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Abstract Deep-sea coral frameworks are understudied in the Red Sea, where conditions in the deep are conspicuously warm and saline compared to other basins. Habitat suitability models can be used to predict the distribution pattern of species or assemblages where direct observation is difficult. Here we show how coral frameworks, built by species within the families Caryophylliidae and Dendrophylliidae, are distributed between water depths of 150 m and 700 m in the northern Red Sea and Gulf of Aqaba. To extrapolate the known (ground-truthed) positions of these deep frameworks, we use environmental and geomorphometric variables to inform well-performing maximum entropy models. Over 250 km2 of seafloor in our study area are identified as suitable for such frameworks, equivalent to at least 35% of the area of photic-zone coral reefs in the same region. We hence contend that deep-water coral frameworks are an important and underappreciated repository of Red Sea biodiversity.

Black corals (Hexacorallia: Antipatharia) are a major component of mesophotic and deep marine ecosystems. Due to their preference for light deprived environments, black corals have historically been considered azooxanthellate, yet recent works have found them in association with dinoflagellates of the family Symbiodiniaceae down to 396 m depth. While corals and Symbiodiniaceae generally establish a symbiotic relationship in shallow water environments, the implications of this association is less well understood at deeper depths, where low light penetration cannot sustain efficient photosynthetic activity for the algae. However, Symbiodinaceae are not obligate autotrophs, and their capacity for heterotrophic feeding categorizes them as mixotrophs. In this study, we investigated the presence and diversity of Symbiodiniaceae associated with the deep-sea black coral Bathypathes thermophila (Antipatharia: Schizopathidae), collected from 204 to 655 m depth in the Saudi Arabian Red Sea. Using high-throughput sequencing of the ITS2 region, we report (1) the deepest record to date of Symbiodiniaceae associated with an anthozoan from 655 m, and (2) the first Red Sea record of Antipatharia in association with Symbiodiniaceae. Our analyses revealed that 14 out of 27 colonies of B. thermophila were associated with Symbiodiniaceae of the genera Cladocopium and Durusdinium. We unveiled 16 novel ITS2 type profiles, possibly unique to black corals and/or to these depths, along with seven profiles that were already known from shallow-water hard corals. No significant pattern was detected in terms of community diversity in relation to depth or sampling locality. Our study supports the existence of black corals-Symbiodiniaceae association and warrants further research to better understand the evolutionary processes and physiological mechanisms driving this association, specifically in light deprived environments.

Coral-dwelling gall crabs (Cryptochiridae) live in an obligate symbiosis with reef-building corals from shallow to deep waters. In particular, crabs of the genus Opecarcinus are known to occur across the tropical belt in association with the scleractinian family Agariciidae, down to a depth of 89 m. The Red Sea is a semi-enclosed basin that has long been recognized as a region of high marine biodiversity and endemism, with more than 230 decapod species recorded so far. There, nine Opecarcinus species are recorded from shallow and mesophotic corals. However, the occurrence of Opecarcinus below 30 m depth is documented only in the Saudi Arabian coast of the Northern Red Sea, and the rest of the Saudi Arabian Red Sea has remained unstudied. In this work, we assess the diversity and distribution of Opecarcinus from the mesophotic waters of the Saudi Arabian Red Sea. Through Remotely Operated Vehicles (ROV) and Submarines explorations, we collected 10 gall crab specimens associated with Agariciidae corals at mesophotic depths between 47 and 89 m. Two mitochondrial and one nuclear markers were used to obtain crab phylogenetic tree showing that three of the Opecarcinus species known from Red Sea shallow-water corals can also live at mesophotic depths. This is the first assessment of the depth distribution of the genus Opecarcinus in the Red Sea. It provides further evidence of the depth range adaptability of the Opecarcinus-Agariciidae association and highlights the need for a more in-depth investigation of the largely overlooked mesophotic coral-associated fauna.

IntroductionThe Red Sea is a narrow rift basin characterized by latitudinal environmental gradients which shape the diversity and distribution of reef-dwelling organisms. Studies on Symbiodiniaceae associated with select hard coral taxa present species- specific assemblages and concordant variation patterns from the North to southeast Red Sea coast at depths shallower than 30 m. At mesophotic depths, however, algal diversity studies are rare. Here, we characterize for the first-time host-associated algal communities of a mesophotic specialist coral species, Leptoseris cf. striatus, along the Saudi Arabian Red Sea coast.MethodsWe sampled 56 coral colonies spanning the eastern Red Sea coastline from the Northern Red Sea to the Farasan Banks in the South, and across two sampling periods, Fall 2020 and Spring 2022. We used Next Generation Sequencing of the ITS2 marker region in conjunction with SymPortal to denote algal assemblages.Results and discussionOur results show a relatively stable coral species-specific interaction with algae from the genus Cladocopium along the examined latitudinal gradient, with the appearance, in a smaller proportion, of presumed thermally tolerant algal taxa in the genera Symbiodinium and Durusdinium during the warmer season (Fall 2020). Contrary to shallow water corals, our results do not show a change in Symbiodiniaceae community composition from North to South in this mesophotic specialist species. However, our study highlights for the first time that symbiont communities are subject to change over time at mesophotic depth, which could represent an important phenomenon to address in future studies.

Octocorals (Cnidaria: Anthozoa) have a global distribution and form benthic assemblages along the depth gradient, from shallow to deep waters. They often occur below SCUBA diving limits, where they can become dominant habitat builders and aggregate different taxa. During a cruise in February 2023, one octocoral specimen was collected at 1453 m depth at Kebrit Deep, in the northern Saudi Arabian Red Sea axis, an area with extremely high temperature and salinity profiles at depth. Morphological analysis coupled with DNA barcoding using two mitochondrial markers (COI and mtMuts), revealed that the coral belongs to Acanthogorgia, a genus of azooxanthellate octocorals known to occur from 3 to 2300 m depths in cold, temperate and tropical waters. In the Red Sea, the genus was previously only known from shallower waters. Hence, we report the deepest record of the genus Acanthogorgia from the warm and saline Red Sea basin. This finding provides novel insights on deep-water octocoral diversity in the Red Sea, a still scantily explored area of the world, while emphasizing the need for further explorations at depth.

Abstract Coral reefs accommodate a myriad of species, many of which live in association with a host organism. Decapod crustaceans make up a large part of this associated fauna on coral reefs. Among these, cryptochirid crabs are obligately associated with scleractinian corals, in which they create dwellings where they permanently reside. These gall crabs show various levels of host specificity, with the majority of cryptochirids inhabiting a specific coral genus or species. Here, we report the first records of gall crabs living in association with two different Porites species in the Red Sea. Crescent‐shaped dwellings were observed in Porites rus and a Porites sp. in situ, and colonies with crabs were collected for further study in the laboratory. Using a combination of morphology and DNA barcoding, the crabs were identified as belonging to Opecarcinus, a genus only known to inhabit Agariciidae corals. The coral skeleton was bleached and studied under a stereo microscope, which revealed that the Porites corals overgrew adjoining agariciid Pavona colonies. We hypothesize that the gall crab originally settled on Pavona, its primary host of choice. Due to coral interspecific competition the Porites colony overgrew the adjacent Pavona colonies, resulting in a secondary and never before reported association of Opecarcinus with Porites. These findings suggest that cryptochirid crabs can adapt to the new microenvironment provided by a different coral host and survive competition for space on coral reefs.

IntroductionThe northern Red Sea has been coined a refuge for reef corals due to the exceptional thermal tolerance of these organisms. With ocean warming threatening coral reefs worldwide, a panoptic characterization of corals living in extreme conditions may provide insights into future responses of corals to environmental change. Among other factors, the genotype of the endosymbiotic algae in the family Symbiodiniaceae has been shown to have major implications on the distribution and resilience of their coral hosts. In this study, we aim at genotyping the Symbiodiniaceae communities associated with three depth generalist and one depth specialist coral species, characterized by the ability to withstand environmental conditions that are apparently limiting for other corals and occurring in a unique geographical region.MethodsWe sampled 50 corals from the northern Saudi Arabian Red Sea and the Gulf of Aqaba, covering a 97 m bathymetric gradient. We used high-throughput ITS2 gene sequencing and recovered different patterns of host–algal associations.Results and discussionThe majority of the recovered algal genotypes appeared host- and environment-specific, while others were more widely distributed. At large, coral specimens were overwhelmingly associated with symbionts from the genus Cladocopium and specifically with many previously undescribed genotypes. This suggests the selection of specific genotypes, which might confer resistance and/or resilience to their host counterparts. Interestingly, we found a limited association with Durusdinium spp. and other known tolerant taxa in mesophotic corals in the northern Red Sea, but not in the Gulf of Aqaba. The broad absence of Durusdinium spp., typically ascribed to be stress tolerant, warrants further investigation into Symbiodiniaceae species that convey environmental resilience. Our data will serve as a baseline to explore the occurrence of specific symbionts that might be contributing to coral acclimation and adaptation and to assay how biodiversity might be impacted if subjected to increasing stressors.

The scleractinian coral family Dendrophylliidae is a major component of shallow and deep-water coral ecosystems worldwide, but our knowledge on the evolutionary history of the family remains scarce. Here, we used ezRAD coupled with Illumina sequencing technology and reconstructed the complete mitochondrial genome of Dendrophyllia minuscula (GenBank accession number OL634845), from mesophotic depths in the Red Sea NEOM area. The mitochondrial genome of D. minuscula consisted of 19,054 bp, organized in 13 protein-coding genes, 2 rRNA genes, and 2 tRNA genes, in agreement with the Scleractinia typical mitogenome organization. This complete mitochondrial genome contributes toward a better knowledge of mesophotic and deep-water coral diversity and evolutionary history.

The diversity and evolution of the Red Sea invertebrates in mesophotic and deep-water benthic ecosystems remain largely unexplored. The Palaemonidae is a diversified family of caridean shrimps with numerous taxa in need of taxonomic revisions based on recent molecular analyses. The Red Sea mesophotic and bathyal palaemonid shrimps are largely unstudied. During recent expeditions off the Red Sea coast of Saudi Arabia, several palaemonid specimens were collected at a depth range of 88–494 m, spanning the mesophotic and bathyal zones. This material was examined morphologically and genetically to infer phylogenetic relationships among the Red Sea taxa and several other palaemonid genera. The concordant morphological and genetic data led to the description of two new genera and two new species. Moreover, one species was recorded in the Red Sea for the first time, with a new host record, whereas three further deep-water species, which do not occur in the Red Sea, were formally transferred to a different genus. As more exploration efforts are deployed, research on the diversity and evolutionary relationships among marine invertebrates from the Red Sea will further underline the uniqueness of its mesophotic and bathyal fauna.

Coral-dwelling gall crabs (Cryptochiridae) are obligate symbionts of stony corals and occur on shallow and deep reefs across the tropical belt. The circumtropical genus Opecarcinus associates with Agariciidae corals, a dominant component of Mesophotic Coral Ecosystems (MCEs). Here, we report the first Red Sea mesophotic record, with 89 m as the deepest record to date, for Opecarcinus—collected from Leptoseris cf mycetoseroides—from the NEOM marine area in Saudi Arabia. This observation reconfirms the depth range flexibility of Opecarcinus species and highlights the need for further mesophotic explorations of reef-associated fauna.

Measurement(s) Point in Time Date and Time Data Type • Spawning Technology Type(s) Observation • digital curation Sample Characteristic - Organism Scleractinia Sample Characteristic - Environment ocean Sample Characteristic - Location Pacific Ocean • Indian Ocean region Machine-accessible metadata file describing the reported data: https://doi.org/10.6084/m9.figshare.13100552

The Stylasteridae, commonly known as lace corals, is a family of colonial calcifying hydrozoans mostly inhabiting deep waters. Stylasterids show a cosmopolitan distribution but, in some areas, they are characterized by low species diversity, such as in the Red Sea, where only a shallow-water species has been reported so far. With this work, we provide the first evidence of a deep-sea stylasterid inhabiting the NEOM region in the northern Saudi Arabian Red Sea, at depths ranging between 166 and 492 m. Morphological examinations revealed that this species was previously unknown and belonging to the genus Stylaster. We, therefore, describe Stylaster tritoni sp. nov., representing the first record of the genus in the Red Sea. Lastly, the phylogenetic position of the species within the Stylasteridae was evaluated, revealing a close relationship with shallow-water Indo-Pacific and Western Atlantic Stylaster species and confirming the polyphyletic nature of the genus Stylaster.

Corals in the genus Porites are among the major framework builders of reef structures worldwide, yet the genus has been challenging to study due to a lack of informative molecular markers. Here, we used ezRAD sequencing to reconstruct the complete mitochondrial genome of Porites fontanesii (GenBank accession number MG754069), a widespread coral species endemic to the Red Sea and Gulf of Aden. The gene arrangement of P. fontanesii did not differ from other Scleractinia and consisted of 18,658 bp, organized in 13 protein-coding genes, 2 rRNA genes, and 2 tRNA genes. This mitochondrial genome contributes essential data to work towards a better understanding of evolutionary relationships within Porites.