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.

Introduction: The 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.Methods: We 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 discussion: Our 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.

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.

Abiotic conditions influence biodiversity and population connectivity, the assessment of which is critical for predicting climate change impacts and guiding conservation efforts. Coral reefs face severe losses, making effective conservation strategies increasingly urgent. Here, we use a seascape genomics approach to examine the population genetic structure of three non-model scleractinian corals around the Arabian Peninsula, a region with distinctive oceanographic features. We reveal contrasting patterns across species, which we integrate with geographic distances, larval dispersal models, and environmental data. Population structure appears shaped both by isolation by distance (IBD) and isolation by resistance (IBR), with environmental gradients often covarying with spatial separation. Our findings clarify coral diversification and evolutionary responses to conditions around the region. This work also provides a methodological framework to advance our understanding of processes structuring reef populations, thereby informing efforts to design robust marine protected area networks in the face of climate change and biodiversity loss.

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.

Coral mortality is occurring worldwide at an alarming rate. Despite the immense and underestimated biodiversity of reef-building corals, very few genomes are available. Further, almost all genomic resources originate from shallow water corals even though photosynthetic, symbiotic corals occur at mesophotic depths deeper than 30 m and even >100 m. We present annotated, de novo genomes for two mesophotic, scleractinian (reef-building) corals Montipora cf. grisea and Leptoseris cf. scabra from American Sāmoa, the latter being the first genome for the widespread genus Leptoseris. We used PacBio continuous long reads and Omni-C data to assemble chromosome-level reference genomes. For Montipora cf. grisea, the final genome size was 1.3 Gb with a completeness level (BUSCO) of 99.9% and 97.2% against the eukaryotic and metazoan databases, respectively. The M. cf. grisea genome had a N50 of 50.2 Mb and the annotation predicted 41,981 genes. For Leptoseris cf. scabra, the final genome size was 794 Mb with a BUSCO of 99.2% and 96.1% against the eukaryotic and metazoan databases, respectively. The L. cf. scabra genome had a N50 of 45.2 Mb and 35,741 predicted genes. These genomes serve as critical references for the analysis of coral gene expression responses to climate change such as ocean warming (i.e., coral bleaching) and ocean acidification impacts. The genomes can be used to investigate the genetic diversity and adaptive divergence of shallow vs. mesophotic coral populations to understand reef resilience and guide conservation strategies.