One new species of Stegocephalus Krøyer, 1842 (Amphipoda, Stegocephalidae) described from a seamount of the Caroline Plate, NW Pacific.

A new species of the subfamily Stegocephalinae, Stegocephaluscarolussp. nov., is described from a seamount in the Caroline Plate. Two related species, S.cascadiensis (Moore, 1992) and S.longicornis (Gurjanova, 1962), were previously reported in the North Pacific. Important morphological characters which differentiate S.carolussp. nov. from S.cascadiensis are found in antenna 1, the mouthparts, pereopod 7 and the length of rami of uropods 2 and 3. The new species differs from S.longicornis by characters of antenna 1, the mouthparts and the shape of epimeral plate 3. Additionally, the morphological differences between the new species and the remaining seven species of Stegocephalus are also presented.

Cryptic diversity begets challenges and opportunities in biodiversity research.

How many species of life are there on Earth? This is a question that we want to know but cannot yet answer. Some scholars speculate that the number of species may reach 2.2 billion when considering cryptic diversity and that each morphology-based insect species may contain an average of 3.1 cryptic species. With nearly two million described species, such high estimates of cryptic diversity would suggest that cryptic species are widespread. The development of molecular species delimitation has led to the discovery of a large number of cryptic species, and cryptic biodiversity has gradually entered our field of vision and attracted more attention. This paper introduces the concept of cryptic species, how they evolve, and methods by which they may be discovered and confirmed, and provides theoretical and methodological guidance for the study of hidden species. A workflow of how to confirm cryptic species is provided. In addition, the importance and reliability of multi-evidence-based integrated taxonomy are reaffirmed as a way to better standardize decision-making processes. Special focus on cryptic diversity and increased funding for taxonomy is needed to ensure that cryptic species in hyperdiverse groups are discoverable and described. An increased focus on cryptic species in the future will naturally arise as more difficult groups are studied, and thereby, we may finally better understand the rules governing the evolution and maintenance of cryptic biodiversity.

Rolling with the punches: Organism-environment interactions shape spatial pattern of adaptive differentiation in the widespread mantis shrimp Oratosquilla oratoria.

Investigating spatial pattern of adaptive variation and its underlying processes can inform the adaptive potential distributed within species ranges, which is increasingly important in the context of a changing climate. A correct interpretation of adaptive variation pattern requires that population history and the ensuing population genetic structure are taken into account. Here we carried out such a study by integrating population genomic analyses, demographic model testing and species distribution modeling to investigate patterns and causes of adaptive differentiation in a widespread mantis shrimp, Oratosquilla oratoria, along a replicated, broad-scale temperature gradient in the northwestern Pacific (NWP). Our results supported a strong hierarchical ecogeographic structure dominated by habitat-linked divergence among O. oratoria populations accompanied with introgressive hybridization. A combined FST outlier and environmental correlation analyses revealed remarkable temperature-associated clines in allele frequency across paired North-South populations on Chinese and Japanese coasts, and identified a suite of loci associated with temperature adaptation. Further demographic model testing revealed the observed clinal variation derived partly from Pleistocene divergence followed by recent secondary contact. More importantly, the likelihood of hybridization is predicted to increase as climate change progresses, which would break barriers to gene flow and enable the spread of adaptive genetic variation. These results support that not only is temperature-driven adaptive differentiation occurs in O. oratoria but that such pattern is likely attributed to ancient adaptive variation, sustained by contemporary ocean conditions and a semi-permeable barrier to gene flow maintained by selection. They moreover provide genomic insights into the distribution of adaptive potential across O. oratoria' s species range. This work can serve as a case study to characterize adaptive diversity of marine species in the NWP by integrating environmental and genetic data at temporal and spatial scales in a population genomic framework, which would improve management and conservation actions under climate change.

The First Genome Survey of the Snail Provanna glabra Inhabiting Deep-Sea Hydrothermal Vents.

The snail P. glabra is an endemic species in deep-sea chemosynthetic ecosystems of the Northwest Pacific Ocean. To obtain more genetic information on this species and provide the basis for subsequent whole-genome map construction, a genome survey was performed on this snail from the hydrothermal vent of Okinawa Trough. The genomic size of P. glabra was estimated to be 1.44 Gb, with a heterozygosity of 1.91% and a repeated sequence content of 69.80%. Based on the sequencing data, a draft genome of 1.32 Gb was assembled. Transposal elements (TEs) accounted for 40.17% of the entire genome, with DNA transposons taking the highest proportion. It was found that most TEs were inserted in the genome recently. In the simple sequence repeats, the dinucleotide motif was the most enriched microsatellite type, accounting for 53% of microsatellites. A complete mitochondrial genome of P. glabra with a total length of 16,268 bp was assembled from the sequencing data. After comparison with the published mitochondrial genome of Provanna sp. from a methane seep, 331 potential single nucleotide polymorphism (SNP) sites were identified in protein-coding genes (PCGs). Except for the cox1 gene, nad2, nad4, nad5, and cob genes are expected to be candidate markers for population genetic and phylogenetic studies of P. glabra and other deep-sea snails. Compared with shallow-water species, three mitochondrial genes of deep-sea gastropods exhibited a higher evolutionary rate, indicating strong selection operating on mitochondria of deep-sea species. This study provides insights into the genome characteristics of P. glabra and supplies genomic resources for further studies on the adaptive evolution of the snail in extreme deep-sea chemosynthetic environments.

A new species of Orchomenella (Amphipoda, Tryphosidae) described from hydrothermal vent in the Okinawa Trough, Northwest Pacific.

A new species of the family Tryphosidae, Orchomenellacompressasp. nov., is described from hydrothermal vents in the Okinawa Trough. This is the first known Orchomenella species found in vent fields. Important morphological characters that differentiate O.compressasp. nov. from its congeners are the absence of eyes, the compressed distal three articles of gnathopod 2, the shape of the posterior margin of epimerons 2 and 3, and the number of dorsal spines on the telson. The genetic divergence of the analyzed COI gene clearly supports this new taxon.

The Diversity and Metabolism of Culturable Nitrate-Reducing Bacteria from the Photic Zone of the Western North Pacific Ocean.

To better understand bacterial communities and metabolism under nitrogen deficiency, 154 seawater samples were obtained from 5 to 200 m at 22 stations in the photic zone of the Western North Pacific Ocean. Total 634 nitrate-utilizing bacteria were isolated using selective media and culture-dependent methods, and 295 of them were positive for nitrate reduction. These nitrate-reducing bacteria belonged to 19 genera and 29 species and among them, Qipengyuania flava, Roseibium aggregatum, Erythrobacter aureus, Vibrio campbellii, and Stappia indica were identified from all tested seawater layers of the photic zone and at almost all stations. Twenty-nine nitrate-reducing strains representing different species were selected for further the study of nitrogen, sulfur, and carbon metabolism. All 29 nitrate-reducing isolates contained genes encoding dissimilatory nitrate reduction or assimilatory nitrate reduction. Six nitrate-reducing isolates can oxidize thiosulfate based on genomic analysis and activity testing, indicating that nitrate-reducing thiosulfate-oxidizing bacteria exist in the photic zone. Five nitrate-reducing isolates obtained near the chlorophyll a-maximum layer contained a dimethylsulfoniopropionate synthesis gene and three of them contained both dimethylsulfoniopropionate synthesis and cleavage genes. This suggests that nitrate-reducing isolates may participate in dimethylsulfoniopropionate synthesis and catabolism in photic seawater. The presence of multiple genes for chitin degradation and extracellular peptidases may indicate that almost all nitrate-reducing isolates (28/29) can use chitin and proteinaceous compounds as important sources of carbon and nitrogen. Collectively, these results reveal culturable nitrate-reducing bacterial diversity and have implications for understanding the role of such strains in the ecology and biogeochemical cycles of nitrogen, sulfur, and carbon in the oligotrophic marine photic zone.

Time-Course and Tissue-Specific Molecular Responses to Acute Thermal Stress in Japanese Mantis Shrimp Oratosquilla oratoria.

Current understanding of adaptability to high temperatures is increasingly important as extreme weather events that can trigger immediate physiological stress in organisms have occurred more frequently. Here, we tracked transcriptomic responses of gills, hepatopancreas, and muscle to acute thermal exposure at 30 °C for 0.5, 6, and 12 h in an economically important crustacean, Oratosquilla oratoria, to gain a preliminary understanding of the tissue-specific and dynamic physiological regulation process under acute heat stress. The unique physiological responses of muscle, hepatopancreas, and gills to acute thermal stress were associated with protein degradation, lipid transport, and energy metabolism in O. oratoria, respectively. Functional enrichment analysis of differentially expressed transcripts and heat-responsive gene clusters revealed a biphasic protective responsiveness of O. oratoria developed from the early responses of signal transduction, immunity, and cytoskeleton reorganization to the responses dominated by protein turnover and energy metabolism at the mid-late stages under acute heat stress. Noteworthy, trend analysis revealed a consistently upregulated expression pattern of high molecular weight heat shock protein (HSP) family members (HSP60, HSP70, and HSP90) during the entire thermal exposure process, highlighting their importance for maintaining heat resistance in O. oratoria. Documenting the whole process of transcriptional responses at fine temporal resolution will contribute to a far-reaching comprehension of plastic responses to acute heat stress in crustaceans, which is critical in the context of a changing climate.

Astrocharis margarita sp. nov., a new snake star (Echinodermata, Ophiuroidea, Euryalidae) from a seamount in the Western Pacific.

During the scientific expedition of the R/V Kexue in 2014, a new species belonging to the genus Astrocharis Koehler, 1904 was collected from a seamount near the Yap Trench. Astrocharis margarita sp. nov. is distinguished from the other species of Astrocharis by the following characters: radial shields are partly naked or completely covered by polygonal ossicles, and the body color is uniformly light pink. Maximum Likelihood Tree and Bayesian Tree, based on a concatenated dataset of COI, 16S, and 18S genes analyses, indicated that the monophyly of Astrocharis was not supported, which suggested the genus needs to be revised.

Microbe-driven elemental cycling enables microbial adaptation to deep-sea ferromanganese nodule sediment fields.

BACKGROUND: Ferromanganese nodule-bearing deep-sea sediments cover vast areas of the ocean floor, representing a distinctive habitat in the abyss. These sediments harbor unique conditions characterized by high iron concentration and low degradable nutrient levels, which pose challenges to the survival and growth of most microorganisms. While the microbial diversity in ferromanganese nodule-associated sediments has been surveyed several times, little is known about the functional capacities of the communities adapted to these unique habitats. RESULTS: Seven sediment samples collected adjacent to ferromanganese nodules from the Clarion-Clipperton Fracture Zone (CCFZ) in the eastern Pacific Ocean were subjected to metagenomic analysis. As a result, 179 high-quality metagenome-assembled genomes (MAGs) were reconstructed and assigned to 21 bacterial phyla and 1 archaeal phylum, with 88.8% of the MAGs remaining unclassified at the species level. The main mechanisms of resistance to heavy metals for microorganisms in sediments included oxidation (Mn), reduction (Cr and Hg), efflux (Pb), synergy of reduction and efflux (As), and synergy of oxidation and efflux (Cu). Iron, which had the highest content among all metallic elements, may occur mainly as Fe(III) that potentially functioned as an electron acceptor. We found that microorganisms with a diverse array of CAZymes did not exhibit higher community abundance. Instead, microorganisms mainly obtained energy from oxidation of metal (e.g., Mn(II)) and sulfur compounds using oxygen or nitrate as an electron acceptor. Chemolithoautotrophic organisms (Thaumarchaeota and Nitrospirota phyla) were found to be potential manganese oxidizers. The functional profile analysis of the dominant microorganisms further indicated that utilization of inorganic nutrients by redox reactions (rather than organic nutrient metabolism) is a major adaptive strategy used by microorganisms to support their survival in the ferromanganese nodule sediments. CONCLUSIONS: This study provides a comprehensive metagenomic analysis of microbes inhabiting metal-rich ferromanganese nodule sediments. Our results reveal extensive redundancy across taxa for pathways of metal resistance and transformation, the highly diverse mechanisms used by microbes to obtain nutrition, and their participation in various element cycles in these unique environments. Video Abstract.

Microplastic sink that cannot be ignored in chemosynthetic organisms.

Marine microplastic (MP) pollution is a widespread concern; however, to date, MP pollution in chemoautotrophic ecosystems remains largely unknown. This study focuses on the cold seep in the South China Sea. Two dominant species, namely mussel (Gigantidas platifrons) and squat lobster (Shinkaia crosnieri), were collected for examining the MP pollution. MPs were present in both mussels and squat lobsters with abundances of 0.13 ± 0.04 and 0.17 ± 0.06 items/ind., respectively. MPs were mainly fibrous (62.5 %) and transparent (45.8 %). The main polymer type was polyester (54.2 %). About 86.5 MPs/m2 were found inhabiting mussels and squat lobsters-a value comparable to those reported in benthos. This pilot report on MP pollution in cold-seep species provides key information for studies on MP pollution in chemoautotrophic ecosystems and evidence regarding a potential biological MP sink. The role of cold-seep organisms in MP retention and transport in the regional sea merits further attention.

A new species of the coral-symbiont crab genus Cymo de Haan, 1833 (Decapoda, Brachyura, Xanthidae) from Nansha Islands, the South China Sea.

A new species of coral-symbiont crab, Cymo mazu sp. nov., is described from the Nansha Islands in the South China Sea. The new species is distinguished from its congeners by several unique morphological characteristics, including a smooth carapace armed with isolated spiny granules, chelipeds featuring large spines and granules, and a strongly concave endopodite of the first maxilliped. Molecular analysis using mitochondrial cytochrome oxidase I DNA barcodes provides further support for the identification of the new species. The relationships between the new species and its congeners were elucidated through a combination of morphological and molecular evidence. Diagnostic characteristics for differentiation among species of Cymo are discussed, and an updated key to the species of the genus is provided.

Going deeper and further: a range and depth extension for the deep-sea feather star Paratelecrinuscubensis (Carpenter, 1881) (Comatulida, Atelecrinidae), first record from the Western Pacific.

A specimen belonging to the deep-sea feather-star family Atelecrinidae was collected in April 2018 at the Kocebu Guyot at 1294 m deep. Based on its morphological characters, the specimen was identified as Paratelecrinuscubensis (Carpenter, 1881). This species of feather star is restricted to the deep sea and was known only from 12 records from the Bahamas and Cuba at depths of 567-892 m. The data represent the first record from the Western Pacific Ocean and the deepest record known, extending the depth where this feather star has been found to beyond 1000 m. The morphological characteristics of the Kocebu Guyot specimen were essentially identical to the morphology of the neotype, with a slight difference in the dorsal spine at the end of the cirri. The phylogenetic analysis based on the mitochondrial cytochrome c oxidase subunit I (COI), 16S rRNA genes, 28S rRNA genes, and 18S rRNA genes reveal a close relationship of P.cubensis with P.wyvilli. Results of our molecular phylogenetic analysis are consistent with our morphological identifications. Our find extends the known geographical distribution of the feather star P.cubensis to the Western Pacific Ocean and provide insights into deep-sea biodiversity in the Kocebu Guyot.

Cryptic diversity and population genetic structure of the mantis shrimp Oratosquilla oratoria in South Korea.

BACKGROUND: The Japanese mantis shrimp Oratosquilla oratoria has two genetic lineages in its Chinese and Japanese populations, which are considered cryptic species. However, the Korean O. oratoria populations have only been studied morphologically, and their genetic diversity pattern remains unknown. OBJECTIVE: To examine the genetic diversity of the Korean O. oratoria populations to explore its cryptic diversity and underlying processes. METHODS: We sequenced the mitochondrial cytochrome c oxidase I gene of 107 O. oratoria individuals from 8 sampling sites in South Korea. Their genetic diversity and haplotype data were compared to previously reported data from 16 Chinese and 6 Japanese populations. RESULTS: The haplotype network, analysis of molecular variance, FST values, and Bayesian skyline plot of the Korean O. oratoria populations confirmed that they were divided into two genetic lineages corresponding to the two previously identified cryptic species, which might be correlated to ocean currents and water temperature. In addition, the population genetic analysis indicated that both lineages had high genetic diversity. Although the Geoje region belongs to the southern group, it only contained lineage N, which could be related to the specific environmental characteristics of Geoje or the hybridization of the two lineages of O. oratoria. CONCLUSION: Korean O. oratoria populations have enough cryptic diversity to be described as cryptic species and are correlated with ocean currents and water temperatures. Lineage N dominance in Geoje might be related to environmental characteristics or hybridization.

Full-length 16S rRNA amplicon sequencing reveals the variation of epibiotic microbiota associated with two shrimp species of Alvinocarididae: possibly co-determined by environmental heterogeneity and specific recognition of hosts.

Shrimps of the family Alvinocarididae, endemic species to deep sea chemosynthetic ecosystems, harbor epibiotic microbes on gills which probably play important roles in the survival of the shrimps. Among them, Alvinocaris longirostris and Shinkaicaris leurokolos occupy different ecological niches within the same hydrothermal vent in Okinawa Trough, and A. longirostris also exists in a methane seep of the South China Sea. In this study, full-length 16S rRNA sequences of the gill associated bacteria of two alvinocaridid species from different chemosynthetically ecological niches were first captured by single-molecule real-time sequencing. Totally, 120,792 optimized circular consensus sequences with ∼1,450 bp in length were obtained and clustered into 578 operational taxonomic units. Alpha diversity analysis showed seep A. longirostris had the highest species richness and evenness (average Chao1 = 213.68, Shannon = 3.39). Beta diversity analysis revealed that all samples were clearly divided into three groups, and microbial community of A. longirostris from seep and vent were more related than the other comparisons. By permutational multivariate analysis of variance, the most significant community compositional variance was detected between seep A. longirostris and vent S. leurokolos (R 2 = 0.731, P = 0.001). The taxon tags were further classified into 21 phyla, 40 classes, 89 orders, 124 families and 135 genera. Overall, the microbial communities were dominated by Campylobacteria and Gammaproteobacteria. Alphaproteobacteria, Bacteroidia, Verrucomicrobiae, Bacilli and other minor groups were also detected at lower abundance. Taxonomic groups recovered from the vent S. leurokolos samples were only dominated by Sulfurovaceae (94.06%). In comparison, gill-associated microbiota of vent A. longirostris consisted of more diverse sulfur-oxidizing bacteria, including Sulfurovaceae (69.21%), Thiotrichaceae (6.77%) and a putative novel Gammaproteobacteria group (14.37%), while in seep A. longirostris, Gammaproteobacteria un-group (44.01%) constituted the major component, following the methane-oxidizing bacteria Methylomonadaceae (19.38%), and Sulfurovaceae (18.66%). Therefore, the gill associated bacteria composition and abundance of alvinocaridid shrimps are closely related to the habitat heterogeneity and the selection of microbiota by the host. However, the interaction between these alvinocaridid shrimps and the epibiotic communities requires further study based on metagenome sequencing and fluorescence in situ hybridization.

Complete mitochondrial genomes of four deep-sea echinoids: conserved mitogenome organization and new insights into the phylogeny and evolution of Echinoidea.

Echinoids are an important component in benthic marine environments, which occur at all depths from the shallow-water hard substrates to abyssal depths. To date, the phylogeny of the sea urchins and the macro-evolutionary processes of deep-sea and shallow water groups have not yet been fully resolved. In the present study, we sequenced the complete mitochondrial genomes (mitogenomes) of four deep-sea sea urchins (Echinoidea), which were the first representatives of the orders Aspidodiadematoida, Pedinoida and Echinothurioida, respectively. The gene content and arrangement were highly conserved in echinoid mitogenomes. The tRNA-Ser AGY with DHU arm was detected in the newly sequenced echinoid mitogenomes, representing an ancestral structure of tRNA-Ser AGY. No difference was found between deep-sea and shallow water groups in terms of base composition and codon usage. The phylogenetic analysis showed that all the orders except Spatangoida were monophyletic. The basal position of Cidaroida was supported. The closest relationship of Scutelloida and Echinolampadoida was confirmed. Our phylogenetic analysis shed new light on the position of Arbacioida, which supported that Arbacioida was most related with the irregular sea urchins instead of Stomopneustoida. The position Aspidodiadematoida (((Aspidodiadematoida + Pedinoida) + Echinothurioida) + Diadematoida) revealed by mitogenomic data discredited the hypothesis based on morphological evidences. The macro-evolutionary pattern revealed no simple onshore-offshore or an opposite hypothesis. But the basal position of the deep-sea lineages indicated the important role of deep sea in generating the current diversity of the class Echinoidea.

Mitogenomics provides new insights into the phylogenetic relationships and evolutionary history of deep-sea sea stars (Asteroidea).

The deep sea (> 200 m) is considered as the largest and most remote biome, which characterized by low temperatures, low oxygen level, scarce food, constant darkness, and high hydrostatic pressure. The sea stars (class Asteroidea) are ecologically important and diverse echinoderms in all of the world's oceans, occurring from the intertidal to the abyssal zone (to about 6000 m). To date, the phylogeny of the sea stars and the relationships of deep-sea and shallow water groups have not yet been fully resolved. Here, we recovered five mitochondrial genomes of deep-sea asteroids. The A+T content of the mtDNA in deep-sea asteroids were significantly higher than that of the shallow-water groups. The gene orders of the five new mitogenomes were identical to that of other asteroids. The phylogenetic analysis showed that the orders Valvatida, Paxillosida, Forcipulatida are paraphyletic. Velatida was the sister order of all the others and then the cladeValvatida-Spinulosida-Paxillosida-Notomyotida versus Forcipulatida-Brisingida. Deep-sea asteroids were nested in different lineages, instead of a well-supported clade. The tropical Western Pacific was suggested as the original area of asteroids, and the temperate water was initially colonized with asteroids by the migration events from the tropical and cold water. The time-calibrated phylogeny showed that Asteroidea originated during Devonian-Carboniferous boundary and the major lineages of Asteroidea originated during Permian-Triassic boundary. The divergence between the deep-sea and shallow-water asteroids coincided approximately with the Triassic-Jurassic extinction. Total 29 positively selected sites were detected in fifteen mitochondrial genes of five deep-sea lineages, implying a link between deep-sea adaption and mitochondrial molecular biology in asteroids.

A review of the common crab genus Macromedaeus Ward, 1942 (Brachyura, Xanthidae) from China Seas with description of a new species using integrative taxonomy methods.

Macromedaeus is one of the most common xanthid genera in shallow waters of the Indo-West Pacific. In this study, we describe a new species, Macromedaeus hainanensis sp. nov., and report on two newly recorded species, M. quinquedentatus (Krauss, 1843) and M. orientalis (Takeda & Miyake, 1969) from Hainan Island, South China Sea. M. hainanensis is most related to M. distinguendus (De Haan, 1833-1850) and M. orientalis on the carapace shape and granular appearance, but can be distinguished by unique morphological characteristics especially its front, pereopods and male first gonopod. Taxonomic identities of the six Macromedaeus species recorded from China seas are discussed, and a phylogenetic analyzation is performed on Macromedaeus and related taxa based on three mitochondrial and two nuclear markers (12S, 16S, COI, H3, 18S). Integrated taxonomic evidence is used to support the taxonomic status of each species.

The first two complete mitogenomes of the order Apodida from deep-sea chemoautotrophic environments: New insights into the gene rearrangement, origin and evolution of the deep-sea sea cucumbers.

The deep-sea ecosystem is considered as the largest and most remote biome of the world. It is meaningful and important to elucidate the life origins by exploring the origin and adaptive genetic mechanisms of the large deep-sea organisms. Sea cucumbers (Holothuroidea) are abundant and economically important group of echinoderms, living from the shallow-waters to deep-sea. In this study, we present the mitochondrial genomes of the sea cucumber Chiridota heheva and Chiridota sp. collected from the deep-sea cold seep and hydrothermal vent, respectively. This is the first reported mitochondrial genomes from the order Apodida. The mitochondrial genomes of C. heheva (17,200 bp) and Chiridota sp. (17,199 bp) display novel gene arrangements with the first protein-coding gene rearrangements in the class Holothuroidea. Bases composition analysis showed that the A + T content of deep-sea holothurians were significantly higher than that of the shallow-water groups. We compared the arrangement of genes from the 24 available holothurian mitogenomes and found that the transposition, reverse transposition and tandem-duplication-random-losses (TDRL) may be involved in the evolution of mitochondrial gene arrangements in Holothuroidea. Phylogenetic analysis revealed that the Apodida clustered with Elasipodida, forming two basal deep-sea holothurian clades. The divergence between the deep-sea and shallow-water holothurians was located at 386.93 Mya, during the Late Devonian. Mitochondrial protein-coding genes of deep-sea holothurians underwent relaxed purifying selection. There are 57 positive selected amino acids sites for some mitochondrial genes of the three deep-sea clades, implying they may involve in the adaption of deep-sea sea cucumbers.

Chromosome-level genome assembly of Paralithodes platypus provides insights into evolution and adaptation of king crabs.

The blue king crab, Paralithodes platypus, which belongs to the family Lithodidae, is a commercially and ecologically important species. However, a high-quality reference genome for the king crab has not yet been reported. Here, we assembled the first chromosome-level blue king crab genome, which contains 104 chromosomes and an N50 length of 51.15 Mb. Furthermore, we determined that the large genome size can be attributed to the insertion of long interspersed nuclear elements and long tandem repeats. Genome assembly assessment showed that 96.54% of the assembled transcripts could be aligned to the assembled genome. Phylogenetic analysis showed the blue king crab to have a close relationship with the Eubrachyura crabs, from which it diverged 272.5 million years ago. Population history analyses indicated that the effective population of the blue king crab declined sharply and then gradually increased from the Cretaceous and Neogene periods, respectively. Furthermore, gene families related to developmental pathways, steroid and thyroid hormone synthesis, and inflammatory regulation were expanded in the genome, suggesting that these genes contributed substantially to the environmental adaptation and unique body plan evolution of the blue king crab. The high-quality reference genome reported here provides a solid molecular basis for further study of the blue king crab's development and environmental adaptation.