Induced spawning with gamete release from body ruptures during reproduction of Xenoturbella bocki.

Xenoturbella is a marine invertebrate with a simple body plan, with recent phylogenomic studies suggesting that it forms the phylum Xenacoelomorpha together with the acoelomorphs. The phylogenetic position of the phylum is still under debate, whether it is an early branching bilaterian or a sister group to the Ambulacraria. Phylogenetic traits often appear during development, and larva resembling the cnidarian planula has been reported for Xenoturbella. However, subsequent developmental studies on Xenoturbella have been scarce. This is mainly due to the difficulties in collecting and keeping adult animals, resulting in the lack of data on the reproduction of the animal, such as the breeding season and the spawning pattern. Here we report on the reproduction of X. bocki and confirm that its breeding season is winter. Spawning induction resulted in gametes being released from body ruptures and not the mouth. No evidence supported the animal as a simultaneous hermaphrodite.

Exploring the trends of adaptation and evolution of sclerites with regards to habitat depth in sea pens.

Octocorals possess sclerites, small elements comprised of calcium carbonate (CaCO3) that are important diagnostic characters in octocoral taxonomy. Among octocorals, sea pens comprise a unique order (Pennatulacea) that live in a wide range of depths. Habitat depth is considered to be important in the diversification of octocoral species, but a lack of information on sea pens has limited studies on their adaptation and evolution across depth. Here, we aimed to reveal trends of adaptation and evolution of sclerite shapes in sea pens with regards to habitat depth via phylogenetic analyses and ancestral reconstruction analyses. Colony form of sea pens is suggested to have undergone convergent evolution and the loss of axis has occurred independently across the evolution of sea pens. Divergences of sea pen taxa and of sclerite forms are suggested to depend on habitat depths. In addition, their sclerite forms may be related to evolutionary history of the sclerite and the surrounding chemical environment as well as water temperature. Three-flanged sclerites may possess the tolerance towards the environment of the deep sea, while plate sclerites are suggested to be adapted towards shallower waters, and have evolved independently multiple times. The common ancestor form of sea pens was predicted to be deep-sea and similar to family Pseudumbellulidae in form, possessing sclerites intermediate in form to those of alcyonaceans and modern sea pens such as spindles, rods with spines, and three-flanged sclerites with serrated edges sclerites, as well as having an axis and bilateral traits.

JAMBIO and Its Coastal Organism Joint Surveys: Network of Marine Stations Explores Japanese Coastal Biota.

Marine stations have continued to contribute significantly to understanding the physiology, taxonomy, development, ecology, and evolution of animals. There are more than 50 marine stations of national universities in Japan, and historically their establishments were closely related to the initial stage of zoology in the country. More than 10 years ago, Japanese Association for Marine Biology (JAMBIO) was established to facilitate the collaboration among marine stations in the activities of research, education and administration. One of the successful activities of JAMBIO that contribute to zoology is the JAMBIO Coastal Organism Joint Surveys, in which scientists and students at multiple marine stations, as well as those from research institutes or museums, stay at a marine station for a few days, and collect and make a record of marine organisms. As of 2021, 22 surveys have been performed and new species have been reported from taxa such as Cnidaria, Nematoda, Platyhelminthes, Annelida, Mollusca, Arthropoda, and Echinodermata.

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.

A New Species of Acoela Possessing a Middorsal Appendage with a Possible Sensory Function.

Acoels, belonging to Xenacoelomorpha, are small worms with a relatively simple body plan and are considered a critical clade for understanding the evolution of bilaterians. Despite acoels' importance, however, many undiscovered species are predicted to be present worldwide. Here, we describe a new marine acoel species, Amphiscolops oni sp. nov., based on materials collected from the intertidal and subtidal zones of rocky shores at several localities along the Japanese Pacific coast. The new species is approximately 3 mm long and shows typical characteristics of the family Convolutidae, such as the presence of eyespots, symbiosis with algae, position of the gonopores, morphology of the bursal nozzles, lack of central singlet microtubules in the axonemes of spermatozoa, and funnel-like posture of the anterior end. Based on morphology and the results of molecular phylogenetic analyses, we assign this species to the genus Amphiscolops. Interestingly, these worms show unique behaviors such as swimming by flapping the lateral sides and actively capturing prey by swinging the anterior funnel. Furthermore, they possess a dorsal appendage-a characteristic previously unreported in Xenacoelomorpha-representing an evolutionary novelty acquired by this species.

Observing Phylum-Level Metazoan Diversity by Environmental DNA Analysis at the Ushimado Area in the Seto Inland Sea.

The dynamics of microscopic marine plankton in coastal areas is a fundamental theme in marine biodiversity research, but studies have been limited because the only available methodology was collection of plankton using plankton-nets and microscopic observation. In recent years, environmental DNA (eDNA) analysis has exhibited potential for conducting comprehensive surveys of marine plankton diversity in water at fixed points and depths in the ocean. However, few studies have examined how eDNA analysis reflects the actual distribution and dynamics of organisms in the field, and further investigation is needed to determine whether it can detect distinct differences in plankton density in the field. To address this, we analyzed eDNA in seawater samples collected at 1 km intervals at three depths over a linear distance of approximately 3.0 km in the Seto Inland Sea. The survey area included a location with a high density of Acoela (Praesagittifera naikaiensis). However, the eDNA signal for this was little to none, and its presence would not have been noticed if we did not have this information beforehand. Meanwhile, eDNA analysis enabled us to confirm the presence of a species of Placozoa that was previously undiscovered in the area. In summary, our results suggest that the number of sequence reads generated from eDNA samples in our project was not sufficient to predict the density of a particular species. However, eDNA can be useful for detecting organisms that have been overlooked using other methods.

Mitochondrial Genome Evolution of Placozoans: Gene Rearrangements and Repeat Expansions.

Placozoans, nonbilaterian animals with the simplest known metazoan bauplan, are currently classified into 20 haplotypes belonging to three genera, Polyplacotoma, Trichoplax, and Hoilungia. The latter two comprise two and five clades, respectively. In Trichoplax and Hoilungia, previous studies on six haplotypes belonging to four different clades have shown that their mtDNAs are circular chromosomes of 32-43 kb in size, which encode 12 protein-coding genes, 24 tRNAs, and two rRNAs. These mitochondrial genomes (mitogenomes) also show unique features rarely seen in other metazoans, including open reading frames (ORFs) of unknown function, and group I and II introns. Here, we report seven new mitogenomes, covering the five previously described haplotypes H2, H17, H19, H9, and H11, as well as two new haplotypes, H23 (clade III) and H24 (clade VII). The overall gene content is shared between all placozoan mitochondrial genomes, but genome sizes, gene orders, and several exon-intron boundaries vary among clades. Phylogenomic analyses strongly support a tree topology different from previous 16S rRNA analyses, with clade VI as the sister group to all other Hoilungia clades. We found small inverted repeats in all 13 mitochondrial genomes of the Trichoplax and Hoilungia genera and evaluated their distribution patterns among haplotypes. Because Polyplacotoma mediterranea (H0), the sister to the remaining haplotypes, has a small mitochondrial genome with few small inverted repeats and ORFs, we hypothesized that the proliferation of inverted repeats and ORFs substantially contributed to the observed increase in the size and GC content of the Trichoplax and Hoilungia mitochondrial genomes.

Development of Xenoturbellida.

Xenoturbellida is a group of benthic marine invertebrates with a very simple bilaterian body plan. Together with Acoelomorpha, it constitutes the clade Xenacoelomorpha, recently interpreted as a sister group to all other extant bilaterians (=Nephrozoa). Therefore, it occupies an important phylogenetic position for studies on origins and evolution of Bilateria. However, due to a number of reasons, developmental and reproductive studies on Xenoturbella have been scarce. In this chapter, I will summarize what is known concerning the reproduction and development of Xenoturbellida, including information from five species that were recently discovered. Gonads are absent in xenoturbellids, and gametes are found in various parts of the body. The gametes are released through ruptures of the body wall, and therefore, the fertilization is presumably external. After holoblastic radial cleavage, embryos hatch as free-swimming hatchlings with uniform ciliation over the surface. Apical tuft forms later in development, and 5 days after hatching, the larvae begin to settle on the substrate. Comparison with the other marine invertebrate larvae suggests that the morphologically simple swimming larvae described in Xenoturbella represent an ancestral larval type of all metazoans and bilaterians.

Microfocus X-ray CT (microCT) Imaging of Actinia equina (Cnidaria), Harmothoe sp. (Annelida), and Xenoturbella japonica (Xenacoelomorpha).

Traditionally, biologists have had to rely on destructive methods such as sectioning in order to investigate the internal structures of opaque organisms. Non-destructive microfocus X-ray computed tomography (microCT) imaging has become a powerful and emerging protocol in biology, due to technological advancements in sample staining methods and innovations in microCT hardware, processing computers, and data analysis software. However, this protocol is not commonly used, as it is in the medical and industrial fields. One of the reasons for this limited use is the lack of a simple and comprehensible manual that covers all of the necessary steps: sample collection, fixation, staining, mounting, scanning, and data analyses. Another reason is the vast diversity of metazoans, particularly marine invertebrates. Because of marine invertebrates' diverse sizes, morphologies, and physiologies, it is crucial to adjust experimental conditions and hardware configurations at each step, depending on the sample. Here, microCT imaging methods are explained in detail using three phylogenetically diverse marine invertebrates: Actinia equina (Anthozoa, Cnidaria), Harmothoe sp. (Polychaeta, Annelida), and Xenoturbella japonica (Xenoturbellida, Xenacoelomorpha). Suggestions on performing microCT imaging on various animals are also provided.

Mitigating Anticipated Effects of Systematic Errors Supports Sister-Group Relationship between Xenacoelomorpha and Ambulacraria.

Xenoturbella and the acoelomorph worms (Xenacoelomorpha) are simple marine animals with controversial affinities. They have been placed as the sister group of all other bilaterian animals (Nephrozoa hypothesis), implying their simplicity is an ancient characteristic [1, 2]; alternatively, they have been linked to the complex Ambulacraria (echinoderms and hemichordates) in a clade called the Xenambulacraria [3-5], suggesting their simplicity evolved by reduction from a complex ancestor. The difficulty resolving this problem implies the phylogenetic signal supporting the correct solution is weak and affected by inadequate modeling, creating a misleading non-phylogenetic signal. The idea that the Nephrozoa hypothesis might be an artifact is prompted by the faster molecular evolutionary rate observed within the Acoelomorpha. Unequal rates of evolution are known to result in the systematic artifact of long branch attraction, which would be predicted to result in an attraction between long-branch acoelomorphs and the outgroup, pulling them toward the root [6]. Other biases inadequately accommodated by the models used can also have strong effects, exacerbated in the context of short internal branches and long terminal branches [7]. We have assembled a large and informative dataset to address this problem. Analyses designed to reduce or to emphasize misleading signals show the Nephrozoa hypothesis is supported under conditions expected to exacerbate errors, and the Xenambulacraria hypothesis is preferred in conditions designed to reduce errors. Our reanalyses of two other recently published datasets [1, 2] produce the same result. We conclude that the Xenacoelomorpha are simplified relatives of the Ambulacraria.

Preoperative radiographic and clinical factors associated with postoperative floating of the lesser toes after resection arthroplasty for rheumatoid forefoot deformity.

BACKGROUND: This study aimed to clarify the characteristics associated with postoperative floating of the lesser toes, especially focusing on the medial and lateral lessor toes, after arthrodesis of the first metatarsophalangeal joint and resection arthroplasty of the lessor toes in rheumatoid forefoot deformity. METHODS: Fourty-seven feet of 43 people who underwent resection arthroplasty of the metatarsal head of the lesser toes for rheumatoid arthritis of the metatarsophalangeal joints were included. We retrospectively evaluated the preoperative radiographic findings and clinical characteristics of the patients, and the occurrence of postoperative floating of the lesser toes. The mean duration of follow-up was 36.5 (range 12 to 114) months. RESULTS: Preoperative dislocation grades of the second and third toes that demonstrated postoperative floating were significantly higher than those of toes that did not experience postoperative floating. The hallux valgus deformity before surgery was significantly more severe in toes with postoperative floating of the second and third lessor toes than those with no floating (p < 0.05). In addition, the Japanese Society for Surgery of the Foot (JSSF) hallux scale scores before surgery in toes with postoperative floating of the fourth and fifth lessor toes were significantly worse than those in non-dislocating toes (p < 0.05). CONCLUSIONS: The preoperative condition of the first metatarsophalangeal joint, including hallux valgus deformity, pain, range of motion, activity of daily living, and function is significantly different between postoperative floating of the lesser toes and non-floating of them after resection arthroplasty for rheumatoid forefoot deformity.

A New Species of Orthonectida That Parasitizes Xenoturbella bocki: Implications for Studies on Xenoturbella.

Orthonectida is a phylum of marine invertebrates known to parasitize many invertebrate animals. Because of its simple body plan, it was suggested that it belong to Mesozoa, together with Dicyemida, and that it represent the evolutionary step between unicellular organisms and multicellular animals. Recent studies, including analyses of its genomes, have clarified its phylogenetic position as a member of the Protostomia, but details such as the species diversity within the phylum and how it infects the host remain unknown. Here we report orthonectids discovered from the marine worm Xenoturbella bocki. Orthonectids were found from sections of four xenoturbellid specimens, collected eight years apart. Live females were also discovered on three separate occasions. These recurring instances of orthonectids found from Xenoturbella show that they are parasitic to the animal and not just chance contaminations. Based on morphological characters such as the presence of sexual dimorphism, the arrangement of oocytes within the female body, and the presence of crystalline inclusions in the male epidermal cells, we regard this orthonectid as a new species, Rhopalura xenoturbellae sp. nov. Since orthonectids are present within the xenoturbellid adult body, caution is needed when interpreting morphological, molecular, and experimental data from X. bocki. Further studies on R. xenoturbellae will yield important information on the fundamental biological details of orthonectids that remain unknown.

Correction to: A new species of Xenoturbella from the western Pacific Ocean and the evolution of Xenoturbella.

After publication of Nakano et al. (2017) [1], the authors became aware of the fact that the new species-group name erected for the two specimens of a Japanese xenoturbellid species in the article is not available because Nakano et al. (2017) [1] does not meet the requirement of the amendment of Article 8.5.3 of the International Code of Zoological Nomenclature (the Code) [2]. The authors therefore describe the two xenoturbellids as a new species again in this correction article. Methods for morphological observation, DNA extraction and sequencing were as described in Nakano et al. (2017) [1]. The holotype and paratype specimens are deposited in the National Museum of Nature and Science, Tsukuba (NSMT), Japan. The DNA sequences obtained were deposited in the International Nucleotide Sequence Database (INSD).

Multiple surveys employing a new sample-processing protocol reveal the genetic diversity of placozoans in Japan.

Placozoans, flat free-living marine invertebrates, possess an extremely simple bauplan lacking neurons and muscle cells and represent one of the earliest-branching metazoan phyla. They are widely distributed from temperate to tropical oceans. Based on mitochondrial 16S rRNA sequences, 19 haplotypes forming seven distinct clades have been reported in placozoans to date. In Japan, placozoans have been found at nine locations, but 16S genotyping has been performed at only two of these locations. Here, we propose a new processing protocol, "ethanol-treated substrate sampling," for collecting placozoans from natural environments. We also report the collection of placozoans from three new locations, the islands of Shikine-jima, Chichi-jima, and Haha-jima, and we present the distribution of the 16S haplotypes of placozoans in Japan. Multiple surveys conducted at multiple locations yielded five haplotypes that were not reported previously, revealing high genetic diversity in Japan, especially at Shimoda and Shikine-jima Island. The observed geographic distribution patterns were different among haplotypes; some were widely distributed, while others were sampled only from a single location. However, samplings conducted on different dates at the same sites yielded different haplotypes, suggesting that placozoans of a given haplotype do not inhabit the same site constantly throughout the year. Continued sampling efforts conducted during all seasons at multiple locations worldwide and the development of molecular markers within the haplotypes are needed to reveal the geographic distribution pattern and dispersal history of placozoans in greater detail.

A new species of Xenoturbella from the western Pacific Ocean and the evolution of Xenoturbella.

BACKGROUND: Xenoturbella is a group of marine benthic animals lacking an anus and a centralized nervous system. Molecular phylogenetic analyses group the animal together with the Acoelomorpha, forming the Xenacoelomorpha. This group has been suggested to be either a sister group to the Nephrozoa or a deuterostome, and therefore it may provide important insights into origins of bilaterian traits such as an anus, the nephron, feeding larvae and centralized nervous systems. However, only five Xenoturbella species have been reported and the evolutionary history of xenoturbellids and Xenacoelomorpha remains obscure. RESULTS: Here we describe a new Xenoturbella species from the western Pacific Ocean, and report a new xenoturbellid structure - the frontal pore. Non-destructive microCT was used to investigate the internal morphology of this soft-bodied animal. This revealed the presence of a frontal pore that is continuous with the ventral glandular network and which exhibits similarities with the frontal organ in acoelomorphs. CONCLUSIONS: Our results suggest that large size, oval mouth, frontal pore and ventral glandular network may be ancestral features for Xenoturbella. Further studies will clarify the evolutionary relationship of the frontal pore and ventral glandular network of xenoturbellids and the acoelomorph frontal organ. One of the habitats of the newly identified species is easily accessible from a marine station and so this species promises to be valuable for research on bilaterian and deuterostome evolution.

What is Xenoturbella?

Xenoturbella is a strange marine worm that can be collected regularly only off the west coast of Sweden. Due to its simple morphology, which lacks a centralized nervous system, coelom, anus, or reproductive organs, its phylogenetic position has long remained obscure. Recent phylogenomic analyses suggest it forms a new phylum, Xenacoelomorpha, together with the Acoelomorpha, but the position of the phylum remains undecided, either as a deuterostome or an early branching bilaterian. Developmental stages exhibit many phylogenetically decisive characters in various animal species, but have remained a mystery for Xenoturbella until recently. Observations of its development showed it has direct development with a very short and simple swimming stage, and that it lacks a feeding larva. Asexual reproduction has never been reported. It has been suggested that Xenoturbella feeds specifically on bivalves, but it still remains unknown whether it feeds on sperm, eggs, larvae, juveniles, carcass, mucus, or feces of bivalves, and direct observations of Xenoturbella feeding on bivalves have not been reported. Endosymbiont bacteria have been found, and their functions are being investigated. The evolutionary scenario of this taxon remains the subject of debate, and our understanding will depend largely on determining its phylogeny. Thus, although recent studies have uncovered many new and crucial facts regarding Xenoturbella, some fundamental biological information, such as phylogeny, complete life cycle, and genome, remain unsolved. Further research on the well-studied Swedish Xenoturbella bocki, as well as the discovery of new species elsewhere, are necessary if we are to more fully understand the nature of Xenoturbella.

Development of ciliary bands in larvae of the living isocrinid sea lily Metacrinus rotundus.

Embryos and larvae of an isocrinid sea lily, Metacrinus rotundus, are described by scanning electron microscopy. Around hatching (35 h after fertilization), the outer surface of the gastrula becomes ubiquitously covered with short cilia. At 40 h, the hatched swimming embryo develops a cilia-free zone of ectoderm on the ventral side. By 3 days, the very early dipleurula larva develops a cilia-free zone ventrally, densely ciliated regions laterally, and a sparsely ciliated region dorsally. At this stage, the posterior and anterior ciliary bands first appear: the former runs along a low ridge separating the densely from the sparsely ciliated epidermal regions, while the latter is visible, at first discontinuously, along the boundary between the densely ciliated lateral regions and the cilia-free ventral zone. In the late dipleurula larva (5 days after fertilization), the anterior and posterior loops of ciliary bands are well defined. The transition from the dipleurula to the semidoliolaria larva occurs at 6 days as the posterior loop becomes rearranged to form incompletely circumferential ciliary bands. The larva becomes competent to settle at this stage. The arrangement of the ciliary bands on the semidoliolaria is maintained during the second week of development, while the larva retains its competence to settle. The larval ciliary patterns described here are compared with those of stalkless crinoids and eleutherozoan echinoderms. The closest morphological similarities are between M. rotundus and the basal eleutherozoan class Asteroidea.

Survey of the Japanese coast reveals abundant placozoan populations in the Northern Pacific Ocean.

Placozoans are the simplest extant free-living animals consisting of only five cell types and lacking neurons and muscle cells. Their phylogenetic position implies they are important for uncovering the origins of metazoans. Although recent studies show multiple groups within the phylum, most placozoan research has been performed on laboratory-cultured clones deriving from a single specimen. Reports of placozoan discovery are concentrated in the tropic and subtropic seas, especially in the Mediterranean and the Caribbean. Here, I report the unexpected abundance of placozoans from the Japanese coast. They were found from all six studied sites, even during winter for two sites, suggesting that they are more tolerant to low temperatures than previously regarded. These results suggest an unknown abundance of placozoans in the Northern Pacific Ocean and further studies on these populations may be essential in solving important biological problems of the phylum.

Xenoturbella bocki exhibits direct development with similarities to Acoelomorpha.

Xenoturbella bocki, a marine animal with a simple body plan, has recently been suggested to be sister group to the Acoelomorpha, together forming the new phylum Xenacoelomorpha. The phylogenetic position of the phylum is still under debate, either as an early branching bilaterian or as a sister group to the Ambulacraria (hemichordates and echinoderms) within the deuterostomes. Although development has been described for several species of Acoelomorpha, little is known about the life cycle of Xenoturbella. Here we report the embryonic stages of Xenoturbella, and show that it is a direct developer without a feeding larval stage. This mode of development is similar to that of the acoelomorphs, supporting the newly proposed phylum Xenacoelomorpha and suggesting that the last common ancestor of the phylum might have been a direct developer.

Crystallization and preliminary X-ray crystallographic analysis of Pz peptidase B from Geobacillus collagenovorans MO-1.

Pz peptidase B is an intracellular M3 metallopeptidase that is found together with Pz peptidase A in the thermophile Geobacillus collagenovorans MO-1 and recognizes collagen-specific tripeptide units (-Gly-Pro-X-). These peptidases have low homology in their primary structures; however, their cleavage patterns towards peptide substrates are similar. In this work, Pz peptidase B was crystallized using the counter-diffusion method. Data were collected to a resolution of 1.6 Šat 100 K from a crystal obtained in the Japanese Experiment Module (JEM; also known as `Kibo') at the International Space Station (ISS). The crystal belonged to the trigonal space group P3(1)21, with unit-cell parameters a = b = 87.64, c = 210.5 Å. A complete data set was also obtained from crystals of selenomethionine-substituted protein.