Irina A. Ekimova

Stygobiotic hydrobiid snails of the western South Caucasus are taxonomically poorly known, and the diagnostic morphological and molecular boundaries within the genus Pontohoratia have not previously been tested using an integrative approach. We provide an integrative taxonomic revision of Pontohoratia based on shell and soft-part morphology and a multilocus dataset of five markers. Phylogenetic analyses recover Pontohoratia s. l. as comprising two deeply divergent, well-supported lineages: Pontohoratia s. str. (including the type species P. birsteini) and the lineage previously identified as ‘P. smyri’. The latter lineage is described as Dimonia gen. nov., with Dimonia smyri comb. nov. redescribed, and two additional molecularly distinct lineages reported as Dimonia spp. pending the discovery of diagnostic morphological characters. Within Pontohoratia s. str. we recognise four species confined to subterranean waters of the southern Bzyb Range: P. birsteini (amended redescription) and three new species, P. styx sp. nov., P. acheron sp. nov. and P. cocytus sp. nov. Both genera show narrow ranges largely restricted to individual karst massifs, with syntopic occurrence at two sites, pointing to prolonged historical isolation among karst massifs as a driver of diversification in this subterranean fauna.

The nudibranch genus Pteraeolidia has traditionally been treated as taxonomically simple, but recent molecular studies suggest that it includes hidden species diversity across the Indo-Pacific. In this study, the Pteraeolidia semperi species complex was investigated from the coastal waters of Vietnam using an integrative approach. Thirty-five specimens from five localities were analysed using mitochondrial (COI, 16S rRNA) and nuclear (H3) markers, together with external and internal morphological data and haplotype network analysis. Our results revealed two deeply divergent clades (Clades A and B) occurring sympatrically in Vietnam. These clades correspond to several molecular operational taxonomic units and show strong geographic structure across the Indo-West Pacific. One of these units (MOTU20), which falls within Clade B, corresponds to Pteraeolidia ianthina (Angas, 1864), a valid species restricted to its type locality, Sydney Harbour, New South Wales; it is treated here as separate from the P. semperi complex. Nuclear data support the separation of the two clades as independent evolutionary lineages, whereas relationships among mitochondrial lineages within each clade remain unresolved. The two clades also differ in external morphology, especially in body colouration, pigmentation pattern, and ceratal arrangement, while internal anatomy is largely similar. Mitochondrial divergence in Pteraeolidia is exceptionally high, including within clades, and exceeds values usually reported between species. This unusual pattern may reflect accelerated mitochondrial evolution, possibly linked to the photosymbiotic lifestyle of these nudibranchs. Our data show that Vietnamese populations of Pteraeolidia include at least two species within a broader and highly diverse species complex. These findings confirm that Pteraeolidia semperi represents a complex of cryptic or semi-cryptic taxa and highlight the need for genomic data to resolve species limits more precisely.

The Cadlina laevis (Linnaeus, 1767) species complex in the White Sea exhibits a high mitochondrial DNA polymorphism, with two divergent haplogroups (HA and HB) showing ~2.5% COI divergence, consistent with minimal interspecific mtDNA differences within this group. This study integrates complete mitochondrial genomes of both haplogroups of the White Sea Cadlina laevis, nuclear markers (18S, ITS1, ITS2, 28S, H3), and species-delimitation analyses to test whether mitochondrial lineages correspond to distinct species. Comparative mitogenomics revealed conserved gene order but heterogeneous divergence, with protein-coding genes showing p-distances from 1.0% (COX2) to 5.1% (ATP6) between haplogroups. Phylogenetic reconstructions based on nuclear markers consistently collapsed HA and HB into single, undifferentiated clade, demonstrating mitonuclear discordance. Similar patterns emerged for other mitochondrial lineages: C. laevis and North Pacific C. vavilovi Korshunova & Martynov, 2024, as well as C. koltzovi Korshunova & Martynov, 2024 and Cadlina sp. 3 showed no nuclear differentiation despite substantial COI p-distances. Species-delimitation methods yielded conflicting results, failing to provide consistent support for mitochondrial-defined species. The COI-wide distance distribution placed the HA–HB comparison within the “grey zone” of ambiguous species boundaries. The study demonstrates that mitochondrial data alone are unreliable for species delimitation in this group, suggesting that the current taxonomy may over-split mitochondrial lineages and require genome-wide nuclear evidence for robust taxonomic revision.

Nudibranch taxonomy is a prime example of the conflict between “lumpers” and “splitters”: with rampant cryptic diversity, difficult to interpret morphological characters, plastic ecology, and variable levels of COI divergence among groups. As a result, it is extremely challenging to reach a consensus on what constitutes a species (not to mention a genus or a family). In this paper, we tackle a particularly problematic group, the family Flabellinidae s.l. Recent studies have shown that this group is polyphyletic and have suggested reclassifying the 84 described species into 29 genera across 7 families. However, further studies indicated the instability of the phylogenetic relationships between some of these species, genera, and families and suggested the need for further revisions of this group. The primary goal of the present study is a critical revision of the Flabellinidae s.l. diversity, by testing the monophyly of currently accepted genera and re-evaluating morphological synapomorphies for high-rank taxa, with the incorporation of previously overlooked taxa and some from poorly studied regions. We conducted an updated phylogenetic analysis based on four molecular markers. We also performed morphological analyses based on light and scanning electron microscopy. The analyses confirm the monophyly of Coryphellidae, Apataidae, and Paracoryphellidae. At the same time, the monophyly of Flabellinidae s.str., Samlidae, Unidentiidae, and Flabellinopsidae and the genera Edmundsella and Samla was questioned. Our results show that Samla rubropurpurata represents a new genus described herein as Launsina gen. nov . We also identify a new phylogenetic clade represented by specimens from the Kurile Islands, which corresponds to the recently established genus Mgueolia. This group is represented by at least four distinct species, three of which are described herein. We demonstrate that the facelinid genus Bajaeolis and the recently described genus Kynaria are members of the family Flabellinopsidae. Finally, we suggest that the family Piseinotecidae likely represents a senior synonym of the family Unidentiidae. Based on our comprehensive integrative analysis, we propose a new taxonomical scheme for Flabellinidae s.l. and identify consistent common traits and synapomorphies for each high-rank group.

Recent integrative taxonomic revisions of the amphiboreal nudibranch Diaphoreolis viridis (Forbes, 1840) suggest that it represents a complex of cryptic or pseudocryptic species. However, species delimitation within trans‐Arctic taxa remains challenging due to the reliance on potentially variable morphological traits (such as coloration) and the limitations of using mtDNA data alone. Therefore, the taxonomic status of several North Pacific populations traditionally attributed to the Diaphoreolis viridis complex ( Diaphoreolis midori (Martynov, Sanamyan & Korshunova, 2015), Diaphoreolis viridis emeraldi Korshunova, Fletcher, Bakken & Martynov, 2023, and the nominal form) requires rigorous re‐evaluation using integrative approaches. The aim of this study is to clarify the identity of chromatic variants of D. viridis across its broad Pacific distribution and to assess the genetic connectivity of its distant populations. By combining detailed morphological examination with mtDNA genotyping and sequencing of nuclear ribosomal markers across a wide geographic range of the complex, this study provides a robust assessment of species boundaries within Diaphoreolis viridis and contributes to a broader understanding of the challenges and solutions in trans‐Arctic nudibranch taxonomy. The phylogeographic analysis included a TCS‐based network analysis, an analysis of molecular variance (AMOVA) and divergence time estimations. The results demonstrate the conspecificity of D. v. emeraldi and D. midori , as well as the conspecificity of the entire complex under the name D. viridis due to the absence of significant morphological and molecular differences. We also identified previously undescribed species from the Kuril Islands, Diaphoreolis shinkaii sp.nov., which differs markedly from other Diaphoreolis species in its molecular, morphological, and bathymetric characteristics.

The main goal of the present paper is to revise the diversity of the nudibranch genus Ziminella and update the geographic ranges of its species, based on historical museum material and newly collected specimens of Ziminella japonica (Volodchenko, 1941) from the Sea of Japan. We provide morphological examinations, a molecular phylogenetic analysis based on four markers, and species delimitation analyses of Ziminella. Our results indicate that Z. salmonacea (Couthouy, 1838) and Z. circapolaris Korshunova et al., 2017 represent a single species but display some intraspecific variation in the COI marker (up to 2.54% between individuals). The species delimitation analyses cannot explicitly support the distinct status of Z. japonica and Z. vrijenhoeki Valdés et al., 2018, but in this case the minimal interspecific p-distance in COI comprised 7.95%. Moreover, several differences in the coloration and external morphology are found in specimens from the type localities of these two species. We also show that Z. japonica displays high morphological diversity across its putative geographical range. Since this variation could be either intraspecific or interspecific, we suggest that specimens collected outside the type localities and lack molecular data, cannot be precisely identified. The molecular phylogenetic analysis does not support close relationships of the shelf species Z. japonica and the abyssal species Z. abyssa Korshunova et al., 2017, thus a hypothesis of a shelf origin of the latter species cannot be confirmed by the presented data. Finally, we provide novel evidence on the hidden diversity of flabellinids in the deep-sea waters of the Sea of Japan.

The life cycle, involving two larvae (planktotrophic and reserve-trophic), is hypothesized to be a synapomorphy of Ambulacraria. Planktotrophic larvae have

The nudibranch genus Acanthodoris Alder et Hancock, 1845 includes 15 recognised species, most of which are distributed in the temperate marine waters of both hemispheres, but only two species were known from the Russian seas: the widely distributed trans-Arctic A. pilosa (Abildgaard in Müller, 1789) and A. uchidai Baba, 1935, which is found from in the Kuril Islands. The main aim of this study is to revise the species composition of Acanthodoris in Russian waters, evaluate the genetic population structure of the trans-Arctic A. pilosa in the context of connectivity among its distant populations, and infer the phylogenetic relationships between the Russian species of Acanthodoris and other representatives of the family Onchidorididae. The material was collected from eight localities in the subarctic and North-West Pacific waters. The methods employed included morphological analyses, particularly, scanning electron microscopy and computer microtomography, as well as molecular analyses of partial fragments of genetic markers: cytochrome c oxidase subunit I, 16S rRNA, histone H3, 28S rRNA, and 18S rRNA. The genus Acanthodoris is represented in the Russian seas by two species: the North Atlantic and subarctic A. pilosa and the North Pacific A. atrogriseata O’Donoghue, 1927, which is recorded here for the first time from Russia. Acanthodoris uchidai represents a separate clade that is not related to other Acanthodoris; therefore, a new genus Acanthomira gen. nov. is proposed herein, with a single representative species Acanthomira uchidai, comb. nov. We also provide evidence for the existence of cryptic biodiversity within North Atlantic A. pilosa. Our current phylogenetic hypothesis suggests an independent loss of the rachidian teeth in both Acanthodoris and Acanthomira gen. nov., although this should be further tested with a larger taxon sampling. The results of this study emphasise the necessity of incorporating molecular analyses to confirm species identity and to prevent the description of new Nudibranchia taxa based solely on subtle morphological differences.