Strength of Sexual Selection and Sex Roles Vary between Social Groups in a Coral Reef Cardinalfish.

AbstractThe strength and direction of sexual selection can vary among populations. However, spatial variability is rarely explored at the level of the social group. Here we investigate sexual selection and sex roles in the paternally mouthbrooding, socially monogamous, and site-attached pajama cardinalfish, Sphaeramia nematoptera. Females were larger and more aggressive and had a longer dorsal fin filament, indicating reversed sex roles. At the scale of social groups, we show that the Bateman gradient and reproductive variance depend on the sex ratio and size of groups. In small and medium-sized groups with balanced or male-biased sex ratios, Bateman gradients were steeper for females, whereas gradients were equally steep for both sexes in large groups or when the sex ratio was female biased. For both sexes, reproductive variance increased with group size and with a higher male-to-female sex ratio. In S. nematoptera, mating opportunities outside the socially monogamous pair appear to impact sexual selection. We conclude that strength and direction of sexual selection can be masked by social dynamics in group-living species when considering only population and large-scale demographic processes.

Symbiodiniaceae diversity varies by host and environment across thermally distinct reefs.

Endosymbiotic dinoflagellates (Symbiodiniaceae) influence coral thermal tolerance at both local and regional scales. In isolation, the effects of host genetics, environment, and thermal disturbances on symbiont communities are well understood, yet their combined effects remain poorly resolved. Here, we investigate Symbiodiniaceae across 1300 km in Australia's Coral Sea Marine Park to disentangle these interactive effects. We identified Symbiodiniaceae to species-level resolution for three coral species (Acropora cf humilis, Pocillopora verrucosa, and Pocillopora meandrina) by sequencing two genetic markers of the symbiont (ITS2 and psbAncr), paired with genotype-by-sequencing of the coral host (DArT-seq). Our samples predominantly returned sequences from the genus Cladocopium, where Acropora cf humilis affiliated with C3k, Pocillopora verrucosa with C. pacificum, and Pocillopora meandrina with C. latusorum. Multivariate analyses revealed that Acropora symbionts were driven strongly by local environment and thermal disturbances. In contrast, Pocillopora symbiont communities were both partitioned 2.5-fold more by host genetic structure than by environmental structure. Among the two Pocillopora species, the effects of environment and host genetics explained four times more variation in symbionts for P. meandrina than P. verrucosa. The concurrent bleaching event in 2020 had variable impacts on symbiont communities, consistent with patterns in P. verrucosa and A. cf humilis, but not P. meandrina. Our findings demonstrate how symbiont macroscale community structure responses to environmental gradients depend on host species and their respective population structure. Integrating host, symbiont, and environmental data will help forecast the adaptive potential of corals and their symbionts amidst a rapidly changing environment.

The effects of marine heatwaves on acute heat tolerance in corals.

Scleractinian coral populations are increasingly exposed to conditions above their upper thermal limits due to marine heatwaves, contributing to global declines of coral reef ecosystem health. However, historic mass bleaching events indicate there is considerable inter- and intra-specific variation in thermal tolerance whereby species, individual coral colonies and populations show differential susceptibility to exposure to elevated temperatures. Despite this, we lack a clear understanding of how heat tolerance varies across large contemporary and historical environmental gradients, or the selective pressures that underpin this variation. Here we conducted standardised acute heat stress experiments to identify variation in heat tolerance among species and isolated reefs spanning a large environmental gradient across the Coral Sea Marine Park. We quantified the photochemical yield (Fv /Fm ) of coral samples in three coral species, Acropora cf humilis, Pocillopora meandrina, and Pocillopora verrucosa, following exposure to four temperature treatments (local ambient temperatures, and + 3°C, +6°C and + 9°C above local maximum monthly mean). We quantified the temperature at which Fv /Fm decreased by 50% (termed ED50) and used derived values to directly compare acute heat tolerance across reefs and species. The ED50 for Acropora was 0.4-0.7°C lower than either Pocillopora species, with a 0.3°C difference between the two Pocillopora species. We also recorded 0.9°C to 1.9°C phenotypic variation in heat tolerance among reefs within species, indicating spatial heterogeneity in heat tolerance across broad environmental gradients. Acute heat tolerance had a strong positive relationship to mild heatwave exposure over the past 35 years (since 1986) but was negatively related to recent severe heatwaves (2016-2020). Phenotypic variation associated with mild thermal history in local environments provides supportive evidence that marine heatwaves are selecting for tolerant individuals and populations; however, this adaptive potential may be compromised by the exposure to recent severe heatwaves.

Macroalgae exhibit diverse responses to human disturbances on coral reefs.

Scientists and managers rely on indicator taxa such as coral and macroalgal cover to evaluate the effects of human disturbance on coral reefs, often assuming a universally positive relationship between local human disturbance and macroalgae. Despite evidence that macroalgae respond to local stressors in diverse ways, there have been few efforts to evaluate relationships between specific macroalgae taxa and local human-driven disturbance. Using genus-level monitoring data from 1205 sites in the Indian and Pacific Oceans, we assess whether macroalgae percent cover correlates with local human disturbance while accounting for factors that could obscure or confound relationships. Assessing macroalgae at genus level revealed that no genera were positively correlated with all human disturbance metrics. Instead, we found relationships between the division or genera of algae and specific human disturbances that were not detectable when pooling taxa into a single functional category, which is common to many analyses. The convention to use percent cover of macroalgae as an indication of local human disturbance therefore likely obscures signatures of local anthropogenic threats to reefs. Our limited understanding of relationships between human disturbance, macroalgae taxa, and their responses to human disturbances impedes the ability to diagnose and respond appropriately to these threats.

Global warming and recurrent mass bleaching of corals.

During 2015-2016, record temperatures triggered a pan-tropical episode of coral bleaching, the third global-scale event since mass bleaching was first documented in the 1980s. Here we examine how and why the severity of recurrent major bleaching events has varied at multiple scales, using aerial and underwater surveys of Australian reefs combined with satellite-derived sea surface temperatures. The distinctive geographic footprints of recurrent bleaching on the Great Barrier Reef in 1998, 2002 and 2016 were determined by the spatial pattern of sea temperatures in each year. Water quality and fishing pressure had minimal effect on the unprecedented bleaching in 2016, suggesting that local protection of reefs affords little or no resistance to extreme heat. Similarly, past exposure to bleaching in 1998 and 2002 did not lessen the severity of bleaching in 2016. Consequently, immediate global action to curb future warming is essential to secure a future for coral reefs.

A connectivity portfolio effect stabilizes marine reserve performance.

Well-managed and enforced no-take marine reserves generate important larval subsidies to neighboring habitats and thereby contribute to the long-term sustainability of fisheries. However, larval dispersal patterns are variable, which leads to temporal fluctuations in the contribution of a single reserve to the replenishment of local populations. Identifying management strategies that mitigate the uncertainty in larval supply will help ensure the stability of recruitment dynamics and minimize the volatility in fishery catches. Here, we use genetic parentage analysis to show extreme variability in both the dispersal patterns and recruitment contribution of four individual marine reserves across six discrete recruitment cohorts for coral grouper (Plectropomus maculatus) on the Great Barrier Reef. Together, however, the asynchronous contributions from multiple reserves create temporal stability in recruitment via a connectivity portfolio effect. This dampening effect reduces the variability in larval supply from individual reserves by a factor of 1.8, which effectively halves the uncertainty in the recruitment contribution of individual reserves. Thus, not only does the network of four marine reserves generate valuable larval subsidies to neighboring habitats, the aggregate effect of individual reserves mitigates temporal fluctuations in dispersal patterns and the replenishment of local populations. Our results indicate that small networks of marine reserves yield previously unrecognized stabilizing benefits that ensure a consistent larval supply to replenish exploited fish stocks.

Coral-bleaching responses to climate change across biological scales.

The global impacts of climate change are evident in every marine ecosystem. On coral reefs, mass coral bleaching and mortality have emerged as ubiquitous responses to ocean warming, yet one of the greatest challenges of this epiphenomenon is linking information across scientific disciplines and spatial and temporal scales. Here we review some of the seminal and recent coral-bleaching discoveries from an ecological, physiological, and molecular perspective. We also evaluate which data and processes can improve predictive models and provide a conceptual framework that integrates measurements across biological scales. Taking an integrative approach across biological and spatial scales, using for example hierarchical models to estimate major coral-reef processes, will not only rapidly advance coral-reef science but will also provide necessary information to guide decision-making and conservation efforts. To conserve reefs, we encourage implementing mesoscale sanctuaries (thousands of km2 ) that transcend national boundaries. Such networks of protected reefs will provide reef connectivity, through larval dispersal that transverse thermal environments, and genotypic repositories that may become essential units of selection for environmentally diverse locations. Together, multinational networks may be the best chance corals have to persist through climate change, while humanity struggles to reduce emissions of greenhouse gases to net zero.

Influence of offshore oil and gas structures on seascape ecological connectivity.

Offshore platforms, subsea pipelines, wells and related fixed structures supporting the oil and gas (O&G) industry are prevalent in oceans across the globe, with many approaching the end of their operational life and requiring decommissioning. Although structures can possess high ecological diversity and productivity, information on how they interact with broader ecological processes remains unclear. Here, we review the current state of knowledge on the role of O&G infrastructure in maintaining, altering or enhancing ecological connectivity with natural marine habitats. There is a paucity of studies on the subject with only 33 papers specifically targeting connectivity and O&G structures, although other studies provide important related information. Evidence for O&G structures facilitating vertical and horizontal seascape connectivity exists for larvae and mobile adult invertebrates, fish and megafauna; including threatened and commercially important species. The degree to which these structures represent a beneficial or detrimental net impact remains unclear, is complex and ultimately needs more research to determine the extent to which natural connectivity networks are conserved, enhanced or disrupted. We discuss the potential impacts of different decommissioning approaches on seascape connectivity and identify, through expert elicitation, critical knowledge gaps that, if addressed, may further inform decision making for the life cycle of O&G infrastructure, with relevance for other industries (e.g. renewables). The most highly ranked critical knowledge gap was a need to understand how O&G structures modify and influence the movement patterns of mobile species and dispersal stages of sessile marine species. Understanding how different decommissioning options affect species survival and movement was also highly ranked, as was understanding the extent to which O&G structures contribute to extending species distributions by providing rest stops, foraging habitat, and stepping stones. These questions could be addressed with further dedicated studies of animal movement in relation to structures using telemetry, molecular techniques and movement models. Our review and these priority questions provide a roadmap for advancing research needed to support evidence-based decision making for decommissioning O&G infrastructure.

Successful validation of a larval dispersal model using genetic parentage data.

Larval dispersal is a critically important yet enigmatic process in marine ecology, evolution, and conservation. Determining the distance and direction that tiny larvae travel in the open ocean continues to be a challenge. Our current understanding of larval dispersal patterns at management-relevant scales is principally and separately informed by genetic parentage data and biological-oceanographic (biophysical) models. Parentage datasets provide clear evidence of individual larval dispersal events, but their findings are spatially and temporally limited. Biophysical models offer a more complete picture of dispersal patterns at regional scales but are of uncertain accuracy. Here, we develop statistical techniques that integrate these two important sources of information on larval dispersal. We then apply these methods to an extensive genetic parentage dataset to successfully validate a high-resolution biophysical model for the economically important reef fish species Plectropomus maculatus in the southern Great Barrier Reef. Our results demonstrate that biophysical models can provide accurate descriptions of larval dispersal at spatial and temporal scales that are relevant to management. They also show that genetic parentage datasets provide enough statistical power to exclude poor biophysical models. Biophysical models that included species-specific larval behaviour provided markedly better fits to the parentage data than assuming passive behaviour, but incorrect behavioural assumptions led to worse predictions than ignoring behaviour altogether. Our approach capitalises on the complementary strengths of genetic parentage datasets and high-resolution biophysical models to produce an accurate picture of larval dispersal patterns at regional scales. The results provide essential empirical support for the use of accurately parameterised biophysical larval dispersal models in marine spatial planning and management.

Minimum size limits and the reproductive value of numerous, young, mature female fish.

Fisheries management relies on various catch and effort controls to preserve spawning stock biomass and maximize sustainable yields while limiting fishery impacts on marine ecosystems. These include species-specific minimum or maximum size limits to protect either small non-reproductive subadults, a portion of reproductively mature adults, or large highly fecund individuals. Protecting size classes of mature fish is expected to yield a viable source of larvae for replenishing populations and reduce the risk of recruitment overfishing, yet size-specific recruitment contributions have rarely been assessed empirically. Here, we apply genetic parentage analysis to measure the reproductive success of a size-structured population of a commercially important species of coral grouper (Plectropomus maculatus-Serranidae) in no-take marine reserves (NTMRs) in the Great Barrier Reef Marine Park, Australia. Although the per capita reproductive success of individual fish increases rapidly with body length, the numerous young mature female fish, below the minimum size limit (MSL) (38 cm total length), were responsible for generating disproportionately large contributions (36%) towards larval replenishment of both fished and reserve reefs. Our findings indicate that MSLs are an effective harvest control measure to safeguard a portion of the spawning stock biomass for coral grouper and supplement recruitment subsidies assured from NTMRs.

Stochastic nature of larval dispersal at sea.

The movement of individuals across landscapes remains a fundamental process in population and community ecology. All species have developed a capacity to disperse but this process remains elusive in organisms with complex life-cycles, and none more so than in the marine environment. Here, most organisms have developed a two-phased life-cycle, leaving the risky business of dispersing through the open ocean to their very small and intractable larval offspring. To this day, quantifying dispersal patterns in marine seascapes remains a significant challenge, and yet it is critical to the way we preserve marine ecosystems and the services they provide. In this issue of Molecular Ecology, Catalano et al. (2021) present one of the first longitudinal studies to demonstrate the stochastic nature of larval dispersal. Their work challenges some of our current ideas about marine population connectivity and provides new methodological insights to study its temporal dimension.

Regional versus latitudinal variation in the life-history traits and demographic rates of a reef fish, Centropyge bispinosa, in the Coral Sea and Great Barrier Reef Marine Parks, Australia.

Environmental temperature is an important determinant of physiological processes and life histories in ectotherms. Over latitudinal scales, variation in temperature has been linked to changes in life-history traits and demographic rates, with growth and mortality rates generally being greatest at low latitudes, and longevity and maximum length being greater at higher latitudes. Using the two-spined angelfish, Centropyge bispinosa, as our focal species, we compared growth patterns, growth rates, longevity, mortality, asymptotic length and maximum length across 22 reefs that span 13° of latitude within the Great Barrier Reef Marine Park (GBRMP) and the Coral Sea Marine Park (CSMP), Australia. We found no predictable latitudinal variation in mortality rates, growth patterns, growth rates, asymptotic or maximum length of C. bispinosa at regional to biogeographic scales. However, C. bispinosa consistently exhibited reduced longevity at lower, warmer latitudes within the CSMP. The greatest differences in mean maximum length of C. bispinosa were between continental (GBRMP) and oceanic (central CSMP) reefs of similar latitude, with individuals being larger on average on continental versus oceanic reefs. The lack of predictable life-history and demographic variation in C. bispinosa across a 13° latitudinal gradient within the CSMP, coupled with differences in mean maximum length between continental and oceanic reefs at similar latitudes, suggest that local environmental conditions have a greater influence than environmental temperature on the demographic rates and life-history traits of C. bispinosa.

Natal philopatry increases relatedness within groups of coral reef cardinalfish.

A central issue in evolutionary ecology is how patterns of dispersal influence patterns of relatedness in populations. In terrestrial organisms, limited dispersal of offspring leads to groups of related individuals. By contrast, for most marine organisms, larval dispersal in open waters is thought to minimize kin associations within populations. However, recent molecular evidence and theoretical approaches have shown that limited dispersal, sibling cohesion and/or differential reproductive success can lead to kin association and elevated relatedness. Here, we tested the hypothesis that limited dispersal explains small-scale patterns of relatedness in the pajama cardinalfish Sphaeramia nematoptera. We used 19 microsatellite markers to assess parentage of 233 juveniles and pairwise relatedness among 527 individuals from 41 groups in Kimbe Bay, Papua New Guinea. Our findings support three predictions of the limited dispersal hypothesis: (i) elevated relatedness within groups, compared with among groups and elevated relatedness within reefs compared with among reefs; (ii) a weak negative correlation of relatedness with distance; (iii) more juveniles than would be expected by chance in the same group and the same reef as their parents. We provide the first example for natal philopatry at the group level causing small-scale patterns of genetic relatedness in a marine fish.

Comparative demography of commercially important species of coral grouper, Plectropomus leopardus and P. laevis, from Australia's great barrier reef and Coral Sea marine parks.

Understanding the spatial and environmental variation in demographic processes of fisheries target species, such as coral grouper (Genus: Plectropomus), is important for establishing effective management and conservation strategies. Herein we compare the demography of Plectropomus leopardus and P. laevis between Australia's Great Barrier Reef Marine Park (GBRMP), which has been subject to sustained and extensive fishing pressure, and the oceanic atolls of Australia's Coral Sea Marine Park (CSMP), where there is very limited fishing for reef fishes. Coral grouper length-at-age data from contemporary and historical otolith collections across 9.4 degrees of latitude showed little difference in lifetime growth between GBRMP and CSMP regions. Plectropomus laevis populations in GBRMP reefs had significantly higher rates of total mortality than populations in the CSMP. Mean maximum lengths and mean maximum ages of P. laevis were also smaller in the GBRMP than in the CSMP, even when considering populations sampled within GBRMP no-take marine reserves (NTMRs). Plectropomus leopardus, individuals were on average smaller on fished reefs than NTMRs in the GBRMP, but all other aspects of demography were broadly similar between regions despite the negligible levels of fishing pressure in the CSMP. Similarities between regions in growth profiles and length-at-age comparisons of P. laevis and P. leopardus suggest that the environmental differences between the CSMP and the GBRMP may not have significant impacts on lifetime growth. Our results show that fishing may have influenced the demography of coral grouper on the GBR, particularly for the slower growing and longer lived species, P. laevis.

Acclimatization of massive reef-building corals to consecutive heatwaves.

Reef-building corals typically live close to the upper limits of their thermal tolerance and even small increases in summer water temperatures can lead to bleaching and mortality. Projections of coral reef futures based on forecasts of ocean temperatures indicate that by the end of this century, corals will experience their current thermal thresholds annually, which would lead to the widespread devastation of coral reef ecosystems. Here, we use skeletal cores of long-lived Porites corals collected from 14 reefs across the northern Great Barrier Reef, the Coral Sea, and New Caledonia to evaluate changes in their sensitivity to heat stress since 1815. High-density 'stress bands'-indicative of past bleaching-first appear during a strong pre-industrial El Niño event in 1877 but become significantly more frequent in the late twentieth and early twenty-first centuries in accordance with rising temperatures from anthropogenic global warming. However, the proportion of cores with stress bands declines following successive bleaching events in the twenty-first century despite increasing exposure to heat stress. Our findings demonstrate an increase in the thermal tolerance of reef-building corals and offer a glimmer of hope that at least some coral species can acclimatize fast enough to keep pace with global warming.

Extra-pair mating in a socially monogamous and paternal mouth-brooding cardinalfish.

Many vertebrates form monogamous pairs to mate and care for their offspring. However, genetic tools have increasingly shown that offspring often arise from matings outside of the monogamous pair bond. Social monogamy is relatively common in coral reef fishes, but there have been few studies that have confirmed monogamy or extra-pair reproduction, either for males or for females. Here, long-term observations and genetic tools were applied to examine the parentage of embryos in a paternally mouth-brooding cardinalfish, Sphaeramia nematoptera. Paternal care in fishes, such as mouth-brooding, is thought to be associated with a high degree of confidence in paternity. Two years of observations confirmed that S. nematoptera form long-term pair bonds within larger groups. However, genetic parentage revealed extra-pair mating by both sexes. Of 105 broods analysed from 64 males, 30.1% were mothered by a female that was not the partner and 11.5% of broods included eggs from two females. Despite the high paternal investment associated with mouth-brooding, 7.6% of broods were fertilized by two males. Extra-pair matings appeared to be opportunistic encounters with individuals from outside the immediate group. We argue that while pair formation contributes to group cohesion, both males and females can maximize lifetime reproductive success by taking advantage of extra-pair mating opportunities.

Widespread hybridization and bidirectional introgression in sympatric species of coral reef fish.

Coral reefs are highly diverse ecosystems, where numerous closely related species often coexist. How new species arise and are maintained in these high geneflow environments have been long-standing conundrums. Hybridization and patterns of introgression between sympatric species provide a unique insight into the mechanisms of speciation and the maintenance of species boundaries. In this study, we investigate the extent of hybridization between two closely related species of coral reef fish: the common coral trout (Plectropomus leopardus) and the bar-cheek coral trout (Plectropomus maculatus). Using a complementary set of 25 microsatellite loci, we distinguish pure genotype classes from first- and later-generation hybrids, identifying 124 interspecific hybrids from a collection of 2,991 coral trout sampled in inshore and mid-shelf reefs of the southern Great Barrier Reef. Hybrids were ubiquitous among reefs, fertile and spanned multiple generations suggesting both ecological and evolutionary processes are acting to maintain species barriers. We elaborate on these finding to investigate the extent of genomic introgression and admixture from 2,271 SNP loci recovered from a ddRAD library of pure and hybrid individuals. An analysis of genomic clines on recovered loci indicates that 261 SNP loci deviate from a model of neutral introgression, of which 132 indicate a pattern of introgression consistent with selection favouring both hybrid and parental genotypes. Our findings indicate genome-wide, bidirectional introgression between two sympatric species of coral reef fishes and provide further support to a growing body of evidence for the role of hybridization in the evolution of coral reef fishes.

The role of marine reserves in the replenishment of a locally impacted population of anemonefish on the Great Barrier Reef.

The development of parentage analysis to track the dispersal of juvenile offspring has given us unprecedented insight into the population dynamics of coral reef fishes. These tools now have the potential to inform fisheries management and species conservation, particularly for small fragmented populations under threat from exploitation and disturbance. In this study, we resolve patterns of larval dispersal for a population of the anemonefish Amphiprion melanopus in the Keppel Islands (southern Great Barrier Reef). Habitat loss and fishing appear to have impacted this population and a network of no-take marine reserves currently protects 75% of the potential breeders. Using parentage analysis, we estimate that 21% of recruitment in the island group was generated locally and that breeding adults living in reserves were responsible for 79% (31 of 39) of these of locally produced juveniles. Overall, the network of reserves was fully connected via larval dispersal; however, one reserve was identified as a critical source of larvae for the island group. The population in the Keppel Islands also appears to be well-connected to other source populations at least 60 km away, given that 79% (145 of 184) of the juveniles sampled remained unassigned in the parentage analysis. We estimated the effective size of the A. melanopus metapopulation to be 745 (582-993 95% CI) and recommend continued monitoring of its genetic status. Maintaining connectivity with populations beyond the Keppel Islands and recovery of local recruitment habitat, potentially through active restoration of host anemone populations, will be important for its long-term persistence.

Large-scale, multidirectional larval connectivity among coral reef fish populations in the Great Barrier Reef Marine Park.

Larval dispersal is the key process by which populations of most marine fishes and invertebrates are connected and replenished. Advances in larval tagging and genetics have enhanced our capacity to track larval dispersal, assess scales of population connectivity, and quantify larval exchange among no-take marine reserves and fished areas. Recent studies have found that reserves can be a significant source of recruits for populations up to 40 km away, but the scale and direction of larval connectivity across larger seascapes remain unknown. Here, we apply genetic parentage analysis to investigate larval dispersal patterns for two exploited coral reef groupers (Plectropomus maculatus and Plectropomus leopardus) within and among three clusters of reefs separated by 60-220 km within the Great Barrier Reef Marine Park, Australia. A total of 69 juvenile P. maculatus and 17 juvenile P. leopardus (representing 6% and 9% of the total juveniles sampled, respectively) were genetically assigned to parent individuals on reefs within the study area. We identified both short-distance larval dispersal within regions (200 m to 50 km) and long-distance, multidirectional dispersal of up to ~250 km among regions. Dispersal strength declined significantly with distance, with best-fit dispersal kernels estimating median dispersal distances of ~110 km for P. maculatus and ~190 km for P. leopardus. Larval exchange among reefs demonstrates that established reserves form a highly connected network and contribute larvae for the replenishment of fished reefs at multiple spatial scales. Our findings highlight the potential for long-distance dispersal in an important group of reef fishes, and provide further evidence that effectively protected reserves can yield recruitment and sustainability benefits for exploited fish populations.

Seascape genetics along environmental gradients in the Arabian Peninsula: insights from ddRAD sequencing of anemonefishes.

Understanding the processes that shape patterns of genetic structure across space is a central aim of landscape genetics. However, it remains unclear how geographical features and environmental variables shape gene flow, particularly for marine species in large complex seascapes. Here, we evaluated the genomic composition of the two-band anemonefish Amphiprion bicinctus across its entire geographical range in the Red Sea and Gulf of Aden, as well as its close relative, Amphiprion omanensis endemic to the southern coast of Oman. Both the Red Sea and the Arabian Sea are complex and environmentally heterogeneous marine systems that provide an ideal scenario to address these questions. Our findings confirm the presence of two genetic clusters previously reported for A. bicinctus in the Red Sea. Genetic structure analyses suggest a complex seascape configuration, with evidence of both isolation by distance (IBD) and isolation by environment (IBE). In addition to IBD and IBE, genetic structure among sites was best explained when two barriers to gene flow were also accounted for. One of these coincides with a strong oligotrophic-eutrophic gradient at around 16-20˚N in the Red Sea. The other agrees with a historical bathymetric barrier at the straight of Bab al Mandab. Finally, these data support the presence of interspecific hybrids at an intermediate suture zone at Socotra and indicate complex patterns of genomic admixture in the Gulf of Aden with evidence of introgression between species. Our findings highlight the power of recent genomic approaches to resolve subtle patterns of gene flow in marine seascapes.