear to be thriving while many shallow reefs in the world are declining. Amid efforts to understand and manage their decline, it was suggested that mesophotic reefs might serve as natural refuges and a possible source of propagules for the shallow reefs. However, our knowledge of how reproductive performance of corals alters with depth is sparse. Here, we present a comprehensive study of the reproductive phenology, fecundity, and abundance of seven reef-building conspecific corals in shallow and mesophotic habitats. Significant differences were found in the synchrony and timing of gametogenesis and spawning between shallow and mesophotic coral populations. Thus, mesophotic populations exhibited delayed or protracted spawning events, which led to spawning of the mesophotic colonies in large proportions at times where the shallow ones had long been depleted of reproductive material. All species investigated demonstrated a substantial reduction in fecundity and/or oocyte sizes at mesophotic depths (40-60 m). Two species (Seriatopora hystrix and Galaxea fascicularis) displayed a reduction in both fecundity and oocyte size at mesophotic depths. Turbinaria reniformis had only reduced fecundity and Acropora squarrosa and Acropora valida only reduced oocyte size. In Montipora verrucosa, reduced fecundity was found during one annual reproductive season while, in the following year, only reduced oocyte size was found. In contrast, reduced oocyte size in mesophotic populations of Acropora squarrosa was consistent along three studied years. One species, Acropora pharaonis, was found to be infertile at mesophotic depths along two studied years. This indicates that reproductive performance decreases with depth; and that although some species are capable of reproducing at mesophotic depths, their contribution to the replenishment of shallow reefs may be inconsequential. Reduced reproductive performance with depth, combined with the possible narrower tolerance to environmental factors, further suggests that mesophotic corals may in fact be more vulnerable than previously conceived. Furthermore, we posit that the observed temporal segregation in reproduction could lead to assortative mating, and this, in turn, may facilitate adaptive divergence across depth.

The unusual case of the widely distributed fiddler crab Minuca rapax (Smith, 1870) from the western Atlantic: an exemplary polytypic species

A classic dilemma in taxonomy is distinguishing intraspecific from interspecific variation. In order to better comprehend the process of divergence and speciation, we examine morphological, genetic, developmental and behavioural variation among related fiddler crab populations from eastern North America, the Caribbean and South America. We chose geographically remote populations that appear related to Minuca rapax (Smith, 1870). First, using females from across the range of the species, we use geometric morphometric techniques to identify regional differences in carapace shape. Second, in the northern portion of the range, the Caribbean into the Gulf of Mexico, we report variation in the relationship between corporal size and cheliped length in males. Third, we examine the major components of the courtship waves produced by males from several locations in the western Gulf of Mexico. Fourth, we compare the structure of the gastric mill between different populations in the Gulf of Mexico, the Caribbean and the Atlantic Ocean. And, fifth, we use mitochondrial 16S rDNA and cytochrome oxidase subunit I as genetic markers to define the phylogeographic relationship among specimens from more than 20 populations. From these studies, we find discrete, distinct populations across the original range of the species. In particular, populations in the northern Gulf of Mexico appear to represent a lineage that has resulted from limited gene flow and sustained selection pressures. On the basis of the observed degree of divergence, it is apparent that some separated populations in M. rapax should be recognised as evolutionary significant units. The geographic range of these populations is consistent with the historical range for Minuca virens (Salmon &amp; Atsaides, 1968), a putative species that otherwise cannot be consistently distinguished from M. rapax based on discrete external morphological characters. This study provides evidence for M. virens as an emergent but possibly not completely isolated subclade of the M. rapax species complex.

Comparative Genomics of Color Morphs In the Coral Montastraea cavernosa

Montastraea cavernosa is a common coral in the Caribbean basin found in several color morphs. To investigate the causes for brown and orange morphs we undertook a genomics approach on corals collected at the same time and depth in the Bahamas. The coral holobiont includes the host, symbiotic dinoflagellates (Symbiodinium spp.), and a diverse microbiome. While the coral host showed significant genetic differentiation between color morphs both the composition of the Symbiodinium spp. communities and the prokaryotic communities did not. Both targeted and global gene expression differences in the transcriptome of the host show no difference in fluorescent proteins while the metatranscriptome of the microbiome shows that pigments such as phycoerythrin and orange carotenoid protein of cyanobacterial origin are significantly greater in orange morphs, which is also consistent with the significantly greater number of cyanobacteria quantified by 16S rRNA reads and flow cytometry. The microbiome of orange color morphs expressed significantly more nitrogenase (nifH) transcripts consistent with their known ability to fix nitrogen. Both coral and Symbiodinium spp. transcriptomes from orange morphs had significantly increased expression of genes related to immune response and apoptosis, which may potentially be involved in maintaining and regulating the unique symbiont population in orange morphs.

Boxer crabs induce asexual reproduction of their associated sea anemones by splitting and intraspecific theft

Crabs of the genus Lybia have the remarkable habit of holding a sea anemone in each of their claws. This partnership appears to be obligate, at least on the part of the crab. The present study focuses on Lybia leptochelis from the Red Sea holding anemones of the genus Alicia (family Aliciidae). These anemones have not been found free living, only in association with L. leptochelis. In an attempt to understand how the crabs acquire them, we conducted a series of behavioral experiments and molecular analyses. Laboratory observations showed that the removal of one anemone from a crab induces a “splitting” behavior, whereby the crab tears the remaining anemone into two similar parts, resulting in a complete anemone in each claw after regeneration. Furthermore, when two crabs, one holding anemones and one lacking them, are confronted, the crabs fight, almost always leading to the “theft” of a complete anemone or anemone fragment by the crab without them. Following this, crabs “split” their lone anemone into two. Individuals of Alicia sp. removed from freshly collected L. leptochelis were used for DNA analysis. By employing AFLP (Fluorescence Amplified Fragments Length Polymorphism) it was shown that each pair of anemones from a given crab is genetically identical. Furthermore, there is genetic identity between most pairs of anemone held by different crabs, with the others showing slight genetic differences. This is a unique case in which one animal induces asexual reproduction of another, consequently also affecting its genetic diversity.

Deep reefs are not universal refuges: Reseeding potential varies among coral species

Deep coral reefs (that is, mesophotic coral ecosystems) can act as refuges against major disturbances affecting shallow reefs. It has been proposed that, through the provision of coral propagules, such deep refuges may aid in shallow reef recovery; however, this "reseeding" hypothesis remains largely untested. We conducted a genome-wide assessment of two scleractinian coral species with contrasting reproductive modes, to assess the potential for connectivity between mesophotic (40 m) and shallow (12 m) depths on an isolated reef system in the Western Atlantic (Bermuda). To overcome the pervasive issue of endosymbiont contamination associated with de novo sequencing of corals, we used a novel subtraction reference approach. We have demonstrated that strong depth-associated selection has led to genome-wide divergence in the brooding species Agaricia fragilis (with divergence by depth exceeding divergence by location). Despite introgression from shallow into deep populations, a lack of first-generation migrants indicates that effective connectivity over ecological time scales is extremely limited for this species and thus precludes reseeding of shallow reefs from deep refuges. In contrast, no genetic structuring between depths (or locations) was observed for the broadcasting species Stephanocoenia intersepta, indicating substantial potential for vertical connectivity. Our findings demonstrate that vertical connectivity within the same reef system can differ greatly between species and that the reseeding potential of deep reefs in Bermuda may apply to only a small number of scleractinian species. Overall, we argue that the "deep reef refuge hypothesis" holds for individual coral species during episodic disturbances but should not be assumed as a broader ecosystem-wide phenomenon.

Contrasting Patterns of Clinal Genetic Diversity and Potential Colonization Pathways in Two Species of Western Atlantic Fiddler Crabs

Fiddler crabs (Brachyura, Ocypodidae), like many other marine organisms, disperse via planktonic larvae. A lengthy pelagic larval duration is generally assumed to result in genetic connectivity even among distant populations. However, major river outflows, such as of the Amazon or Orinoco, or strong currents may act as phylogeographic barriers to ongoing gene flow. For example, the Mona Passage, located between Puerto Rico and Hispaniola, has been postulated to impair larval exchange of several species. In this study, Cox1 mtDNA data was used to analyze population genetic structure of two fiddler crab species from the western Atlantic, comparing the continental coastline and Caribbean islands. The results indicate genetic homogeneity in Minuca rapax among Atlantic (continental) populations (Suriname, Brazil), whereas Caribbean populations show significantly restricted gene flow among the constituent islands and towards continental populations. Our data support the hypothesis of the Mona Passage hindering larval exchange. Contrastingly, Caribbean Leptuca leptodactyla populations appear to be devoid of detectable variation, while Atlantic-continental (i.e. Brazilian) populations show much higher haplotype and nucleotide diversities and display slight genetic differentiation among populations within the Atlantic region, though not statistically significant. Both species show a pronounced divergence between regions, supporting the presence of a phylogeographic barrier.

Structure of Mesophotic Reef Fish Assemblages in the Northwestern Hawaiian Islands

Mesophotic coral ecosystems (MCEs) support diverse communities of marine organisms with changes in community structure occurring along a depth gradient. In recent years, MCEs have gained attention due to their depths that provide protection from natural and anthropogenic stressors and their relative stability over evolutionary time periods, yet ecological structures of fish assemblages in MCEs remain largely un-documented. Here, we investigated composition and trophic structure of reef fish assemblages in the Northwestern Hawaiian Islands (NWHI) along a depth gradient from 1 to 67 m. The structure of reef fish assemblages as a whole showed a clear gradient from shallow to mesophotic depths. Fish assemblages at mesophotic depths had higher total densities than those in shallower waters, and were characterized by relatively high densities of planktivores and invertivores and relatively low densities of herbivores. Fishes that typified assemblages at mesophotic depths included six species that are endemic to the Hawaiian Islands. The present study showed that mesophotic reefs in the NWHI support unique assemblages of fish that are characterized by high endemism and relatively high densities of planktivores. Our findings underscore the ecological importance of these undersurveyed ecosystems and warrant further studies of MCEs.

Population Genetic Structure, Abundance, and Health Status of Two Dominant Benthic Species in the Saba Bank National Park, Caribbean Netherlands: Montastraea cavernosa and Xestospongia muta

Saba Bank, a submerged atoll in the Caribbean Sea with an area of 2,200 km², has attained international conservation status due to the rich diversity of species that reside on the bank. In order to assess the role of Saba Bank as a potential reservoir of diversity for the surrounding reefs, we examined the population genetic structure, abundance and health status of two prominent benthic species, the coral Montastraea cavernosa and the sponge Xestospongia muta. Sequence data were collected from 34 colonies of M. cavernosa (nDNA ITS1-5.8S-ITS2; 892 bp) and 68 X. muta sponges (mtDNA I3-M11 partition of COI; 544 bp) on Saba Bank and around Saba Island, and compared with published data across the wider Caribbean. Our data indicate that there is genetic connectivity between populations on Saba Bank and the nearby Saba Island as well as multiple locations in the wider Caribbean, ranging in distance from 100s–1000s km. The genetic diversity of Saba Bank populations of M. cavernosa (π = 0.055) and X. muta (π = 0.0010) was comparable to those in other regions in the western Atlantic. Densities and health status were determined along 11 transects of 50 m² along the south-eastern rim of Saba Bank. The densities of M. cavernosa (0.27 ind. m^-2, 95% CI: 0.12–0.52) were average, while the densities of X. muta (0.09 ind. m^-2, 95% CI: 0.02–0.32) were generally higher with respect to other Caribbean locations. No disease or bleaching was present in any of the specimens of the coral M. cavernosa, however, we did observe partial tissue loss (77.9% of samples) as well as overgrowth (48.1%), predominantly by cyanobacteria. In contrast, the majority of observed X. muta (83.5%) showed signs of presumed bleaching. The combined results of apparent gene flow among populations on Saba Bank and surrounding reefs, the high abundance and unique genetic diversity, indicate that Saba Bank could function as an important buffer for the region. Either as a natural source of larvae to replenish genetic diversity or as a storehouse of diversity that can be utilized if needed for restoration practices.

In four shallow and mesophotic tropical reef sponges from Guam the microbial community largely depends on host identity

Sponges (phylum Porifera) are important members of almost all aquatic ecosystems, and are renowned for hosting often dense and diverse microbial communities. While the specificity of the sponge microbiota seems to be closely related to host phylogeny, the environmental factors that could shape differences within local sponge-specific communities remain less understood. On tropical coral reefs, sponge habitats can span from shallow areas to deeper, mesophotic sites. These habitats differ in terms of environmental factors such as light, temperature, and food availability, as well as anthropogenic impact. In order to study the host specificity and potential influence of varying habitats on the sponge microbiota within a local area, four tropical reef sponges, Rhabdastrella globostellata, Callyspongia sp., Rhaphoxya sp., and Acanthella cavernosa, were collected from exposed shallow reef slopes and a deep reef drop-off. Based on 16S rRNA gene pyrosequencing profiles, beta diversity analyses revealed that each sponge species possessed a specific microbiota that was significantly different to those of the other species and exhibited attributes that are characteristic of high- and/or low-microbial-abundance sponges. These findings emphasize the influence of host identity on the associated microbiota. Dominant sponge- and seawater-associated bacterial phyla were Chloroflexi, Cyanobacteria, and Proteobacteria. Comparison of individual sponge taxa and seawater samples between shallow and deep reef sites revealed no significant variation in alpha diversity estimates, while differences in microbial beta diversity (variation in community composition) were significant for Callyspongia sp. sponges and seawater samples. Overall, the sponge-associated microbiota is significantly shaped by host identity across all samples, while the effect of habitat differentiation seems to be less predominant in tropical reef sponges.

Long distance dispersal and vertical gene flow in the Caribbean brooding coral Porites astreoides

To date, most assessments of coral connectivity have emphasized long-distance horizontal dispersal of propagules from one shallow reef to another. The extent of vertical connectivity, however, remains largely understudied. Here, we used newly-developed and existing DNA microsatellite loci for the brooding coral Porites astreoides to assess patterns of horizontal and vertical connectivity in 590 colonies collected from three depth zones (≤10 m, 15-20 m and ≥25 m) at sites in Florida, Bermuda and the U.S. Virgin Islands (USVI). We also tested whether maternal transmission of algal symbionts (Symbiodinium spp.) might limit effective vertical connectivity. Overall, shallow P. astreoides exhibited high gene flow between Florida and USVI, but limited gene flow between these locations and Bermuda. In contrast, there was significant genetic differentiation by depth in Florida (Upper Keys, Lower Keys and Dry Tortugas), but not in Bermuda or USVI, despite strong patterns of depth zonation in algal symbionts at two of these locations. Together, these findings suggest that P. astreoides is effective at dispersing both horizontally and vertically despite its brooding reproductive mode and maternal transmission of algal symbionts. In addition, these findings might help explain the ecological success reported for P. astreoides in the Caribbean in recent decades.

Environmental drivers of microbial community shifts in the giant barrel sponge, Xestospongia muta, over a shallow to mesophotic depth gradient

The giant barrel sponge, Xestospongia muta, is a high microbial abundance sponge found on Caribbean coral reefs along shallow to mesophotic depth gradients where multiple abiotic factors change with depth. Sponges were collected along a depth gradient at Little Cayman (LC) and Lee Stocking Island (LSI), and the microbiome of these samples was analysed using 16S rRNA amplicon sequencing. Statistically significant shifts in community structure and dissimilarity (∼ 40%) were detected from 10 to 90 m in LC sponges, but a similar shift was not identified in sponges from 10 to 60 m at LSI (only 17% dissimilar). Additionally, inorganic nutrient levels steadily increased with depth at LSI but not at LC. Based on bulk stable isotopic variability, sponges collected from LC were generally more enriched in (15) N and less enriched in (13) C as depth increased, suggesting a transition from dependency on photoautotrophy to heterotrophy as depth increased. Patterns of stable isotopic enrichment were largely invariant at LSI, which is also reflected in the more stable microbial community along the depth gradient. It appears that environmental factors that change with depth may contribute to differences in X. muta microbial assemblages, demonstrating the importance of contemporaneous environmental sampling in studies of the microbiome of sponges.

Population Structure of Montastraea cavernosa on Shallow versus Mesophotic Reefs in Bermuda

Mesophotic coral reef ecosystems remain largely unexplored with only limited information available on taxonomic composition, abundance and distribution. Yet, mesophotic reefs may serve as potential refugia for shallow-water species and thus understanding biodiversity, ecology and connectivity of deep reef communities is integral for resource management and conservation. The Caribbean coral, Montastraea cavernosa, is considered a depth generalist and is commonly found at mesophotic depths. We surveyed abundance and size-frequency of M. cavernosa populations at six shallow (10m) and six upper mesophotic (45m) sites in Bermuda and found population structure was depth dependent. The mean surface area of colonies at mesophotic sites was significantly smaller than at shallow sites, suggesting that growth rates and maximum colony surface area are limited on mesophotic reefs. Colony density was significantly higher at mesophotic sites, however, resulting in equal contributions to overall percent cover. Size-frequency distributions between shallow and mesophotic sites were also significantly different with populations at mesophotic reefs skewed towards smaller individuals. Overall, the results of this study provide valuable baseline data on population structure, which indicate that the mesophotic reefs of Bermuda support an established population of M. cavernosa.

Baseline Assessment of Mesophotic Reefs of the Vitória-Trindade Seamount Chain Based on Water Quality, Microbial Diversity, Benthic Cover and Fish Biomass Data

Seamounts are considered important sources of biodiversity and minerals. However, their biodiversity and health status are not well understood; therefore, potential conservation problems are unknown. The mesophotic reefs of the Vitória-Trindade Seamount Chain (VTC) were investigated via benthic community and fish surveys, metagenomic and water chemistry analyses, and water microbial abundance estimations. The VTC is a mosaic of reef systems and includes fleshy algae dominated rhodolith beds, crustose coralline algae (CCA) reefs, and turf algae dominated rocky reefs of varying health levels. Macro-carnivores and larger fish presented higher biomass at the CCA reefs (4.4 kg per frame) than in the rhodolith beds and rocky reefs (0.0 to 0.1 kg per frame). A larger number of metagenomic sequences identified as primary producers (e.g., Chlorophyta and Streptophyta) were found at the CCA reefs. However, the rocky reefs contained more diseased corals (>90%) than the CCA reefs (~40%) and rhodolith beds (~10%). Metagenomic analyses indicated a heterotrophic and fast-growing microbiome in rocky reef corals that may possibly lead to unhealthy conditions possibly enhanced by environmental features (e.g. light stress and high loads of labile dissolved organic carbon). VTC mounts represent important hotspots of biodiversity that deserve further conservation actions.

Potential for adaptive evolution at species range margins: contrasting interactions between red coral populations and their environment in a changing ocean

Studying population-by-environment interactions (PEIs) at species range margins offers the opportunity to characterize the responses of populations facing an extreme regime of selection, as expected due to global change. Nevertheless, the importance of these marginal populations as putative reservoirs of adaptive genetic variation has scarcely been considered in conservation biology. This is particularly true in marine ecosystems for which the deep refugia hypothesis proposes that disturbed shallow and marginal populations of a given species can be replenished by mesophotic ones. This hypothesis therefore assumes that identical PEIs exist between populations, neglecting the potential for adaptation at species range margins. Here, we combine reciprocal transplant and common garden experiments with population genetics analyses to decipher the PEIs in the red coral, Corallium rubrum. Our analyses reveal partially contrasting PEIs between shallow and mesophotic populations separated by approximately one hundred meters, suggesting that red coral populations may potentially be locally adapted to their environment. Based on the effective population size and connectivity analyses, we posit that genetic drift may be more important than gene flow in the adaptation of the red coral. We further investigate how adaptive divergence could impact population viability in the context of warming and demonstrate differential phenotypic buffering capacities against thermal stress. Our study questions the relevance of the deep refugia hypothesis and highlights the conservation value of marginal populations as a putative reservoir of adaptive genetic polymorphism.

Prevalent endosymbiont zonation shapes the depth distributions of scleractinian coral species

Bathymetric distributions of photosynthetic marine invertebrate species are relatively well studied, however the importance of symbiont zonation (i.e. hosting of distinct algal endosymbiont communities over depth) in determining these depth distributions still remains unclear. Here, we assess the prevalence of symbiont zonation in tropical scleractinian corals by genotyping the Symbiodinium of the 25 most common species over a large depth range (down to 60 m) on a Caribbean reef. Symbiont depth zonation was found to be common on a reef-wide scale (11 out of 25 coral species), and a dominant feature in species with the widest depth distributions. With regards to reproductive strategy, symbiont zonation was more common in broadcasting species, which also exhibited a higher level of polymorphism in the symbiont zonation (i.e. number of different Symbiodinium profiles involved). Species with symbiont zonation exhibited significantly broader depth distributions than those without, highlighting the role of symbiont zonation in shaping the vertical distributions of the coral host. Overall, the results demonstrate that coral reefs can consist of highly structured communities over depth when considering both the coral host and their obligate photosymbionts, which probably has strong implications for the extent of connectivity between shallow and mesophotic habitats.

Vertical and horizontal genetic connectivity in Chromis verater, an endemic damselfish found on shallow and mesophotic reefs in the Hawaiian Archipelago and adjacent Johnston Atoll

Understanding vertical and horizontal connectivity is a major priority in research on mesophotic coral ecosystems (30-150 m). However, horizontal connectivity has been the focus of few studies, and data on vertical connectivity are limited to sessile benthic mesophotic organisms. Here we present patterns of vertical and horizontal connectivity in the Hawaiian Islands-Johnston Atoll endemic threespot damselfish, Chromis verater, based on 319 shallow specimens and 153 deep specimens. The mtDNA markers cytochrome b and control region were sequenced to analyze genetic structure: 1) between shallow (< 30 m) and mesophotic (30-150 m) populations and 2) across the species' geographic range. Additionally, the nuclear markers rhodopsin and internal transcribed spacer 2 of ribosomal DNA were sequenced to assess connectivity between shallow and mesophotic populations. There was no significant genetic differentiation by depth, indicating high levels of vertical connectivity between shallow and deep aggregates of C. verater. Consequently, shallow and deep samples were combined by location for analyses of horizontal connectivity. We detected low but significant population structure across the Hawaiian Archipelago (overall cytochrome b: ΦST = 0.009, P = 0.020; control region: ΦST = 0.012, P = 0.009) and a larger break between the archipelago and Johnston Atoll (cytochrome b: ΦST = 0.068, P < 0.001; control region: ΦST = 0.116, P < 0.001). The population structure within the archipelago was driven by samples from the island of Hawaii at the southeast end of the chain and Lisianski in the middle of the archipelago. The lack of vertical genetic structure supports the refugia hypothesis that deep reefs may constitute a population reservoir for species depleted in shallow reef habitats. These findings represent the first connectivity study on a mobile organism that spans shallow and mesophotic depths and provide a reference point for future connectivity studies on mesophotic fishes.

Allelopathy in the tropical alga Lobophora variegata (Phaeophyceae): mechanistic basis for a phase shift on mesophotic coral reefs?

Macroalgal phase shifts on Caribbean reefs have been reported with increasing frequency, and recent reports of these changes on mesophotic coral reefs have raised questions regarding the mechanistic processes behind algal population expansions to deeper depths. The brown alga Lobophora variegata is a dominant species on many shallow and deep coral reefs of the Caribbean and Pacific, and it increased in percent cover (>50%) up to 61 m on Bahamian reefs following the invasion of the lionfish Pterois volitans. We examined the physiological and ecological constraints contributing to the spread of Lobophora on Bahamian reefs across a mesophotic depth gradient from 30 to 61 m, pre- and post-lionfish invasion. Results indicate that there were no physiological limitations to the depth distribution of Lobophora within this range prior to the lionfish invasion. Herbivory by acanthurids and scarids in algal recruitment plots at mesophotic depths was higher prior to the lionfish invasion, and Lobophora chemical defenses were ineffective against an omnivorous fish species. In contrast, Lobophora exhibited significant allelopathic activity against the coral Montastraea cavernosa and the sponge Agelas clathrodes in laboratory assays. These data indicate that when lionfish predation on herbivorous fish released Lobophora from grazing pressure at depth, Lobophora expanded its benthic cover to a depth of 61 m, where it replaced the dominant coral and sponge species. Our results suggest that this chemically defended alga may out-compete these species in situ, and that mesophotic reefs may be further impacted in the near future as Lobophora continues to expand to its compensation point.

Depth-dependent mortality of reef corals following a severe bleaching event: implications for thermal refuges and population recovery

Coral bleaching caused by rising sea temperature is a primary cause of coral reef degradation. However, bleaching patterns often show significant spatial variability, therefore identifying locations where local conditions may provide thermal refuges is a high conservation priority. Coral bleaching mortality often diminishes with increasing depth, but clear depth zonation of coral communities and putative limited overlap in species composition between deep and shallow reef habitats has led to the conclusion that deeper reef habitats will provide limited refuge from bleaching for most species. Here, we show that coral mortality following a severe bleaching event diminished sharply with depth. Bleaching-induced mortality of Acropora was approximately 90% at 0-2m, 60% at 3-4 m, yet at 6-8m there was negligible mortality. Importantly, at least two-thirds of the shallow-water (2-3 m) Acropora assemblage had a depth range that straddled the transition from high to low mortality. Cold-water upwelling may have contributed to the lower mortality observed in all but the shallowest depths. Our results demonstrate that, in this instance, depth provided a refuge for individuals from a high proportion of species in this Acropora-dominated assemblage. The persistence of deeper populations may provide a critical source of propagules to assist recovery of adjacent shallow-water reefs.

Depth-dependent mortality of reef corals following a severe bleaching event: implications for thermal refuges and population recovery

Coral bleaching caused by rising sea temperature is a primary cause of coral reef degradation. However, bleaching patterns often show significant spatial variability, therefore identifying locations where local conditions may provide thermal refuges is a high conservation priority. Coral bleaching mortality often diminishes with increasing depth, but clear depth zonation of coral communities and putative limited overlap in species composition between deep and shallow reef habitats has led to the conclusion that deeper reef habitats will provide limited refuge from bleaching for most species. Here, we show that coral mortality following a severe bleaching event diminished sharply with depth. Bleaching-induced mortality of Acropora was approximately 90% at 0-2m, 60% at 3-4 m, yet at 6-8m there was negligible mortality. Importantly, at least two-thirds of the shallow-water (2-3 m) Acropora assemblage had a depth range that straddled the transition from high to low mortality. Cold-water upwelling may have contributed to the lower mortality observed in all but the shallowest depths. Our results demonstrate that, in this instance, depth provided a refuge for individuals from a high proportion of species in this Acropora-dominated assemblage. The persistence of deeper populations may provide a critical source of propagules to assist recovery of adjacent shallow-water reefs.

Depth-dependent mortality of reef corals following a severe bleaching event: implications for thermal refuges and population recovery

Coral bleaching caused by rising sea temperature is a primary cause of coral reef degradation. However, bleaching patterns often show significant spatial variability, therefore identifying locations where local conditions may provide thermal refuges is a high conservation priority. Coral bleaching mortality often diminishes with increasing depth, but clear depth zonation of coral communities and putative limited overlap in species composition between deep and shallow reef habitats has led to the conclusion that deeper reef habitats will provide limited refuge from bleaching for most species. Here, we show that coral mortality following a severe bleaching event diminished sharply with depth. Bleaching-induced mortality of Acropora was approximately 90% at 0-2m, 60% at 3-4 m, yet at 6-8m there was negligible mortality. Importantly, at least two-thirds of the shallow-water (2-3 m) Acropora assemblage had a depth range that straddled the transition from high to low mortality. Cold-water upwelling may have contributed to the lower mortality observed in all but the shallowest depths. Our results demonstrate that, in this instance, depth provided a refuge for individuals from a high proportion of species in this Acropora-dominated assemblage. The persistence of deeper populations may provide a critical source of propagules to assist recovery of adjacent shallow-water reefs.