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In this paper we perform an integrative investigation of the Eubranchus odhneri -Eubranchus sanjuanensis species complex. Several specimens morphologically similar to E. sanjuanensis were collected in the Sea of Japan. However, preliminary molecular data indicated that they were closer to the Arctic species E. odhneri. For our analysis we obtained new data on mitochondrial COI and 16S rRNA, and nuclear histone H3 and 18S rRNA molecular markers to reconstruct their phylogeny (Bayesian Inference and Maximum Likelihood approaches), delimit species with ASAP, bPTP and GMYC analyses and estimate divergence times of putative species. We also studied morphological and ecological features including external morphology, colouration, jaw and radular morphology, configuration of the reproductive system, ultrastructure of cnidosacs, and determination of possible diet preferences. Our results show that specimens from the Sea of Japan represent a new species described herein, Eubranchus malakhovi sp.n. It differs both morphologically and genetically from its closely related species, the Arctic E. odhneri and the North-East Pacific E. sanjuanensis. The new species is closely related to E. odhneri with low mtDNA divergence, and our divergence time estimations suggest these species split ~1.5 Mya.

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Conformity of trans-Arctic Coryphellidae genera with the most recent revised taxonomy for the group was tested. Both morphological and molecular data were used to compare more than 200 specimens from various localities in the North Pacific, North Atlantic and Arctic Oceans. The molecular study included four markers (COI, 16S, H3 and 28S) and implemented Bayesian and maximum likelihood phylogenetic approaches. Species delimitation was investigated with automatic barcode gap discovery (ABGD), Poisson tree processes (PTP) and generalized mixed Yule coalescent (GMYC) methods. The inter- and intraspecific divergence were studied using the TCS-based haplotype analysis and calculation of p-distances. Morphological analysis included standard anatomical techniques and scanning electron microscopy. Based on integrative analysis, we conclude that Coryphella verrucosa and C. pseudoverrucosa represent a single species (C. verrucosa), which is highly heterogeneous at morphological and molecular levels. For Microchlamylla gracilis and M. amabilis, their synonymization under the name M. gracilis is suggested, but increased sample size is needed for further clarification of their identities. Finally, the amphiboreal species Borealea nobilis and the North Pacific B. sanamyanae are distinct species displaying secondary sympatry in the north-west Pacific. Based on these results, we propose a new classification system for the family Coryphellidae uniting all currently considered genera in the single genus Coryphella, and suggest a new approach for further revision of Flabellinidae s.l. taxa.

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Buccal complex of Gastropoda is a complex structure consisting of the radula, odontophore and the buccal muscles. The general morphology and function of the buccal complex of Gastropoda was well-studied in several aspects. However, there are only a few integrated studies on both general and fine morphology, and the mechanism of feeding performed on opisthobranchs. Opisthobranchs' feeding mechanisms are very specific and diverse, because opisthobranch molluscs have highly-specified feeding preferences. Unlike the majority of opisthobranchs, Flabellina verrucosa (Gastropoda: Opisthobranchia) has a wide range of feeding objects. The feeding mechanism of this species can be an example of the non-specified feeding mode. General and fine morphology of the buccal complex of F. verrucosa is studied in the present work. Based on three-dimensional reconstruction of the buccal complex and data on the fine morphology of muscles, we suggest the mechanism of the functioning of the food-obtaining apparatus. Prey is pulled into the buccal cavity due to blowing negative pressure and triturated using the radula. This feeding mechanism is suggested for Gastropoda for the first time and could be compared only with that in Tochuina tetraquetra and Dendronotus iris (Nudibranchia: Dendronotida), although the morphology of radula in these three species differs considerably.

Gastropoda is morphologically highly variable and broadly distributed group of mollusks. Due to the high morphological and functional diversity of the feeding apparatus gastropods follow a broad range of feeding strategies: from detritivory to highly specialized predation. The feeding apparatus includes the buccal armaments: jaw(s) and radula. The radula comprises a chitinous ribbon with teeth arranged in transverse and longitudinal rows. A unique characteristic of the radula is its continuous renewal during the entire life of a mollusk. The teeth and the membrane are continuously synthesized in the blind end of the radular sac and are shifted forward to the working zone, while the teeth harden and are mineralized on the way. Despite the similarity of the general mechanism of the radula formation in gastropods, some phylogenetically determined features can be identified in different phylogenetic lineages. These mainly concern shape, size, and number of the odontoblasts forming a single tooth. The radular morphology depends on the shape of the formation zone and the morphology of the subradular epithelium. The radula first appears at the pre‐ and posttorsional veliger stages as an invagination of the buccal epithelium of the larval anterior gut. The larval radular sac is lined with uniform undifferentiated cells. Each major phylogenetic lineage is characterized by a specific larval radula type. Thus, the docoglossan radula of Patellogastropoda is characterized by initially three and then five teeth in a transverse row. The larval rhipidoglossan radula has seven teeth in a row with differentiation into central, lateral, and marginal teeth and later is transformed into the adult radula morphology by the addition of lateral and especially marginal teeth. The taenioglossan radula of Caenogastropoda is nearly immediately formed in adult configuration with seven teeth in a row.