Correction to: 'Revisiting Stygocapitella (Annelida, Parergodrilidae) in Japan, with insights into their amphi-Pacific diversification' (2024), by Natsumi Hookabe et al.

[This corrects the article DOI: 10.1098/rsos.231782.].

Revisiting Stygocapitella (Annelida, Parergodrilidae) in Japan, with insights into their amphi-Pacific diversification.

Polychaetes are typically found in marine environments with limited species adapting to semi-terrestrial habitats. The genus Stygocapitella comprises interstitial polychaetes dwelling in sandy beach areas around or above the high-water line. Based on molecular data, previous studies suggested the presence of multiple cryptic species in some different localities in the world lumped together as Stygocapitella subterranea. In Japan, reports on Stygocapitella were scarce, with only one species having been documented 40 years ago at Ishikari Beach in Hokkaido by the name of S. subterranea. We revisited these earlier findings and uncovered the presence of two distinct species in Stygocapitella. One of these species is herein named Stygocapitella itoi sp. nov., while the other corresponds to S. budaevae, originally described from the Russian Far East. Stygocapitella itoi sp. nov. possesses a chaetal pattern similar to that of S. australis, S. furcata and S. pacifica but can be distinguished from the congeners by two characters: a slightly forked pygidium and forked chaetae consisting of two teeth and two outer prongs. Our multi-locus phylogenetic analysis showed close relationships across the Pacific Ocean in two separated lineages in the genus, suggesting ancient dispersal or allopatric speciation after vicariance events.

Discovery of deep-sea acoels from a chemosynthesis-based ecosystem.

Chemosynthesis-based ecosystems such as hydrothermal vents and hydrocarbon seeps harbour various endemic species, each uniquely adapted to the extreme conditions. While some species rely on obligatory relationships with bacterial symbionts for nutrient uptake, scavengers and predators also play important roles in food web dynamics in these ecosystems. Acoels, members of the phylum Xenacoelomorpha, are simple, worm-like invertebrates found in marine environments worldwide but are scarcely understood taxa. This study presents a novel genus and species of acoel from a deep-sea hydrocarbon seep off Hatsushima, Japan, Hoftherma hatsushimaensis gen. et sp. nov. Our multi-locus phylogenetic analysis revealed that the acoels are nested within Hofsteniidae, a family previously known exclusively from shallow waters. This finding suggests that at least two independent colonization events occurred in the chemosynthesis-based environments from the phylum Xenoacoelomorpha, represented by hofsteniid acoels and Xenoturbella. Previous reports of hofsteniid species from low-oxygen and sulfide-rich environments, including intertidal habitats with decomposing leaves, in addition to H. hatsushimaensis gen. et sp. nov. from a deep-sea hydrocarbon seep, imply a common ancestral adaptation to sulfide-rich ecosystems within Hofsteniidae. Moreover, the sister relationship between solenofilomorphid acoels predominating in sulfide-rich habitats indicates common ancestral adaptation to sulfide-rich ecosystems between these two families.

A new genus and species of nudibranch-mimicking Syllidae (Annelida, Polychaeta).

Nudibranch mollusks, which are well-known for their vivid warning coloration and effective defenses, are mimicked by diverse invertebrates to deter predation through both Müllerian and Batesian strategies. Despite extensive documentation across different taxa, mimickers have not been detected among annelids, including polychaetes, until now. This study described a new genus and species of polychaete living on Dendronephthya octocorals in Vietnam and Japan. Belonging to Syllidae, it exhibits unique morphological adaptations such as a low number of body segments, simple chaetae concealed within the parapodia and large and fusiform antennae and cirri. Moreover, these appendages are vividly colored, featuring an internal dark red area with numerous terminal white spots and bright yellow tips, effectively contributing to mimicking the appearance of a nudibranch. This discovery not only documents the first known instance of such mimicry among annelids, but also expands our understanding of evolutionary adaptation and ecological strategies in marine invertebrates.

Two new species of Parahesione (Annelida: Hesionidae) associated with ghost shrimps (Crustacea: Decapoda) and their phylogenetic relationships.

Two new species of Hesionidae, Parahesione pulvinata sp. nov. and Parahesione apiculata sp. nov. are described based on materials collected at tidal flats in Okinawa (Japan) from burrows of the ghost shrimps Neocallichirus jousseaumei and Glypturus armatus. The two new species are characterized by having eight enlarged cirri, dorsal cirrophores with dorsal foliose lobe and biramous parapodia, and by lacking median antenna. Parahesione apiculata sp. nov. has digitate lobes on the posterior margin of the dorsal foliose lobe (absent in P. pulvinata sp. nov.). The two new species were never found outside the ghost shrimp burrows, suggesting they are obligate symbionts. Phylogenetic analyses based on four concatenated genes suggest that the symbiotic lifestyle has evolved several times in Hesionidae.

A New Species of Branchellion (Hirudinea: Piscicolidae) Parasitizing the Gills of Short-Tail Stingrays (Batoidea: Dasyatidae) from the West Pacific.

A new fish leech, Branchellion brevicaudatae sp. n., is described based on specimens parasitizing the gills of the short-tail stingray, Bathytoshia brevicaudata (Hutton, 1875), collected from Japanese waters. The new species can be distinguished from other congeners by having: i) pulsating vesicles emerging from posterior base of branchiae, one pair per somite; ii) dorsal white spots, not arranged in longitudinal row; and iii) blackish body. A phylogenetic tree based on partial sequences of the mitochondrial cytochrome c oxidase subunit I gene from the new species and other piscicolid worms showed that the new species is sister to Branchellion torpedinis Savigny, 1822. This is the first record of Branchellion Savigny, 1822 from Japanese waters.

From commensalism to parasitism within a genus-level clade of barnacles.

Understanding how animals evolve to become parasites is key to unravelling how biodiversity is generated as a whole, as parasites could account for half of all species richness. Two significant impediments to this are that parasites fossilize poorly and that they retain few clear shared morphological features with non-parasitic relatives. Barnacles include some of the most astonishingly adapted parasites with the adult body reduced to just a network of tubes plus an external reproductive body, but how they originated from the sessile, filter-feeding form is still a mystery. Here, we present compelling molecular evidence that the exceedingly rare scale-worm parasite barnacle Rhizolepas is positioned within a clade comprising species currently assigned to Octolasmis, a genus exclusively commensal with at least six different phyla of animals. Our results imply that species in this genus-level clade represent an array of species at various transitional stages from free-living to parasitic in terms of plate reduction and host-parasite intimacy. Diverging only about 19.15 million years ago, the route to parasitism in Rhizolepas was associated with rapid modifications in anatomy, a pattern that was likely true for many other parasitic lineages.

Theama japonica sp. nov., an Interstitial Polyclad Flatworm Showing a Wide Distribution along Japanese Coasts.

We establish a new interstitial polyclad species, Theama japonica sp. nov., based on specimens collected from coarse-sandy habitats in three Japanese main islands (Hokkaido, Honshu, and Shikoku) along the coasts of the Pacific Ocean and the Sea of Japan. Theama japonica is characterized by i) two pairs of cerebral eyespots and four to six precerebral eyespots; ii) eosinophilic secretion glands distributed in the distal half of the inner ventral part of the prostatic vesicle; iii) a conical penis papilla, bent up dorsally, with a sclerotized inner wall; iv) the prostatic sheath with an inner angular fold on the dorso-distal side; and v) the external cilia longer dorsally than ventrally. Partial sequences of the cytochrome c oxidase subunit I (COI) gene from 20 specimens collected at eight localities along Japanese coasts represented 19 haplotypes. The uncorrected p-distances among these COI haplotypes fell within intraspecific variations observed in other polyclads. A network analysis based on these COI haplotypes suggested a geographically non-cohesive genetic structure of the species, possibly indicating the species' high dispersibility. Molecular phylogenetic analyses based on a concatenated dataset of 18S and 28S rDNA sequences showed T. japonica formed a clade with other Theama species. The resulting tree also indicates that our new species is more closely related to Theama sp. from Colombia than species from Panama and Croatia.

Another piece for the syllid puzzle: A new species from Japan and its mitochondrial genome reveal the enigmatic Clavisyllis (Phyllodocida: Syllidae) as a member of Eusyllinae.

The phylogenetic relationships of Syllidae have been analyzed in several studies during the last decades, resulting in highly congruent topologies. Most of the subfamilies were found to be monophyletic, while other groups (Eusyllinae and several genera) have been reorganized attending their phylogenetic relationships. However, there are still several enigmatic genera, which could not be assigned to any of the established subgroups. These enigmatic genera usually show a combination of characters indicating relationships with several different groups, and some show morphological traits unique to Syllidae. One of the most intriguing genera, still unclassified within Syllidae is Clavisyllis Knox. Herein, we provide a complete description of a new species Clavisyllis tenjini n. sp. from Japan. We sequence the complete mitochondrial genome, compare with the available data from other syllids, and perform a phylogenetic analysis of three genes (18S, 16S, COI), traditionally used in previous studies. Clavisyllis shows a unique combination of characters within Syllidae, such as nuchal lappets and large ovoid dorsal cirri. The new species has additional anterior appendages that have not been found in any other syllid. Our results show the genus is a member of Eusyllinae, closely related to Pionosyllis Malmgren. The mitochondrial gene order agrees with the considered plesiomorphic gene order in Annelida, which is present in all members of Eusyllinae investigated so far. Clavisyllis reproduces by epigamy, the reproductive mode of members of Eusyllinae. The present study contributes to the systematics of Syllidae, a complex group with a large number of species and striking reproductive modes.

Investigating the diversity of bioluminescent marine worm Polycirrus (Annelida), with description of three new species from the Western Pacific.

Bioluminescence, a phenomenon observed widely in organisms ranging from bacteria to metazoans, has a significant impact on the behaviour and ecology of organisms. Among bioluminescent organisms, Polycirrus, which has unique emission wavelengths, has received attention, and advanced studies such as RNA-Seq have been conducted, but they are limited to a few cases. In addition, accurate species identification is difficult due to lack of taxonomic organization. In this study, we conducted comprehensive taxonomic survey of Japanese Polycirrus based on multiple specimens from different locations and described as three new species: Polycirrus onibi sp. nov., P. ikeguchii sp. nov. and P. aoandon sp. nov. The three species can be distinguished from the known species based on the following characters: (i) arrangement of mid-ventral groove, (ii) arrangement of notochaetigerous segments, (iii) type of neurochaetae uncini, and (iv) arrangement of nephridial papillae. By linking the bioluminescence phenomenon with taxonomic knowledge, we established a foundation for future bioluminescent research development. We also provide a brief phylogenetic tree based on cytochrome c oxidase subunit I (COI) sequences to discuss the evolution of bioluminescence and the direction of future research.

The Abyssal Parasitic Flatworm Fecampia cf. abyssicola: New Records, Anatomy, and Molecular Phylogeny, with a Discussion on Its Systematic Position.

AbstractThe order Fecampiida, a group of parasitic turbellarians, has been poorly studied in terms of its species diversity, morphology, and ecology. Fecampiida is positioned within the monophyletic clade Adiaphanida, along with Tricladida and Prolecithophora, but their phylogenetic relationships are not well understood. Although the nervous and muscular systems of only two species in Fecampiida have been studied, recent research inferred morphological similarities between Fecampiida and Prolecithophora. In this study, we collected fecampiid cocoons and juveniles at depths of 1861-4438 m in Japanese waters. We identified the species on the basis of swimming juvenile specimens and by using histological and molecular methods, while we also examined its musculature and nervous system. Our study revealed a more complex nervous system than previously reported, with dorsal, lateral, and ventral pairs of longitudinal nerve cords connected through an anterior neuropile and posterior transverse commissures. While the nervous and muscular morphology suggested similarities with Prolecithophora, our phylogenetic analysis did not support a close relationship between Fecampiida and Prolecithophora.

Two new species of Branchinotogluma (Polynoidae: Annelida) from chemosynthesis-based ecosystems in Japan.

The widely distributed polychaete family Polynoidae Kinberg, 1856 is found across all oceans and from shallow to deep waters, including deep-sea hydrothermal vents and hydrocarbon seeps. Taxa inhabiting chemosynthesis-based ecosystems are often endemic to those specific habitats commonly targeted by deep-sea mining, and understanding their species diversity is essential for shaping conservation plans. Here, we report two previously undescribed scale-worms in the genus Branchinotogluma Pettibone, 1985 from the Off Hatsushima hydrocarbon seep of Sagami Bay and the Nikko Seamount hydrothermal vent on the Izu-Ogasawara Arc, and describe them as B. nikkoensis sp. nov. and B. sagamiensis sp. nov. Branchinotogluma nikkoensis sp. nov. is distinguished from the known species by the following characters: i) ventral segmental lamellae near ventral bases of neuropodia present on segments 1317, ii) dorsal tentacular cirri being longer than ventral tentacular cirri, iii) absence of dorsal tubercles. Branchinotogluma sagamiensis sp. nov. can be differentiated from other congeners by i) 20 segments, ii) dorsal tentacular cirri being longer than ventral tentacular cirri, iii) ventral segmental lamellae near ventral bases of neuropodia present on segments 1318, and iv) thin median antennae. The two new species are distinct in both morphology and four gene sequences from the only two species previously known from Japan including Branchinotogluma japonicus (Miura Hashimoto, 1991) and B. elytropapillata Zhang, Chen Qiu, 2018, originally described from Kaikata Seamount vent on the Izu-Ogasawara Arc and Okinawa Trough, respectively.

Worm on worm: Two rare genera of Calamyzinae (Annelida, Chrysopetalidae), with a description of new species.

Marine annelids in the subfamily Calamyzinae (family Chrysopetalidae) are either symbiotic or free-living forms that have been mainly reported from deep-sea chemosynthetic environments. Symbiotic calamyzines predominantly live in the mantle cavity of bivalves distributing at hydrothermal vents or methane seeps except for two species inhabiting the epidermis of polychaetes and octopuses. In this study, we describe a new species, Calamyzas crambon sp. nov., from Japan and report a new record of Nautiliniella calyptogenicola from the Mariana Trench. We also provide the phylogenetic position of the two species within Chrysopetalidae based on four gene markers (COI, 16S, 18S, and H3).

Four new species of Ctenodrilus, Raphidrilus, and Raricirrus (Cirratuliformia, Annelida) in Japanese waters, with notes on their phylogenetic position.

Four new species of annelids, Ctenodrilus japonicus sp. nov., Raphidrilus misakiensis sp. nov., Raphidrilus okinawaensis sp. nov., and Raricirrus anubis sp. nov., are described based on specimens collected from Japanese waters. Ctenodrilus japonicus sp. nov. inhabits the interstitial environment and can be distinguished from the other congeners by the following features: (i) total of 16 chaetigers, (ii) chaetigers 1-3 with stout hooks, (iii) minute body (approximately 1 mm in length), (iv) all parapodia with the same number of chaetae (two notochaetae; two neurochaetae), and (v) presence of dorsal and ventral papillae. Raphidrilus misakiensis sp. nov. lives under intertidal stones and can be distinguished from other congeners by having pectinate neurochaetae. Raphidrilus okinawaensis sp. nov. inhabits the interstitial environment and can be distinguished from other congeners by: (i) absence of annulation on the peristomium and achaetous segment and (ii) presence of a heart body in chaetigers 4-5. Raricirrus anubis sp. nov. inhabits whale bones and can be distinguished from other congeners by the following features: (i) presence of a heart body in chaetigers 9-14, (ii) presence of capillary neurochaetae on chaetiger 1, and (iii) presence of simple curved spines. A phylogenetic tree based on partial sequences of cytochrome c oxidase subunit I and 16S rRNA from the new species and other cirratulid worms showed that Raphidrilus is included in Cirratuliformia. This is the first record of Raphidrilus and Raricirrus from Japanese waters.

First record of the decapod-egg predator Ovicides paralithodis (Nemertea, Carcinonemertidae) from the snow crab Chionoecetes opilio (Decapoda, Brachyura).

The carcinonemertid monostiliferan Ovicides paralithodis Kajihara and Kuris, 2013 was originally described as an egg predator of the red-king crab Paralithodes camtschaticus (Tilesius, 1815) in the Sea of Okhotsk, the Bering Sea, and the Gulf of Alaska. In the present study, several carcinonemertid specimens were obtained from the egg mass of the snow crab Chionoecetes opilio (O. Fabricius, 1788) in the Sea of Japan. Partial sequences of the mitochondrial cytochrome c oxidase subunit I gene (COI) determined from two specimens of the carcinonemertid were identical with a barcode sequence from the holotype of O. paralithodis, indicating that the host range of the species covers at least the two decapod species, P. camtschaticus and C. opilio.

Sinking Down or Floating Up? Current State of Taxonomic Studies on Marine Invertebrates in Japan Inferred from the Number of New Species Published between the Years 2003 and 2020.

Taxonomic studies on marine invertebrates have been prominent in Japan since the 19th century. Globally, taxonomy was reported to have been in recession since the early 21st century, but it is not clear if it is still in hardship or is making a recovery in recent years in Japan. In order to comprehend the status quo of taxonomic studies on marine invertebrates in Japan, we compiled a list of marine invertebrate species newly described from the exclusive economic zone of this country during the period between 2003 and 2020 and investigated trends by making comparisons of higher taxa and academic journals in terms of the numbers of new species and taxonomic authors. We noticed that recruitment of new researchers into taxonomic studies is evident in some taxa. We also found that certain articles with descriptions of new species are now being published in journals aimed at general biology/zoology, not oriented principally to taxonomy. We consider from our analyses that taxonomic studies on marine invertebrates in Japan show signs of resurgence, but development of new taxonomists is awaited in certain taxa.

The Phylogenetic Position of Branchamphinome (Annelida, Amphinomidae) with a Description of a New Species from the North Pacific Ocean.

A new species of amphinomid polychaete, Branchamphinome kohtsukai sp. nov., is described from Japanese waters, 29-211 m in depth. The species is distinguishable from other congeners by the following features: i) branchiae with four-six filaments in midbody segments; ii) two pairs of eyes not coalescent; iii) the ventral side of the first four chaetigers broadly pigmented. This is the first record of Branchamphinome from the North Pacific Ocean. We provide a phylogenetic tree based on the sequences of four genes (COI, 16S, 18S, 28S) and discuss amphinomids' relationships.

Unearthing the diversity of Japanese Magelona (Annelida: Magelonidae); three species new to science, and a redescription of Magelona japonica.

Whilst seven species of magelonids have been originally described from the North-Western Pacific Ocean, only two have been from Japanese waters. Given the often high diversity of magelonid species within relatively small regions, the number of Japanese Magelona species is likely to be higher. The validity of several recorded species from the region has been additionally called into question, and the urgent need for a review of magelonids of Japan highlighted. Newly collected samples of magelonids have emphasised the presence of three species new to science occurring off Japan, herein described: Magelona alba sp. nov., Magelona armatis sp. nov., and Magelona boninensis sp. nov. A redescription of Magelona japonica is additionally provided, along with notes on a fifth species, which approaches Magelona cornuta. A dichotomous identification key to magelonid species of the North-Western Pacific Ocean, along with a table of characters for all five observed species is provided.

A new species of the Spirobranchuskraussii complex, S.akitsushima (Annelida, Polychaeta, Serpulidae), from the rocky intertidal zone of Japan.

A new species of Spirobranchus (Annelida: Serpulidae) is described based on specimens collected at the coastal Shonan area of Sagami Bay and the adjacent areas of Honshu, Japan. Spirobranchusakitsushima sp. nov. forms large aggregations in the intertidal rocky zone of warm-temperate Japanese shores. This species was referred to as Pomatoleioskraussii (Baird, 1864) until the monotypic genus Pomatoleios was synonymized with Spirobranchus. This new species is formally described based on morphologically distinct Japanese specimens with supporting DNA sequence data. The calcareous opercular endplate of Spirobranchusakitsushima sp. nov. lacks a distinct talon, but some specimens have a slight rounded swelling on the endplate underside, while in other species of the S.kraussii complex a talon is present, usually extended, and with bulges. We examined sub-fossil tube aggregations of the new species and suggest that such aggregation stranded ashore is a good indicator of vertical land movements (uplift and subsidence) resulting from past events, such as earthquakes, in Honshu, Japan.

Violet bioluminescent Polycirrus sp. (Annelida: Terebelliformia) discovered in the shallow coastal waters of the Noto Peninsula in Japan.

Terebellidae worms have large numbers of tentacles responsible for various biological functions. Some Terebellidae worms whose tentacles emit light are found around the world, including exceptional violet-light-emitting Polycirrus spp. found in Europe and North America. However, there is no video-recorded observation of the luminous behavior of such unique species in nature, and the genetic information related to their ecology are lacking. Here, for the first time, we video-recorded the violet-light-emitting behavior of an undescribed Japanese worm in its natural habitat. The worm was designated as Polycirrus sp. ISK based on morphological observations, and the luminescence spectrum showed a peak at 444 nm, which is an exceptionally short wavelength for bioluminescence in a shallow coastal water environment. An analysis of differentially expressing genes based on separate RNA-Seq analysis for the tentacles and the rest of body revealed the specific expression of genes that are probably involved in innate immunity in the tentacles exposed to predators. We also found a Renilla luciferase homologous gene, but coelenterazine was not detected in the worm extract by analyses using a liquid chromatography and a recombinant Renilla luciferase. These results will promote an understanding of the ecology and luminescence mechanisms of luminous Polycirrus spp.