Pelagic gastropods, including holoplanktonic and neustonic species, have been considered potential sources of trematodes that parasitize marine fishes. However, the taxonomy and life cycles of metacercariae detected in pelagic gastropods remain poorly understood. This study examined metacercaria infections in 10 species of pelagic gastropods (N = 415) collected from Japanese coastal waters between 2016 and 2025. Metacercariae were detected in four pelagic gastropod species and were identified as seven trematode species based on molecular analyses of the 28S ribosomal RNA and cytochrome c oxidase subunit 1 regions. In holoplanktonic gastropods, six species within the family Lepocreadiidae were molecularly identified as Cephalolepidapedon saba, Prodistomum orientale, Prodistomum sp., Opechona sp., Preptetos sp., and Lepocreadiidae gen. sp., all of which represent new host-parasite records. A metacercaria molecularly identified as Dinurus tornatus (Hemiuridae) was detected in the neustonic gastropod Glaucus atlanticus. These results confirm that pelagic gastropods can act as the second intermediate or paratenic hosts and suggest predator-prey relationships with marine fishes. In addition, rediae obtained from benthic gastropods (cowries) were examined molecularly and identified as Prodistomum sp., thereby clarifying aspects of its life cycle.

How symbionts acquired hosts and diversified phylogenetically during their evolutionary history is a focus of attention in many symbiotic taxa. Marine polyclad flatworms are usually free-living, but some are symbiotic, using animals as hosts. However, the history of their acquisition of symbiotic systems is not well understood. Therefore, we focused on mollusc symbiotic flatworms in the suborder Acotylea and investigated the host specificity and phylogenetic history of the acquisition of symbiosis. Field surveys revealed that symbiotic flatworms utilized certain molluscs as hosts. In particular, Stylochoplana pusilla and Stylochoplana parasitica utilized different molluscan species as hosts sympatrically. The phylogenetic analysis and the ancestral state reconstruction indicate that the mollusc symbiotic flatworms formed a monophyletic group and that their common ancestor shifted from free-living to mollusc symbiosis. These results suggest that each of the flatworms did not independently acquire a symbiotic system with molluscan hosts during its phylogenetic history, but that their common ancestor acquired a mollusc symbiotic system, which then underwent acquisition of host specificity and speciation. This study emphasizes that multiple host use can be a driving force for niche advancement and speciation in the symbionts.

Most species of sea spiders (pycnogonids) have been identified as predators, scavengers, or parasites, with some consuming polychaetes, although their prey is unknown in most species. A tiny (similar to 3 mm) pycnogonid preying on a juvenile polychaete was found in Aomori, Japan. The pycnogonid and polychaete were identified as Phoxichilidiidae sp. (a member of Anoplodactylus or Phoxiphilyra) and Thelepus japonicus, respectively, based on their morphological characteristics and mitochondrial 16S rRNA and/or COI gene sequence analysis. Our video demonstrates the pycnogonid feeding on the polychaete at around chaetiger 1, which is consistent with a previous report of a pycnogonid feeding on a nereidid polychaete. Our findings provide additional evidence of pycnogonids preying on polychaetes and signify the importance of polychaetes as a food source for pycnogonids.

Little is known about the diversity and ecology of digenean parasites in the abyssal ocean (3500-6500 m deep) with previous occurrence records down only to 4877 m. Here, we report a 28S rDNA sequence of the fish digenean genus Lepidapedon (Lepidapedidae), amplified from DNA extract of a velutinid gastropod collected at 6185-6221 m in the Kuril-Kamchatka Trench, Northwest Pacific. This represents the deepest record for the platyhelminth class Digenea and, to our knowledge, for any group of fish parasites. Our finding also verifies for the first time that abyssal lepidapedids exploit benthic snails as intermediate hosts. The present species of Lepidapedon presumably accomplishes its life cycle in the lowermost abyssal zone with macrourid fish as the definitive host.

The tideland snails Pirenellanipponica and Pirenellaasiatica are distributed north of the central Ryukyu Islands and in South Ryukyu, respectively, in Japan. To reveal their distribution and genetic characteristics in Taiwan, we sampled Pirenella snails along the western coast of Taiwan Island and analyzed the nucleotide sequences of their mitochondrial DNAs. Pirenella nipponica and P. asiatica inhabit the northern and southern parts of the western coast of Taiwan, respectively, and coexist only in the central part. Taiwanese and Japanese populations of P. asiatica showed significant genetic differentiation. The former showed higher genetic diversity and a larger effective population size than the latter. However, the Taiwanese population of P. nipponica was not genetically deviated from the local Japanese population on Kyushu Island. Both the Taiwan and Kyushu populations of P. nipponica showed significant genetic differences from local populations in other regions of Japan, namely, Honshu Island (the Japanese mainland) and Central Ryukyu. They also showed higher genetic diversity and a larger effective population size than the others. The Taiwanese populations of both species might be part of a large panmictic population with individuals from the Asian continent and Kyushu Island.

The nearly complete mitochondrial genome of the threatened tideland snail Pirenella pupiformis (Mollusca: Cerithioidea: Potamididae) was determined by shotgun next-generation sequencing. The mitogenome is comprised of 13 protein-coding genes (PCGs), two ribosomal RNA (12S and 16S) genes, and 22 transfer RNA genes (tRNAs). This gene order is consistent with the previously published mitochondrial genomes of other species belonging to the family Potamididae. The family Potamididae including P. pupiformis was recovered as a monophyletic group in the superfamily Cerithioidea.

Terpios hoshinota is a thin encrusting sponge that overgrows live scleractinian corals and it is linked to coral loss in many reefs. However, our knowledge of the species associated with this sponge species is poor. During a periodical survey of T. hoshinota in 2020, we found tiny snails crawling on the sponge in the subtropical waters around Okinawa Island, Japan. We observed egg capsules inside the sponge tissue and veliger larvae released from the egg capsules. Molecular analyses of both the snails and veliger larvae (cytochrome oxidase I, COI) showed that they were identical and belonged to Joculator sp. (family Cerithiopsidae). There was no direct observation of predation on the sponge by this snail; however, to the best of our knowledge, this is the first report on a close association between a snail and the sponge T. hoshinota.

Deep-sea fishes were collected by the R/V Sonne from the Kurile-Kamchatka Trench during the summer of 2016. A total 17 specimens were collected by Agassiz trawl and multi-plankton nets. We identified them and provided a species list that includes 2 bristlemouths (Cyclothone atraria and Sigmops gracilis), 2 lanternfishes (Stenobrachius leucopsarus and S. nannochir) and 2 grenadiers (Coryphaenoides acrolepis and C. cinereus) with systematic and/or geographic information. Although C. acrolepis was captured by Agassiz trawl net that reached the sea floor at 5134-8729 m depths, the specimens were thought to be collected from midwaters based on analysis of trimethylamine N-oxide (TMAO) and stomach contents. A single macrourid larva collected by Agassiz trawl net was identified as C. cinereus by morphological and molecular identification. New collection data were compared with those of the 39th voyage of the R/V Vityaz III in the Kurile-Kamchatka Trench during the summer of 1966.

1 Graduate School of Science and Engineering, Kagoshima University, Kagoshima 890-0065, Japan E-mail: sato@sci.kagoshima-u.ac.jp 2 National Institute for Environmental Studies, Tsukuba, Ibaraki 305-8506, Japan 3 Atmosphere and Ocean Research Institute, The University of Tokyo, Kashiwanoha, Kashiwa, Chiba 277-8564, Japan 4 Sesoko Station, Tropical Biosphere Research Center, University of the Ryukyus, Sesoko, Motobu, Okinawa 905-0227, Japan 5 Marine and Coastal Resources Institute, Prince of Songkla University, Hat Yai, Songkhla 90110, Thailand 6 Corresponding author

The mitochondrial genome of the neritid snail Nerita (Heminerita) japonica (Mollusca: Neritimorpha) from Kumamoto, Japan was determined by whole-genome sequencing. This mitogenome is comprised of 13 protein-coding genes, 2 ribosomal RNA (12S and 16S) genes, and 22 transfer RNA genes, with the same gene order as in the other species of the family Neritidae. A likelihood-based phylogenetic reconstruction recovered the subgenus Heminerita (including N. japonica as its type and N. yoldii from China) as monophyletic and sister to a clade with four species of the subgenera Nerita and Theliostyla.

The joint German-Russian expedition KuramBio II in 2016 provided a significant opportunity to reinvestigate the species composition and distribution of the abyssal and hadal gastropod fauna in the Kuril-Kamchatka Trench, which was sampled previously only by the Russian research vessel (R/V) Vityaz in the 1940-1960s. During the cruise of the German R/V Sonne, benthic animals were collected using an Agassiz trawl, an epibentic sledge, a multiple corer and a giant box corer. A total catch of 592 live specimens of non-heterobranch gastropods were classified into 86 species and 22 families of the subclasses Neomphaliones, Vetigastropoda, and Caenogastropoda. The abundance, richness and phylogenetic diversity were lower in the hadal zone than in the abyssal zone. Seven hadal stations (6,442-9,584 m) yielded a total of 101 specimens belonging to 22 species in 11 families, whereas catches from four abyssal stations on the landward and seaward slopes (5,102-6,221 m) included 491 specimens and 70 species in 21 families. The abundance, richness and diversity for the hadal zone are, however, particularly high in comparison with other oceanic trenches, due, presumably, to high primary production in the surface layer of the subarctic Northwest Pacific. The uniqueness of species composition increased with depth; sixteen out of the 22 species from the hadal stations (72.7%) were found exclusively in this depth zone. Two unnamed species of Neomphalidae and Sahlingia from 9,577 to 9,584 m represent the world's deepest records for Neomphaliones and Vetigastropoda and suggest the as yet unidentified presence of the world-deepest cold-seep habitats in the Kuril-Kamchatka Trench.

The mitochondrial genome of the cowry snail Monetaria annulus (Caenogastropoda: Cypraeoidea: Cypraeidae) was determined by whole-genome next-generation sequencing. The mitogenome is composed of 13 protein-coding genes (PCGs), 2 ribosomal RNA (12S and 16S) genes, and 22 transfer RNA genes (tRNAs). This gene order is consistent with the previously published mitochondrial genomes of other species belonging to the order Littorinimorpha. The superfamily Cypraeoidea was recovered as a sister clade to the group of Tonnoidea and Neogastropoda.

Benthic invertebrates at deep-sea hydrothermal vents and cold seeps have attracted considerable attention regarding their spatiotemporal distributions, colonization pathways, geological origins and morphological, ecological and physiological adaptations. Here we first reconstruct a molecular phylogeny for vent and seep species in the gastropod subclass Neritimorpha based on combined mitochondrial and nuclear gene sequences. The resulting tree, in conjunction with anatomical and palaeontological evidence, indicates that neritimorph snails and limpets in the Cenozoic deep-sea chemosynthetic habitats belong to the monophyletic subfamily Shinkailepadinae (family Phenacolepadidae). Confamilial shallow-water species form its sister clade, the subfamily Phenacolepadinae. There were probably at least three independent shifts from the coiled snail with a functional operculum to the limpet form in the evolutionary history of the family, twice in the deep-sea Shinkailepadinae and once in the shallow-water Phenacolepadinae. Physiological and ecological characteristics including the presence of red blood cells and larval migration seem to have facilitated their early Cenozoic colonization of vents and seeps. The type specimens of type species were investigated for most nominal genera to amend generic classification of this long-neglected family. A monotypic Divia gen. nov. is proposed for Shinkailepas briandi; Shinkailepas and Thalassonerita pre-date Olgasolaris and Bathynerita, respectively.

Macrofauna (46,343 invertebrates from 41 taxa) were collected by means of a camera-epibenthic sledge (CEBS) during the expedition SokhoBio (Sea of Okhotsk Biodiversity Studies) on board of the RV Akademik M.A. Lavrentyev in 2015. In total 11 sites and 22 stations were sampled at bathyal and abyssal depths ranging from 1696 m to 4798 m in the Kuril Basin of the Sea of Okhotsk (16 stations), the Bussol Strait (two stations) and western slope of the Kuril-Kamchatka Trench (KKT) (four stations). Polychaetes occurred most frequently in the samples with 17,546 individuals, followed by Peracarida with 14,099 individuals (5625 isopods, 3887 amphipods, 3225 tanaids, 1269 cumaceans, and 90 mysids), Copepoda (4679 ind.) and Bivalvia (3999 ind.). Numbers of individuals ranged between 140 at station 3-10-6383 at station 7-3. The numbers of taxa per station were smallest at station 3-10 (4 taxa) and highest at station 2-7 (31 taxa). Peracarida were the most common malacostracan crustaceans recorded, with Isopoda comprising 40% of all peracarids within this fraction, followed by Amphipoda with 27%, Tanaidacea with 23%, Cumacea with 9% and Mysida with 1%. At station 3-10, not a single peracarid was sampled. Isopoda dominated most of the stations, especially in the abyssal sites of the Kuril Basin. Amphipoda occurred in higher numbers in the Bussol Strait at site 8 as well as at Pacific station 9-7. In the SokhoBio area, studied macrofauna was distributed very patchily. In terms of diversity and composition the abyssal macrofauna of the Kuril Basin shows an intermediate state between the adjacent studied abyssal areas: the semi-isolated Sea of Japan and the open abyssal plain of the Kuril-Kamchatka Trench (KKT) area.

Evolutionary transitions from free-living to symbiotic lifestyles often lead to dramatic changes in morphological, ecological and physiological characteristics. Galeommatoidea represents a highly diverse superfamily of Bivalvia, with > 620 described species in multiple free-living and symbiotic Glades and, thereby, provides a unique opportunity to investigate the character evolution associated with lifestyle transitions. Barrimysia cumingii is a commensal galeommatoidean that lives in the deep burrow of the strahlaxiid shrimp Neaxius acanthus. The burrow is a sulphiderich reducing environment owing to poor water circulation and decay of seagrass leaves accumulated by the shrimp. In this study, we found that abundant spherical red blood cells (RBCs; similar to 10 mu m in diameter) with one prominent nucleus occur in the haemocoel of B. cu,ningii. This is the first report of RBCs for the superfamily. Our phylogenetic reconstruction based on four-gene DNA sequences indicates a relatively recent divergence of B. cumingii from free-living ancestors without RBCs and the origin of RBCs in association with the colonization of the shrimp burrow. Other bivalve lineages with RBCs tend to occur in hypoxic habitats, including mangrove swamps and deep-sea hydrothermal vents and cold seeps, suggesting that B. cumingii has also obtained RBCs as a physiological adaptation to within-host-burrow hypoxia.

Based on material from deep-sea expeditions including SokhoBio, KuramBio I, and KuramBio II, five species of ringiculid gastropods were found to inhabit the upper bathyal zone of the Sea of Okhotsk, lower bathyal and abyssal depths of the Kuril–Kamchatka Trench and adjacent waters. Three species are described here as new to science: Ringicula minichevi sp. nov., Microglyphis hasegawai sp. nov., and Ringiculoides vityazi sp. nov. Data on the anatomy of Microglyphis are provided for the first time. Two species occur in the bathyal zone (Microglyphis furukawai and R. minichevi sp. nov.), while the other three are exclusively abyssal (Ringiculoides kurilensis, R. vityazi sp. nov.) or eurybathic (M. hasegawai sp. nov.). The two species of Ringiculoides have closely similar shells and overlapping geographic and bathymetric distributions, thus highlighting the importance of combined molecular and morphological analyses for accurate taxonomy of the group. Our literature review suggests that the Northwest Pacific has the highest taxonomic diversity of bathyal and abyssal ringiculids, whereas the Sea of Okhotsk is inhabited solely by M. furukawai (≤ 591m).

The red-blooded limpet ‘Shinkailepas’ briandi (Neritimorpha: Phenacolepadidae) is one of the commonest gastropod species at deep-sea hydrothermal vents on the Mid-Atlantic Ridge (MAR). We investigated its population connectivity along MAR as the first such study for gastropods and explored the importance of larval migration for the distribution of vent-endemic animals. Our analyses, based on 1.3-kbp DNA sequences from the mitochondrial COI gene, showed a panmictic population throughout its geographic and bathymetric ranges that span from the northernmost and shallowest Menez Gwen vent field (38°N; 814–831 m depth) to the southernmost and deepest Ashadze field (13°N; 4090 m). Early development of this species is presumed to have a long pelagic duration as a planktotrophic larva; the hatchling with a shell diameter of 170–180 μm attains a constant settlement size of 706 ± 8 μm (mean ± SD). Retention of eye pigmentation in newly settled juveniles, along with the genetic panmixia, suggests that the hatched larva of ‘S.’ briandi migrates vertically to the surface water, presumably to take advantage of richer food supplies and stronger currents for dispersal, as has been shown for confamilial species at hydrothermal vents and cold methane seeps.