Genome sizes and repeatome evolution in zoantharians (Cnidaria: Hexacorallia: Zoantharia).

Across eukaryotes, large variations of genome sizes have been observed even between closely related species. Transposable elements as part of the repeated DNA have been proposed and confirmed as one of the most important contributors to genome size variation. However, the evolutionary implications of genome size variation and transposable element dynamics are not well understood. Together with phenotypic traits, they are commonly referred to as the "C-value enigma". The order Zoantharia are benthic cnidarians found from intertidal zones to the deep sea, and some species are particularly abundant in coral reefs. Despite their high ecological relevance, zoantharians have yet to be largely studied from the genomic point of view. This study aims at investigating the role of the repeatome (total content of repeated elements) in genome size variations across the order Zoantharia. To this end, whole-genomes of 32 zoantharian species representing five families were sequenced. Genome sizes were estimated and the abundances of different repeat classes were assessed. In addition, the repeat overlap between species was assessed by a sequence clustering method. The genome sizes in the dataset varied up to 2.4 fold magnitude. Significant correlations between genome size, repeated DNA content and transposable elements, respectively (Pearson's correlation test R2 = 0.47, p = 0.0016; R2 = 0.22, p = 0.05) were found, suggesting their involvement in the dynamics of genome expansion and reduction. In all species, long interspersed nuclear elements and DNA transposons were the most abundant identified elements. These transposable elements also appeared to have had a recent expansion event. This was in contrast to the comparative clustering analysis which revealed species-specific patterns of satellite elements' amplification. In summary, the genome sizes of zoantharians likely result from the complex dynamics of repeated elements. Finally, the majority of repeated elements (up to 70%) could not be annotated to a known repeat class, highlighting the need to further investigate non-model cnidarian genomes. More research is needed to understand how repeated DNA dynamics relate to zoantharian evolution and their biology.

Preliminary study on the acute effects of hydrogen sulfide on Amphipoda (Lysianassoidea; Pseudorchomene sp. and Anonyx sp.) collected from deep-sea floors in the Sea of Japan.

To study the environmental impact of the assessment technologies for the development of shallow methane hydrate zones in the Sea of Japan, deep-sea amphipods (Pseudorchomene sp. and Anonyx sp.) were collected from a depth of approximately 1000 m and were tested for H2S toxicity. At 0.57 mg L-1 H2S, all specimens of Pseudorchomene sp. were dead after 96 h, whereas all individuals survived at 0.18 mg L-1. Moreover, Anonyx sp. had a survival rate of 17 % after 96 h at 0.24 mg L-1. A similar toxicity test was conducted with the coastal amphipod Merita sp., a detritivore, and all individuals died within 24 h at 0.15 mg L-1. These results suggested that compared with coastal detritivorous amphipods, deep-sea detritivorous amphipods, which also live near biomats with sediment H2S concentrations exceeding 10 mg L-1, showed a higher tolerance to H2S.

A new hexactinellid-sponge-associated zoantharian (Porifera, Hexasterophora) from the northwestern Pacific Ocean.

Symbiotic associations between zoantharians and sponges can be divided into two groups: those that associate with Demospongiae and those that associate with Hexactinellida. Parachurabanashinseimaruae Kise, gen. nov. et sp. nov., a new genus and a new species of Hexactinellida-associated zoantharian from Japanese waters, is described. It is characterized by a combination of the following: i) its host hexactinellid sponge, ii) very flat polyps, iii) cteniform endodermal marginal muscles, and iv) characteristic mutations in three mitochondrial regions (including a unique 26-bp deletion in 16S ribosomal DNA) and three nuclear regions. Parachurabanashinseimaruae Kise, gen. nov. et sp. nov. is the third genus in the family Parazoanthidae that is reported to be associated with Hexasterophora sponges. Although specimens have so far only been collected on Takuyo-Daigo Seamount off Minami-Torishima Island in Japan, unidentified zoantharians of similar description have been reported from the waters around Australia, indicating that the species might be widespread across the Pacific.

The complete mitochondrial genome of the invasive cyanobacteriosponge Terpios hoshinota (Demospongiae, Suberitida, Suberitidae).

The cyanobacteriosponge Terpios hoshinota occurs on tropical reefs throughout the Indo-Pacific. The species encrusts live coral, and other benthos, and is considered a pest species that can threaten the health and productivity of locally native benthic communities on coral reefs. Here we assemble a complete mitochondrial genome to aid further research into the range expansion of this species. The circular genome was 20,504 bp in length and encoded 14 protein-coding genes, two ribosomal RNA (rRNA) genes, and 25 transfer RNA (tRNA) genes. A phylogenetic analysis based on the concatenated sequences of 14 protein-coding genes of 12 members of the subclass Heteroscleromorpha including the newly sequenced T. hoshinota, suggests further taxonomic revisions within the order Suberitida may be warranted.

The Complete Genome Sequences of 7 Species of Epizoanthus (Epizoanthidae, Zoantharia, Hexacorallia, Cnidaria).

We present the complete genome sequences of 7 species of the zoantharian genus Epizoanthus. Illumina sequencing was performed on genetic material from single wild-collected individuals. The reads were assembled using a de novo method followed by a finishing step. The raw and assembled data are publicly available via GenBank.

The Complete Genome Sequences of 13 Species of Brachycnemina (Cnidaria, Hexacorallia, Anthozoa, Zoantharia).

We present the complete genome sequences of 13 species of the zoantharian suborder Brachycnemina (Haddon, and Shackleton, 1891; Poliseno et al., 2020). Genome sequencing was performed on genetic material from single wild-collected individuals. For each species the sequence reads were assembled using a de novo method followed by a finishing step. The raw and assembled data are publicly available via GenBank.

Genetic population structures of common scavenging species near hydrothermal vents in the Okinawa Trough.

Deep-sea mining of hydrothermal deposits off the coast of Japan is currently under consideration, and environmental baseline studies of the area are required to understand possible impacts. The aim of this study is to clarify population structures of dominant benthic megafaunal species near hydrothermal vent fields in the Okinawa Trough, using a population genetics approach. We examined dominant deep-sea scavenging species including eels, several amphipods, and a decapod and performed population genetic analyses based on the mitochondrial cytochrome c oxidase subunit I region. Several sites were sampled within Okinawa Trough to examine intra-population diversity while two other locations 1400-2400 km away were chosen for inter-population comparisons. For synaphobranchid eels Simenchelys parasitica and Synaphobranchus kaupii, our results showed significant intra-population diversity but no inter-population genetic differentiation, suggesting strong genetic connectivity and/or large population sizes. In addition, single nucleotide polymorphism analysis also confirmed strong genetic connectivity for Simenchelys parasitica. Among scavenging amphipods, we detected seven putative species using molecular phylogenetic analysis. We evaluated population structures of the most abundant species of amphipods and a decapod species (Nematocarcinus lanceopes). Our results provide basic information on the genetic population structures of benthic megafaunal species near hydrothermal vent fields, which can be used to select candidate species for future connectivity analysis with high-resolution genetic markers and aid understanding of the potential population impacts of environmental disturbances.

Evolutionary patterns of host switching, lifestyle mode, and the diversification history in symbiotic zoantharians.

Symbioses play important roles in forming the structural and distributional patterns of marine diversity. Understanding how interspecies interactions through symbioses contribute to biodiversity is an essential topic. Host switching has been considered as one of the main drivers of diversification in symbiotic systems. However, its process and patterns remain poorly investigated in the marine realm. Hexacoral species of the order Zoantharia (=zoantharians) are often epizoic on other marine invertebrates and generally use specific taxa as hosts. The present study investigates the patterns of host switching and the diversification history of zoantharians based on the most comprehensive molecular phylogenetic analyses to date, using sequences from three mitochondrial and three nuclear markers from representatives of 27 of 29 genera. Our results indicate that symbiotic zoantharians, in particular those within suborder Macrocnemina, diversified through repeated host switching. In addition, colonization of new host taxa appears to have driven morphological and ecological specialization in zoantharians. These findings have important implications for understanding the role of symbioses in the morphological and ecological evolution of marine invertebrates.

A new Antipathozoanthus species (Cnidaria, Hexacorallia, Zoantharia) from the northwest Pacific Ocean.

A new species of zoantharian within the genus Antipathozoanthus is described based on specimens collected from the coast of mainland Japan, northwest Pacific Ocean. Antipathozoanthustubus sp. nov. is characterized by its substrate (epibiotic on polychaete tube) and habitat (exposed rock). As well, the results of molecular phylogenetic analyses using concatenated multiple genetic markers also support the distinction between A.tubus sp. nov. and its congenerics. Antipathozoanthustubus sp. nov. is the first species of Antipathozoanthus species reported to be epibiotic on polychaete tubes, and is the second species in the genus that is not associated with antipatharians.

A New Epizoanthus Species (Cnidaria: Anthozoa: Epizoanthidae) Associated with the Gastropod Mollusk Guildfordia triumphans from Southern Japan.

From previous research, it is known that hermit crabs predominantly dwell in vacated gastropod shells. Several epibiotic taxa are known to live on gastropod shells inhabited by hermit crabs, including some species of the zoantharian genus Epizoanthus. Although many previous taxonomic studies have focused on hermit-crab-associated Epizoanthus species, and have resulted in the description of several species, gastropod-associated species have received comparatively much less attention. At least five Epizoanthus species associated with gastropods have been formally described, but some species have not been found or examined since their original description. In Japan, specimens on the gastropod mollusk Guildfordia triumphans have been found and examined in previous studies, but no formal taxonomic conclusions were made. In this study, we formally describe Epizoanthus rinbou sp. n. from southern Japan based on molecular phylogenetic analyses combined with morphological observations. Epizoanthus rinbou sp. n. is located within an Epizoanthus clade consisting of species associated with gastropods, hermit crabs, sea urchins, and barnacles, as well as non-associated Epizoanthus species. The present study highlights the utility of molecular phylogeny for understanding the diversity and relationships of gastropod-associated Epizoanthus species.

A phylogeny and the evolution of epizoism within the family Hydrozoanthidae with description of a new genus and two new species.

The Family Hydrozoanthidae are macrocnemic zoantharians, however their phylogenetic position is closer to brachycnemic zoantharians than to other macrocnemic zoantharians. Previous studies have indicated the presence of undescribed Hydrozoanthidae species from various locations in the Indo-Pacific Ocean. In this study, two new Hydrozoanthidae species, Aenigmanthus segoi gen. n., sp. n. and Hydrozoanthus sils sp. n., are described from Japanese and Palauan waters based on combined morphological and molecular phylogenetic analyses utilizing multiple genetic markers. Additionally, Hydrozoanthidae consists of species with an obligate epizoic relationship with hydroids (Hydrozoanthus) and of species with facultative epizoic relationships (Aenigmanthus gen. n. and Terrazoanthus). Results of ancestral state reconstruction analyses indicate that Hydrozoanthus gained obligate epizoic relationships in their evolutionary history perhaps due to structural differences of host invertebrates.

Three new species and the molecular phylogeny of Antipathozoanthus from the Indo-Pacific Ocean (Anthozoa, Hexacorallia, Zoantharia).

In this study, three new species of macrocnemic zoantharians (Hexacorallia, Zoantharia) are described from localities in the Indo-Pacific Ocean including the Red Sea, the Maldives, Palau, and southern Japan: Antipathozoanthus obscurussp. n., A. remengesauisp. n., and A. cavernussp. n. Although the genus Antipathozoanthus is currently restricted to species living on antipatharians, A. obscurussp. n. is not associated with any living substrate and instead is found on coral reef carbonate substrate within narrow caves or cracks. The two new species that have association with antipatharians, A. remengesauisp. n. and A. cavernussp. n., can be distinguished by their relative coenenchyme development and the antipatharian species that each uses as substrate. Additionally, all new species described in this study have unique nuclear internal transcribed spacer region of ribosomal DNA (ITS-rDNA) sequences. Our results indicate that more phylogenetic studies focusing on increasing the numbers of species examined within each of the genera of Parazoanthidae are required in order to better understand the evolutionary history of substrate specificity within the family Parazoanthidae.

Unexpected diversity and a new species of Epizoanthus (Anthozoa, Hexacorallia) attached to eunicid worm tubes from the Pacific Ocean.

Epizoanthus species are generally found in association with other marine invertebrates such as hermit crabs and gastropods. Although Epizoanthus spp. are relatively common, there is limited information about their diversity and ecology due to their habitats or hosts, often being below the depths of SCUBA diving (>~50 m). In particular, the Epizoanthus fauna of the Indo-Pacific Ocean remains poorly understood. In this study, the diversity of Epizoanthus species associated with eunicid worm tubes from shallow waters in the Pacific Ocean we investigated using molecular analyses (mitochondrial cytochrome oxidase subunit 1 = COI, mitochondrial 16S ribosomal DNA = mt 16S-rDNA, nuclear internal transcribed spacer region of ribosomal DNA = ITS-rDNA) combined with morphological and ecological data. The combined data set leads us to describe two new species; Epizoanthus inazuma sp. n. and Epizoanthus beriber sp. n. Both new species are found in low-light environments: Epizoanthus inazuma sp. n. on mesophotic coral reef slopes and reef floors, or on the sides of overhangs; Epizoanthus beriber sp. n. has only been found in caves. Morphological characteristics of these two new species are very similar to Epizoanthus illoricatus Tischbierek, 1930 but the two new species are genetically distinct. Mesentery numbers and coloration of polyps may be useful diagnostic characteristics among eunicid-associated Epizoanthus species. These results demonstrate that there is high potential for other potentially undescribed zoantharian species, particularly in underwater cave habitats.