Postembryonic development and lifestyle shift in the commensal ribbon worm.

BACKGROUND: Various morphological adaptations are associated with symbiotic relationships between organisms. One such adaptation is seen in the nemertean genus Malacobdella. All species in the genus are commensals of molluscan hosts, attaching to the surface of host mantles with a terminal sucker. Malacobdella possesses several unique characteristics within the order Monostilifera, exhibiting the terminal sucker and the absence of eyes and apical/cerebral organs, which are related to their adaptation to a commensal lifestyle. Nevertheless, the developmental processes that give rise to these morphological characteristics during their transition from free-living larvae to commensal adults remain uncertain. RESULTS: In the present study, therefore, we visualized the developmental processes of the internal morphologies during postembryonic larval stages using fluorescent molecular markers. We demonstrated the developmental processes, including the formation of the sucker primordium and the functional sucker. Furthermore, our data revealed that sensory organs, including apical/cerebral organs, formed in embryonic and early postembryonic stages but degenerated in the late postembryonic stage prior to settlement within their host using a terminal sucker. CONCLUSIONS: This study reveals the formation of the terminal sucker through tissue invagination, shedding light on its adhesion mechanism. Sucker muscle development likely originates from body wall muscles. Notably, M. japonica exhibits negative phototaxis despite lacking larval ocelli. This observation suggests a potential role for other sensory mechanisms, such as the apical and cerebral organs identified in the larvae, in facilitating settlement and adhesive behaviors. The loss of sensory organs during larval development might reflect a transition from planktonic feeding to a stable, host-associated lifestyle. This study also emphasizes the need for further studies to explore the phylogenetic relationships within the infraorder Amphiporiina and investigate the postembryonic development of neuromuscular systems in closely related taxa to gain a more comprehensive understanding of ecological adaptations in Nemertea.

Three new species in Tetrastemma Ehrenberg, 1828 (Nemertea, Monostilifera) from sublittoral to upper bathyal zones of the northwestern Pacific.

Monostiliferous nemerteans in the genus Tetrastemma Ehrenberg, 1828 are generally characterized as having four eyes, and they occur worldwide, from the intertidal zone to the deep-sea bottom. Recent extensive sampling of Tetrastemma has explored the high species diversity, including many undescribed forms, but phylogenic analysis has revealed non-monophyly of the genus. We herein describe three new species of the genus (T.albumsp. nov., T.personasp. nov., and T.shohoensesp. nov.) from northwestern Pacific waters based on specimens collected by dredging or by use of a remotely operated vehicle at depths of 116-455 m. Since anatomical and histological characters traditionally used in systematics of the genus are sometimes interspecifically uniform, a histology-free approach is applied for the species descriptions in this study. To confirm the generic affiliation of the new species, a molecular phylogenetic analysis based on partial sequences of cytochrome c oxidase subunit I, 16S rRNA, 18S rRNA, 28S rRNA, and histone H3 genes was performed. Our result shows that all three new species are nested in a subclade formed by species from the North Pacific and American Atlantic, inferring that geographic distribution does not reflect the cladogenesis of Tetrastemma. Furthermore, two Tetrastemma species with a cylindrical stylet basis, T.freyaeChernyshev et al., 2020 from off the coast of India and Hawaii and T.shohoensesp. nov. from Shoho Seamount, Japan, constitute a clade in the resulting tree.

First eumonostiliferous nemertean from the Nishi-Shichito Ridge, Genrokunemertes obesa gen. et sp. nov. (Eumonostilifera, Nemertea).

Nemerteans are mostly marine, benthic invertebrates, inhabiting intertidal to hadal zones. Recently, they have been recognized from deep sea with environmental DNA (eDNA) metabarcoding of sediment samples whereas any records from the locations and/or the water depth have not been documented in nemertean taxonomic publications to date. It suggests that there are major gaps between deep-sea nemertean fauna observed with eDNA and taxonomic knowledge. During a research expedition in 2021, we obtained a single specimen of eumonostiliferous nemertean from the southern part of Genroku Seamount Chain, the Nishi-Shichito Ridge, where any nemertean species have never been reported. Subsequent morphological and molecular examination reveal that the species is placed in a new genus and herein described as Genrokunemertes obesa gen. et sp. nov. It resembles shallow-water-dwelling Kurilonemertes and Typhloerstedia, but differs from the former in lacking eyes and the latter in possessing well-developed cephalic glands and lacking accessory nerves of lateral nerve cords. In terms of genetic distances based on partial sequences of the cytochrome c oxidase subunit I gene, G. obesa gen. et sp. nov. is closest to Monostilifera sp. Owase collected from Japan, which is characterized by large four eyes; the COI distance is higher than commonly observed interspecific divergences in eumonostiliferans.

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.

First Detailed Record of Symbiosis Between a Sea Anemone and Homoscleromorph Sponge, With a Description of Tempuractis rinkai gen. et sp. nov. (Cnidaria: Anthozoa: Actiniaria: Edwardsiidae).

A new species in a new genus of sea anemone, Tempuractis rinkai gen. et sp. nov., was discovered at several localities along the temperate rocky shores of Japan. The new species is approximately 4 mm in length and has been assigned to family Edwardsiidae, because it has eight macrocnemes, lacks sphincter and basal muscles, and possesses rounded aboral end. The sea anemone, however, also has a peculiar body shape unlike that of any other known taxa. This new species resembles some genera, especially Drillactis and Nematostella, in smooth column surface without nemathybomes or tenaculi, but is distinguishable from them by several morphological features: the presence of holotrichs and absence of nematosomes. Furthermore, this edwardsiid species exhibits a peculiar symbiotic ecology with sponges. Therefore, a new genus, Tempuractis, is proposed for this species. In the field, T. rinkai sp. nov. was always found living inside homosclerophorid sponge of the genus Oscarella, which suggests a possible obligate symbiosis between Porifera and Actiniaria. The benefit of this symbiosis is discussed on the basis of observations of live specimens, both in the aquarium and field. This is the first report of symbiosis between a sea anemone and a homoscleromorph sponge.

Species richness of Eurasian Zephyrus hairstreaks (Lepidoptera: Lycaenidae: Theclini) with implications on historical biogeography: An NDM/VNDM approach.

AIM: A database based on distributional records of Eurasian Zephyrus hairstreaks (Lepidoptera: Lycaenidae: Theclini) was compiled to analyse their areas of endemism (AoEs), species richness and distribution patterns, to explore their locations of past glacial refugia and dispersal routes. METHODS: Over 2000 Zephyrus hairstreaks occurrences are analysed using the NDM/VNDM algorithm, for the recognition of AoEs. Species richness was calculated by using the option 'Number of different classes' to count the different classes of a variable presented in each 3.0°×3.0° grid cell, and GIS software was used to visualize distribution patterns of endemic species. RESULTS: Centres of species richness of Zephyrus hairstreaks are situated in the eastern Qinghai-Tibet Plateau (EQTP), Hengduan Mountain Region (HDMR) and the Qinling Mountain Region (QLMR). Latitudinal gradients in species richness show normal distribution with the peak between 25° N and 35° N in the temperate zone, gradually decreasing towards the poles. Moreover, most parts of central and southern China, especially the area of QLMR-EQTP-HDMR, were identified as AoEs that may have played a significant role as refugia during Quaternary global cooling. There are four major distributional patterns of Zephyrus hairstreaks in Eurasia: Sino-Japanese, Sino-Himalayan, high-mountain and a combined distribution covering all three patterns. CONCLUSIONS: Zephyrus hairstreaks probably originated at least 23-24 Myr ago in E. Asia between 25° N to 35° N in the temperate zone. Cenozoic orogenies caused rapid speciation of this tribe and extrusion of the Indochina block resulted in vicariance between the Sino-Japanese and the Sino-Himalayan patterns. The four distribution patterns provided two possible dispersal directions: Sino-Japanese dispersal and Sino-Himalayan dispersal.

Systematic review of diplommatinid land snails (Caenogastropoda, Diplommatinidae) endemic to the Palau Islands. (3) Description of eight new species and two new subspecies of Hungerfordia.

Hungerfordia Beddome, 1889 is a land snail genus of the family Diplommatinidae, which shows extensive endemic radiation in the Palau (Belau) islands. Although 42 species or subspecies of the genus have been named to date, many species remain to be described. In this article, we describe ten new taxa of the genus: H. goniobasis exserta subsp. nov., H. ngereamensis sp. nov., H. spiroperculata sp. nov., H. fragilipennis sp. nov., H. brachyptera sp. nov., H. elegantissima anomphala subsp. nov., H. nodulosa sp. nov., H. irregularis sp. nov., H. chilorhytis sp. nov., H. globosa sp. nov.

Systematic review of diplommatinid land snails (Caenogastropoda, Diplommatinidae) endemic to the Palau Islands. (2) Taxonomic revision of Hungerfordia species with low axial ribs.

Hungerfordia Beddome, 1889 is a terrestrial caenogastropod genus of the family Diplommatinidae endemic to the Palau (Belau) islands. Hungerfordia is radiated within the Palau islands and exhibits substantial morphological diversity. In this paper, the taxonomy of Hungerfordia species with low axial ribs is revised. Hungerfordia polymorpha (Crosse, 1866), H. inflatula (Crosse, 1866), H. crassilabris crassilabris (Beddome, 1889), H. lutea lutea (Beddome, 1889), H. ringens ringens (Crosse, 1866), H. pyramis pyramis (Crosse, 1866), and H. aurea (Beddome, 1889) are redescribed based on the syntypes and newly collected materials. The following new taxa are described: H. aspera sp. nov., H. basodonta sp. nov., H. microbasodonta sp. nov., H. rudicostata sp. nov., H. unisulcata sp. nov., H. longissima sp. nov., H. eurystoma sp. nov., H. crenata sp. nov., H. crassilabris tridentata subsp. nov., H. crassilabris attenuata subsp. nov., H. lutea hemilaevis subsp. nov., H. loxodonta sp. nov., H. omphaloptyx sp. nov., H. robiginosa sp. nov., H. angaurensis sp. nov., H. ringens rotundata subsp. nov., H. ringens ventrinodus subsp. nov., H. pyramis pteroma subsp. nov., H. spinoscapula sp. nov.

Systematic review of diplommatinid land snails (Caenogastropoda, Diplommatinidae) endemic to the Palau Islands. (1) Generic classification and revision of Hungerfordia species with highly developed axial ribs.

Diplommatinidae is a family of terrestrial caenogastropod snails that shows extensive species diversity and endemic radiation within the Palau (Belau) Islands. In this paper, we revised the taxonomy of Palauan endemic diplommatinids based on the comparative morphology of shell, operculum, radula, and genitalia. Although the generic classification of Palauan diplommatinids has been confusing, they are currently classified into two genera, Hungerfordia Beddome, 1889 and Palaina Semper, 1865. Palauan diplommatinids of these two genera are primarily distinguished by the radular, genital and operculum morphology. However, shell morphology, on which taxonomy has traditionally been based, does not provide definitive characters for generic identification, because shell shape and sculpture are highly variable within each genus. Although Hungerfordia has long been known as a monotypic genus, the current study redefines the endemic genus to include a wider range of species that were formerly placed in Diplommatina or Palaina. Following species are transferred to Hungerfordia: H. alata (Crosse, 1866) comb. nov.; H. lamellata (Crosse, 1866) comb. nov.; H. pyramis (Crosse, 1866) comb. nov.; H. ringens (Crosse, 1866) comb. nov.; H. polymorpha (Crosse, 1866) comb. nov.; H. inflatula (Crosse, 1866) comb. nov.; H. lutea (Beddome, 1889) comb. nov.; H. aurea (Beddome, 1889) comb. nov.; H. gibboni (Beddome, 1889) comb. nov.; H. crassilabris (Beddome, 1889) comb. nov. Pseudopalaina Moellendorff in Kobelt & Moellendorff, 1898 (synonym nov.) is demoted as a junior synonym of Hungerfordia. Palaina is also redefined based on the internal shell features and operculum morphology, in addition to the external shell characters. Furthermore, Eupalaina Kobelt & Moellendorff, 1898 is restored as a subgenus of Palaina for Palauan species on the basis of a genital character. The taxonomy of Hungerfordia species with highly developed axial ribs is revised. H. pelewensis Beddome, 1889, H. lamellata (Crosse, 1866) comb. nov., and H. alata (Crosse, 1866) comb. nov. are redescribed with new morphological data. Following new taxa are described: H. triplochilus sp. nov., H. expansilabris sp. nov., H. nudicollum sp. nov., H. echinata echinata sp. et subsp. nov., H. echinata tubulispina subsp. nov., H. elegantissima sp. nov., H. goniobasis goniobasis sp. et subsp. nov., H. goniobasis dmasechensis subsp. nov., H. subalata sp. nov., H. pteropurpuroides sp. nov., H. papilio papilio sp. et subsp. nov., H. papilio stenoptera subsp. nov.. 

Novel repetitive structures, deviant protein-encoding sequences and unidentified ORFs in the mitochondrial genome of the brachiopod Lingula anatina.

Complete sequence determination of the brachiopod Lingula anatina mtDNA (28,818 bp) revealed an organization that is remarkably atypical for an animal mt-genome. In addition to the usual set of 37 animal mitochondrial genes, which make up only 57% (16,555 bp) of the entire sequence, the genome contains lengthy unassigned sequences. All the genes are encoded in the same DNA strand, generally in a compact way, whereas the overall gene order is highly divergent in comparison with known animal mtDNA. Individual genes are generally longer and deviate considerably in sequence from their homologues in other animals. The genome contains two major repeat regions, in which 11 units of unassigned sequences and six genes (atp8, trnM, trnQ, trnV, and part of cox2 and nad2) are found in repetition, in the form of nested direct repeats of unparalleled complexity. One of the repeat regions contains unassigned repeat units dispersed among several unique sequences, novel repetitive structure for animal mtDNAs. Each of those unique sequences contains an open reading frame for a polypeptide between 80 and 357 amino acids long, potentially encoding a functional molecule, but none of them has been identified with known proteins. In both repeat regions, tRNA genes or tRNA gene-like sequences flank major repeated units, supporting the view that those structures play a role in the mitochondrial gene rearrangements. Although the intricate repeated organization of this genome can be explained by recurrent tandem duplications and subsequent deletions mediated by replication errors, other mechanisms, such as nonhomologous recombinations, appear to explain certain structures more easily.