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.

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.

Molecular phylogeny of Myriapoda provides insights into evolutionary patterns of the mode in post-embryonic development.

Myriapoda, a subphylum of Arthropoda, comprises four classes, Chilopoda, Diplopoda, Pauropoda, and Symphyla. While recent molecular evidence has shown that Myriapoda is monophyletic, the internal phylogeny, which is pivotal for understanding the evolutionary history of myriapods, remains unresolved. Here we report the results of phylogenetic analyses and estimations of divergence time and ancestral state of myriapods. Phylogenetic analyses were performed based on three nuclear protein-coding genes determined from 19 myriapods representing the four classes (17 orders) and 11 outgroup species. The results revealed that Symphyla whose phylogenetic position has long been debated is the sister lineage to all other myriapods, and that the interordinal relationships within classes were consistent with traditional classifications. Ancestral state estimation based on the tree topology suggests that myriapods evolved from an ancestral state that was characterized by a hemianamorphic mode of post-embryonic development and had a relatively low number of body segments and legs.

Mitochondrial genome of a Japanese placozoan.

Placozoans are marine invertebrates found in tropical and subtropical waters. Their body plan is among the simplest of free-living animals. The present study determined the mitochondrial genome sequence of a placozoan collected on the coast of Shirahama, Wakayama, Honshu, Japan, and compared it with those of Trichoplax adhaerens from the Red Sea and of three strains from the Caribbean Sea. The sequences of mitochondrial respiratory chain of the Japanese placozoan genes are very similar to those of the BZ49 strain from the Caribbean Sea. However, there are distinct differences in gene arrangement, such as the location of two open reading frames. This Japanese placozoan is therefore distinguishable from the other strains. Based on current knowledge of placozoan 16S diversity our 'Shirahama' strain most likely represents the H15 lineage, known from the Philippines. In the mitochondrial genome of placozoans, substitution rates are slower than in bilaterians, whereas the rate of rearrangements is faster.

Nucleation of lamellar domains from a sponge phase under shear flow: Shape selection of nuclei in a nonequilibrium steady state.

It is a fundamental physical problem how a state is selected in a nonequilibrium steady state where the energy is continuously dissipated. This problem is common to phase transitions in liquids under shear flow and those in solids under deformation or electric current. In particular, soft matter often exhibits a strong nonlinear response to an external field, since its structural susceptibility to the external field is extremely large due to its softness and flexibility. Here we study the nucleation and growth process of the lamellar phase from the sponge phase under shear flow in a bilayer-forming surfactant system. We found an interesting shape selection of lamellar nuclei under shear flow between multilamellar vesicles (onions) and cylinders (leeks). These two types of behavior are separated sharply at a critical shear rate: a slight change of the shear rate is enough to switch one behavior to the other. We also found that, under a sufficiently strong shear flow, nucleated onions decrease their size with time, and eventually transform into leeks. This suggests that leeks may be the stable morphology under steady shear flow. However, the stability is limited only to the lamellar-sponge coexistence region. When a system enters into the lamellar phase region by further cooling, leeks lose their stability and break up into rather monodisperse onions, presumably via Rayleigh-like instability of a fluid tube. On the basis of these results, we draw a dynamic state diagram of smectic membrane organization under shear flow.

Shear-induced discontinuous and continuous topological transitions in a hyperswollen membrane system.

Here we demonstrate that both discontinuous and continuous transition between the sponge and lamellar phase can be induced by steady shear flow for a hyperswollen membrane system. The discontinuous nature of the transition is revealed by a distinct hysteresis in the rheological behavior between shear-rate increasing and decreasing measurements at a constant temperature. This discontinuity becomes weaker with an increase in the shear rate and temperature, and the transition eventually becomes a continuous one without any hysteresis. We also found another shear-induced transition in a one-phase lamellar region. The dynamic phase diagram in a nonequilibrium steady state under shear is constructed for the sponge-lamellar transition as well as another transition in a stable lamellar phase. Possible physical mechanisms for these shear-induced transitions are discussed.