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The gastropod infraclass Euthyneura comprises at least 30,000 species of snails and slugs, including nudibranch sea slugs, sea hares and garden snails, that flourish in various environments on earth. A unique morphological feature of Euthyneura is the presence of two pairs of sensory head tentacles with different shapes and functions: the anterior labial tentacles and the posterior rhinophores or eyestalks. Here we combine molecular phylogenetic and microanatomical evidence that suggests the two pairs of head tentacles have originated by splitting of the original single tentacle pair (with two parallel nerve cords in each tentacle) as seen in many other gastropods. Minute deep-sea snails of Tjaernoeia and Parvaplustrum, which in our phylogeny belonged to the euthyneurans’ sister group (new infraclass Mesoneura), have tentacles that are split along much of their lengths but associated nerves and epidermal sense organs are not as specialized as in Euthyneura. We suggest that further elaboration of cephalic sense organs in Euthyneura closely coincided with their ecological radiation and drastic modification of body plans. The monotypic family Parvaplustridae nov., superfamily Tjaernoeioidea nov. (Tjaernoeiidae + Parvaplustridae), and new major clade Tetratentaculata nov. (Mesoneura nov. + Euthyneura) are also proposed based on their phylogenetic relationships and shared morphological traits.

The thecosome pteropods Limacina helicina and L. retroversa are important contributors to the zooplankton community in high-latitude environments but little is known about their distribution and life cycle under polar conditions. We collected the early life stages (< 1 mm) of the thecosome population in 2012 and 2013 at a bi-weekly to monthly resolution in fjord highly influenced by Arctic waters as well as Atlantic inflows (Adventfjorden, Svalbard, 78°N), together with environmental parameters. L. retroversa only occurred episodically, in association with the inflow of Atlantic water, with low numbers and random size distributions. This suggests that this boreal species does not fulfill its life cycle in Adventfjorden. In contrast, young specimens of L. helicina were present during the entire study. Veligers hatched in late summer/autumn and measured 0.14 mm on average. They grew with rates of 0.0006 mm day− 1 over the 10–11 months of development. Only thereafter, growth accelerated by one order of magnitude and maximal rates were reached in autumn (0.0077 mm day− 1). Our results indicate that L. helicina reaches a size of 1 mm after approximately 1.5 years in Adventfjorden. We therefore suggest that L. helicina overwinters the first year as a small juvenile and that it needs at least 2 years to reach an adult size of 5 mm in Adventfjorden. This reveals an complex and delicate aspect of the life-cycle of L. helicina and further research is needed to determine if it makes the population especially vulnerable towards climate changes.

Gastropods (slugs and snails) are prominent and species-rich faunal elements in marine and terrestrial habitats of the tropics. While several clades of snails inhabit freshwater systems, slugs are extremely rare in freshwater: only the centimeter-sized Acochlidiidae, with currently three genera, contain more than one species and live in the lower reaches of island streams in an area comprising Eastern Indonesia, Fiji, and Palau. Where known, the species of this unique group are specialized predators of other amphidromous snails’ egg capsules (Neritidae) and their reproductive biology and adaptations to life in freshwater are complex. Acochlidiidae are thus of special interest for evolutionary biology and ecology. We here describe a new genus of unusually bluish-green acochlidiid to date known only from a single locality on the island of Ambon, Indonesia. Previous molecular data found this charismatic species to link slender Strubellia slugs with broad and flattened Acochlidium and Palliohedyle. We establish Wallacellia siputbiru n. gen. n. sp., the “blue slug” in Bahasa Indonesia, by using scanning electron microscopy of cuticular elements, light microscopy of serial semithin histological sections of the soft body, and 3D reconstruction of all organ systems based on these sections. Special structures of this seemingly rare endemic species include the enlarged kidney and the huge copulatory organ. Our data now clarify that, in the invasion of freshwater habitats in Acochlidiidae, sexual selection (the anterior three-part copulatory organ) preceded ecological selection (posterior flattened habitus with branched or multiplied internal organs).

Dorids are one of the largest nudibranch groups, encompassing more than 2000 species. One of the crucial problems with tracing the evolution of dorids is the relationship between cryptobranch dorids (gill cavity present) and phanerobranch dorids (gill cavity absent). Integrative morphological and molecular studies of the enigmatic Japanese dorid species of the cryptobranch genus Cadlina, C. japonica and ‘C.’ sagamiensis, which were collected by the Emperor of Japan (Shōwa era), are presented here for the first time. It is shown that while C. japonica does belong to the Cadlina clade, another Japanese species ‘C.’ sagamiensis is not part of the cryptobranch dorids of the family Cadlinidae, but is related to both the phanerobranch dorid family, Hexabranchidae, and to the cryptobranch family Cadlinellidae stat. nov. A new genus, Showajidaia gen. nov., and new family, Showajidaiidae fam. nov., are proposed for ‘C.’ sagamiensis, and four new species of the genus Cadlina are described based on a dorid-wide molecular phylogenetic analysis, which is the first substantial update of the dorid family system since 2010. Integration of phylogenetic data with an ontogenetic model of dorid evolution suggests that cryptobranch organization can be most reliably assessed as the ancestral state for the majority of dorids.

Sampling impediments and paucity of suitable material for molecular analyses have precluded the study of speciation and radiation of deep-sea species in Antarctica. We analyzed barcodes together with genome-wide single nucleotide polymorphisms obtained from double digestion restriction site-associated DNA sequencing (ddRADseq) for species in the family Antarctophilinidae. We also reevaluated the fossil record associated with this taxon to provide further insights into the origin of the group. Novel approaches to identify distinctive genetic lineages, including unsupervised machine learning variational autoencoder plots, were used to establish species hypothesis frameworks. In this sense, three undescribed species and a complex of cryptic species were identified, suggesting allopatric speciation connected to geographic or bathymetric isolation. We further observed that the shallow waters around the Scotia Arc and on the continental shelf in the Weddell Sea present high endemism and diversity. In contrast, likely due to the glacial pressure during the Cenozoic, a deep-sea group with fewer species emerged expanding over great areas in the South-Atlantic Antarctic Ridge. Our study agrees on how diachronic paleoclimatic and current environmental factors shaped Antarctic communities both at the shallow and deep-sea levels, promoting Antarctica as the center of origin for numerous taxa such as gastropod mollusks Harvard University | Ref. NSF DEB-1754278 Fundación Ramón Areces | Ref. beca postdoctoral National Science Foundation | Ref. NSF ANT-1043749

BackgroundThe “Spanish” slug,Arion vulgarisMoquin-Tandon, 1855, is considered to be among the 100 worst pest species in Europe. It is common and invasive to at least northern and eastern parts of Europe, probably benefitting from climate change and the modern human lifestyle. The origin and expansion of this species, the mechanisms behind its outstanding adaptive success and ability to outcompete other land slugs are worth to be explored on a genomic level. However, a high-quality chromosome-level genome is still lacking.FindingsThe final assembly ofA. vulgariswas obtained by combining short reads, linked reads, Nanopore long reads, and Hi-C data. The genome assembly size is 1.54 Gb with a contig N50 length of 8.6 Mb. We found a recent expansion of transposable elements (TEs) which results in repetitive sequences accounting for more than 75% of theA. vulgarisgenome, which is the highest among all known gastropod species. We identified 32,518 protein coding genes, and 2,763 species specific genes were functionally enriched in response to stimuli, nervous system and reproduction. With 1,237 single-copy orthologs fromA. vulgarisand other related mollusks with whole-genome data available, we reconstructed the phylogenetic relationships of gastropods and estimated the divergence time of stylommatophoran land snails (Achatina) andArionslugs at around 126 million years ago, and confirmed the whole genome duplication event shared by them.ConclusionsTo our knowledge, theA. vulgarisgenome is the first land slug genome assembly published to date. The high-quality genomic data will provide valuable genetic resources for further phylogeographic studies ofA. vulgarisorigin and expansion, invasiveness, as well as molluscan aquatic-land transition and shell formation.

Background Heterobranchia is a diverse clade of marine, freshwater, and terrestrial gastropod molluscs. It includes such disparate taxa as nudibranchs, sea hares, bubble snails, pulmonate land snails and slugs, and a number of (mostly small-bodied) poorly known snails and slugs collectively referred to as the “lower heterobranchs”. Evolutionary relationships within Heterobranchia have been challenging to resolve and the group has been subject to frequent and significant taxonomic revision. Mitochondrial (mt) genomes can be a useful molecular marker for phylogenetics but, to date, sequences have been available for only a relatively small subset of Heterobranchia. Results To assess the utility of mitochondrial genomes for resolving evolutionary relationships within this clade, eleven new mt genomes were sequenced including representatives of several groups of “lower heterobranchs”. Maximum likelihood analyses of concatenated matrices of the thirteen protein coding genes found weak support for most higher-level relationships even after several taxa with extremely high rates of evolution were excluded. Bayesian inference with the CAT + GTR model resulted in a reconstruction that is much more consistent with the current understanding of heterobranch phylogeny. Notably, this analysis recovered Valvatoidea and Orbitestelloidea in a polytomy with a clade including all other heterobranchs, highlighting these taxa as important to understanding early heterobranch evolution. Also, dramatic gene rearrangements were detected within and between multiple clades. However, a single gene order is conserved across the majority of heterobranch clades. Conclusions Analysis of mitochondrial genomes in a Bayesian framework with the site heterogeneous CAT + GTR model resulted in a topology largely consistent with the current understanding of heterobranch phylogeny. However, mitochondrial genomes appear to be too variable to serve as good phylogenetic markers for robustly resolving a number of deeper splits within this clade.

Molecular and morphological evidence support the view that the widely distributed species Berthella stellata (Risso, 1826) is a species complex of at least eight different species. The closely related species Berthella plumula (Montagu, 1803), examined for comparison, is also a complex of two species; the name B. plumula is retained for the Atlantic species and the name Berthella perforata (Philippi, 1844) is proposed for the Mediterranean species. The B. stellata species complex forms a monophyletic group when the Eastern Pacific species Berthella strongi (MacFarland, 1966) is included. Based on a critical review of the literature, the name Berthella stellata is retained for the Eastern Atlantic and Mediterranean species, and the name Berthella pellucida (Pease, 1860) is resurrected for a species found in the Hawaiian Islands. Two new species from the Caribbean region (Berthella nebula sp. nov., Berthella vialactea sp. nov.) and one from the Eastern Pacific (Berthella andromeda sp. nov.) are described herein, but the status of the Brazilian species B. tupala Er. Marcus, 1957 remains uncertain. Two possible new species from the Eastern Pacific, represented by one specimen each, were recovered in the phylogenetic analyses but not formally described. It is hypothesized that additional species of this complex may occur in other parts of the Indo-Pacific tropics, particularly in the Indian Ocean.

Relationships among the major lineages of Mollusca have long been debated. Morphological studies have considered the rarely collected Monoplacophora (Tryblidia) to have several plesiomorphic molluscan traits. The phylogenetic position of this group is contentious as morphologists have generally placed this clade as the sister taxon of the rest of Conchifera whereas earlier molecular studies supported a clade of Monoplacophora + Polyplacophora (Serialia) and phylogenomic studies have generally recovered a clade of Monoplacophora + Cephalopoda. Phylogenomic studies have also strongly supported a clade including Gastropoda, Bivalvia, and Scaphopoda, but relationships among these taxa have been inconsistent. In order to resolve conchiferan relationships and improve understanding of early molluscan evolution, we carefully curated a high-quality data matrix and conducted phylogenomic analyses with broad taxon sampling including newly sequenced genomic data from the monoplacophoran Laevipilina antarctica. Whereas a partitioned maximum likelihood (ML) analysis using site-homogeneous models recovered Monoplacophora sister to Cephalopoda with moderate support, both ML and Bayesian inference (BI) analyses using mixture models recovered Monoplacophora sister to all other conchiferans with strong support. A supertree approach also recovered Monoplacophora as the sister taxon of a clade composed of the rest of Conchifera. Gastropoda was recovered as the sister taxon of Scaphopoda in most analyses, which was strongly supported when mixture models were used. A molecular clock based on our BI topology dates diversification of Mollusca to ~546 MYA (+/− 6 MYA) and Conchifera to ~540 MYA (+/− 9 MYA), generally consistent with previous work employing nuclear housekeeping genes. These results provide important resolution of conchiferan mollusc phylogeny and offer new insights into ancestral character states of major mollusc clades.

The Flabellinidae, a heterogeneous assembly of supposedly plesiomorphic to very derived sea slug groups, have not yet been addressed by integrative studies. Here novel material of rarely seen Arctic taxa as well as North Atlantic, North and South Pacific, and tropical Indo-West Pacific flabellinid species is investigated morpho-anatomically and with multi-locus markers (partial COI, 16S rDNA, 28S rDNA and H3) which were generated and analysed in a comprehensive aeolid taxon sampling. It was found that the current family Flabellinidae is polyphyletic and its phylogeny and taxonomic patterns cannot be understood without considering members from all the Aeolidacean families and, based on a robust phylogenetic hypothesis, morpho-anatomical evolution of aeolids is more complex than suspected in earlier works and requires reclassification of the taxon. Morphological diversity of Flabellinidae is corroborated by molecular divergence rates and supports establishing three new families (Apataidae fam. n., Flabellinopsidae fam. n., Samlidae fam. n.), 16 new genera, 13 new species, and two new subspecies among the former Flabellinidae. Two families, namely Coryphellidae and Paracoryphellidae, are restored and traditional Flabellinidae is considerably restricted. The distinctness of the recently described family Unidentiidae is confirmed by both morphological and molecular data. Several species complexes among all ex-“Flabellinidae” lineages are recognised using both morphological and molecular data. The present study shows that Facelinidae and Aeolidiidae, together with traditional “Tergipedidae”, deeply divide traditional “Flabellinidae.” Diagnoses for all aeolidacean families are therefore provided and additionally two new non-flabellinid families (Abronicidae fam. n. and Murmaniidae fam. n.) within traditional tergipedids are established to accommodate molecular and morphological disparity. To address relationships and disparity, we propose a new family system for aeolids. Here the aeolidacean species are classified into at least 102 genera and 24 families. Operational rules for integration of morphological and molecular data for taxonomy are suggested. © The Author(s). 2017. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Large parts of the discovered diversity of sea slugs are still undescribed, despite their potential contribution to a better understanding of the evolution of the group. Here, we describe an enigmatic tiny sea slug that was discovered in the 1970s on subtidal reef flats on Guam (Mariana Islands, Indo-West Pacific). The slug presents a long, coiled visceral sac, unique within the highly diverse body plans of sea slugs, which are usually characterized by different degrees of external body symmetry, linked to the reduction of the shell. Its initial assignment to the order Acochlidiida is confirmed by our 3D-microanatomical study based on a histological section series. Helicohedyle dikiki n. gen., n. sp. shows a number of putative plesiomorphic characters such as a simple excretory system-including a sac-like kidney combined with a short nephroduct-and an androdiaulic, hermaphroditic reproductive system, which bears an unarmed copulatory organ. Based on external morphology and microanatomical data it can be assigned to the Hedylopsacea and shows affinities both with marine and brackish Pseudunelidae, and also with limnic Tantulidae. We discuss the different potential scenarios of the evolution of a helicoid visceral sac as a secondary adaptation to the benthic marine lifestyle vs its interpretation as a plesiomorphic character from a coiled and shell-bearing ancestor, predating the transition to the interstices of sandy habitats associated with (progenetic) modifications in body plans.

Rhodopemorphs are small, interstitial or psammobiotic heterobranch slugs, which have been troubling to place phylogenetically. Their small size and habit of living in or among sediment have led to a correlated reduction and simplification of morphology, and consequently to contradictory phylogenetic signal from anatomical features. When morphological data have previously been used to generate phylogenetic hypotheses, these were vulnerable to the effects of homoplasy. We collected multiple species of Rhodope, along with another rhodopemorph genus Helminthope, to produce DNA sequence data for the first time to test their monophyly. We sequenced mitochondrial and nuclear genes, and analysed the data under maximum likelihood and Bayesian inference. By analysing these data in a broader heterobranch dataset, we also examined the placement of Rhodopemorpha within Heterobranchia. Despite rhodopemorphs showing aspects of a euthyneurous (and pentaganglionate) condition, their placement in a molecular phylogeny occurs outside the taxon defined by this term (i.e. Euthyneura = Pentaganglionata). Instead, model-based inferences placed rhodopemorphs among basal heterobranch taxa, usually as a clade sister to a shelled group (Murchisonellidae) that was recently removed from the Pyramidelloidea. The Rhodopemorpha + Murchisonellidae clade is herein termed Allomorpha. Three-dimensional reconstruction methods have elucidated potential morphological homologies for Rhodopemorpha and comparisons with Murchisonellidae have also uncovered morphological support for this placement. Thus, we consider the phylogenetic placement of Rhodopemorpha solved, although relationships among lower Heterobranchia lineages remain challenging.

Polyclad flatworms have a troubled classification history, with two contradicting systems in use. They both rely on a ventral adhesive structure to define the suborders Acotylea and Cotylea, but superfamilies were defined according to eyespot arrangement (Prudhoe’s system) or prostatic vesicle characters (Faubel’s system). Molecular data available cover a very limited part of the known polyclad family diversity and have not allowed testing morphology-based classification systems on Polycladida yet. We thus sampled a suitable marker, partial 28S ribosomal DNA (rDNA), from Polycladida (19 families and 32 genera), generating 136 new sequences and the first comprehensive genetic dataset on polyclads. Our maximum likelihood (ML) analyses recovered Polycladida, but the traditional suborders were not monophyletic, as the supposedly acotyleans Cestoplana and Theama were nested within Cotylea; we suggest that these genera should be included in Cotylea. The partial 28S rDNA trees were generally well supported and robust but in conflict with both Faubel’s and Prudhoe’s superfamilies. Therefore, we compiled morphological and anatomical characters for all taxa used and examined their distribution on our molecular tree. Combining morphological and molecular evidence, we redefined polyclad superfamilies. Acotylea contain tentaculated and atentaculated groups and is now divided in three superfamilies. The suborder Cotylea can be divided in five superfamilies. In general, there is a trait of anteriorization of sensory structures, from the plesiomorphic acotylean body plan to the cotylean gross morphology. Traditionally used characters, such as prostatic vesicle, eyespot distribution, and type of pharynx, are all homoplastic and likely have misled polyclad systematics so far.

The widespread sea hare species Aplysia parvula includes four genetically distinct lineages, containing a total of ten different species. While the four lineages can be differentiated by their external characteristics, species in each clade are often morphologically indistinguishable. A review of literature and type material revealed that several available names exist for species recognized herein: Aplysia parvula is retained for a species from the north-eastern Atlantic; A. atromarginata, A. elongata, A. nigrocincta and A. japonica are resurrected for species from the western Pacific Ocean, the Hawaiian Islands, the Indian and western Pacific Oceans, and Japan and Korea, respectively. Two new species names are introduced for animals from the eastern Pacific, and the Atlantic and Indian Oceans. Mitochondrial sequences from Mediterranean and eastern Atlantic specimens identified as A. parvula, resulted to be A. punctata. However, two specimens were heterozygotes of histone H3 alleles of A. punctata and of a new Atlantic species described herein, suggesting they could be hybrids. These results contradict the hypothesis that the Mediterranean was colonized by A. parvula. If an invasion occurred, it was a limited introgression of nDNA from an Atlantic species into native A. punctata populations.

The Splanchnotrophidae is a family of highly modified endoparasitic copepods known to infest nudibranch or sacoglossan sea slug hosts. Most splanchnotrophid species appear to be specific to a single host, but some were reported from up to nine different host species. However, splanchnotrophid taxonomy thus far is based on external morphology, and taxonomic descriptions are, mostly, old and lack detail. They are usually based on few specimens, with intraspecific variability rarely reported. The present study used molecular data for the first time to test (1) the current taxonomic hypotheses, (2) the apparently strict host specificity of the genusIsmailaand (3) the low host specificity of the genusSplanchnotrophuswith regard to the potential presence of cryptic species. Phylogenetic analyses herein used sequences of the barcoding region of the cytochrome oxidase I (COI) gene from 40 specimens representing 13 species of five genera. Species delimitation approaches include distance and barcoding gap analyses, haplotype networks and diagnostic nucleotides. Molecular results are largely compatible with the commonly accepted, morphology-based taxonomy of the Splanchnotrophidae. Strict host specificity could be confirmed for twoIsmailaspecies. COI analyses also supported the idea thatSplanchnotrophus angulatusis host-promiscuous. InIsmaila, morphology seems more suitable than barcoding to display speciation events via host switches in a recent Chilean radiation. InSplanchnotrophus, some genetic structure suggests ongoing diversification, which should be investigated further given the inadequate morphology-based taxonomy. The present study thus supports the presence of two different life history strategies in splanchnotrophids, which should be explored integratively.

For a long time, Diaphanidae has been considered a basal family within Cephalaspidea, based on the presence of plesiomorphic morphological features within this taxon. Traditionally, the family contained the genera Bogasonia, Colobocephalus, Colpodaspis, Diaphana, Newnesia, Toledonia and Woodbridgea. Some phylogenetic analyses of several of these genera support the basal position of Diaphanidae within Cephalaspidea sensu stricto. However, the family is presently confirmed to be a polyphyletic taxon in which only the genus Diaphana is included. Several genera previously embraced within the family, such as the monotypic Newnesia, have never been previously analysed in molecular studies. Here, we provide an extensive morphological, anatomical and histological description of a new species of Newnesia from Antarctic deep waters (967–1227 m depth) in the Drake Passage. We also discuss the similarities to the traditional Diaphanidae genera to try to shed light into this phylogenetic conundrum. We sequenced cytochrome c oxidase subunit I, 16S rRNA, 28S rRNA and histone H3 markers of Newnesia antarctica and Newnesia joani n. sp. We analysed a comprehensive dataset of sequenced genera to evaluate the placement of both Newnesia species within the cephalaspidean families. Maximum-likelihood and Bayesian phylograms support the monophyly of N. joani n. sp. and suggest cryptic speciation in N. antarctica specimens. Newnesia is recovered as the most basal offshoot of Cephalaspidea, suggesting the establishment of a new family restricted to Antarctic waters, named Newnesiidae n. fam., to hold both species. The possible Antarctic origin of Cephalaspidea is discussed.

Stylommatophora is one of the most speciose orders of Gastropoda, including terrestrial snails and slugs, some of which are economically important as human food, agricultural pests, vectors of parasites or due to invasiveness. Despite their great diversity and relevance, the internal phylogeny of Stylommatophora has been debated. To date, only 34 stylommatophoran mitogenomes were sequenced. Here, the complete mitogenome of an invasive pest slug,Arion vulgarisMoquin-Tandon, 1855 (Stylommatophora: Arionidae), was sequenced using next generation sequencing, analysed and compared with other stylommatophorans. The mitogenome ofA. vulgarismeasures 14,547 bp and contains 13 protein-coding, two rRNA, 22 tRNA genes, and one control region, with an A + T content of 70.20%. All protein coding genes (PCGs) are initiated with ATN codons except forCOX1, ND5andATP8and all are ended with TAR or T-stop codons. All tRNAs were folded into a clover-leaf secondary structure except fortrnCandtrnS1(AGN). Phylogenetic analyses confirmed the position ofA. vulgariswithin the superfamily Arionoidea, recovered a sister group relationship between Arionoidea and Orthalicoidea, and supported monophyly of all currently recognized superfamilies within Stylommatophora except for the superfamily Helicoidea. Initial diversification time of the Stylommatophora was estimated as 138.55 million years ago corresponding to Early Cretaceous. The divergence time ofA. vulgarisandArion rufus(Linnaeus, 1758) was estimated as 15.24 million years ago corresponding to one of Earth’s most recent, global warming events, the Mid-Miocene Climatic Optimum. Furthermore, selection analyses were performed to investigate the role of different selective forces shaping stylommatophoran mitogenomes. Although purifying selection is the predominant selective force shaping stylommatophoran mitogenomes, six genes (ATP8,COX1,COX3,ND3,ND4andND6) detected by the branch-specific aBSREL approach and three genes (ATP8,CYTBandND4L) detected by codon-based BEB, FUBAR and MEME approaches were exposed to diversifying selection. The positively selected substitutions at the mitochondrial PCGs of stylommatophoran species seems to be adaptive to environmental conditions and affecting mitochondrial ATP production or protection from reactive oxygen species effects. Comparative analysis of stylommatophoran mitogenome rearrangements using MLGO revealed conservatism in Stylommatophora; exceptions refer to potential apomorphies for several clades including rearranged orders oftrnW-trnYand oftrnE-trnQ-rrnS-trnM-trnL2-ATP8-trnN-ATP6-trnRclusters for the genusArion. Generally, tRNA genes tend to be rearranged and tandem duplication random loss, transitions and inversions are the most basic mechanisms shaping stylommatophoran mitogenomes.

Pteropods are holopelagic marine snails and slugs that are of particular interest to science due to their role in in marine food webs, global carbon cycle and their potential sensitivity to ocean change. Due to their pelagic, often exclusively offshore occurrence, samples are difficult to obtain, resulting in a lack of knowledge about their physiology, ecology, anatomy, geographical ranges, phylogenetic relationships and reproductive biology. Despite a recent increase in interest surrounding pteropod taxonomy, many evolutionary uncertainties remain due to limited taxon sampling and inavailability of molecular vouchers, in particular for the lesser investigated groups Pseudothecosomata and Gymnosomata. The Northwest Pacific Ocean is one of the least investigated areas for pteropods and in the adjacent semi-enclosed Sea of Okhotsk basin, current knowledge is restricted to the epipelagic zone. We summarize results from plankton hauls (from up to 5900 m depth) conducted during the joint German/Russian SokhoBio and KuramBio II cruises to the Sea of Okhotsk and the Kuril-Kamchatka-Trench region. This study presents an integrative taxonomic overview of six pteropod species identified by detailed morphological methods, including serial semithin sectioning, µCT and SEM scanning supported by multimarker (COI, 28S, and H3) genetic barcoding. We found four species of Gymnosomata slugs (Clione limacina, Clione okhotensis, Notobranchaea grandis and Thliptodon sp.), three species of Euthecosomata snails (Limacina helicina and two genetically delimited Clio spp.) and one shelled pseudothecosome species that is probably new to science (Peracle n. sp.). Multilocus phylogenetic analyses support monophyly of major traditional groups such as Pteropoda, Thecosomata, Pseudothecosomata and Gymnosomata. Micro-CT scanning was applied for the first time on pteropod soft bodies, allowing direct comparison between detailed anatomical peculiarities and molecular barcodes of the respective species. Furthermore, taxonomic positions, geographical ranges and potential dispersal barriers are discussed, with implications for future biodiversity comparisons. This study serves as a solid foundation for monitoring pteropods in a changing ocean.

We define unimodular measures on the space of rooted simplicial complexes and associate to each measure a chain complex and a trace function. As a consequence, we can define $\ell^2$-Betti numbers of unimodular random rooted simplicial complexes and show that they are continuous under Benjamini-Schramm convergence., Added references and Example 19; small corrections in the proof of Lemma 4

Polyclads are a conspicuous group of marine invertebrates, the most charismatic members of the phylum Platyhelminthes. From Brazil, a total of 71 polyclad species were reported or described. Only three of them were recently described, five are recent records for the Brazilian coast, and 55 were described by Ernest and Eveline Marcus, who were by far the most productive workers. However, they quite often published in Portuguese or German, rather than English, and have not designated type material or specified material deposited in museum collections. Most of the polylcad material studied by the Marcus was found to be in the Swedish Museum of Natural History. Here we summarize the knowledge about Brazilian polyclad biodiversity, give information about deposited material in different museums for future reference, and designate type material for the species that did not have any. We examined 58 polyclad species reported from Brazil and designated type material and information available on type series of 52 species. Lectotypes (89 vouchers) were designated for 30 species and paralectotypes (73 specimens / 70 vouchers) were designated for 22 Brazilian species. Among the 261 type vouchers examined in this work, 22 species (77 vouchers) had material recognized as holotypes and 2 vouchers were recognized as paratypes. Of the total number of species reported from Brazil, 10 species remain without information about type material. In the present paper we also propose a new combination (Lurymare cynarium nov. comb.). Eleven species have their geographical distribution range broadened and 42 were photographed for the first time, five of those were photographed live as well. The number of Brazilian polyclad species is expected to rise when different regions and environments are surveyed.

Our study focuses on the distribution and diversity of different cephalaspidean species collected from the Kuril-Kamchatka Trench (KKT) during the deep-sea expeditions KuramBioI (2012), SokhoBio (2015) and KuramBioII (2016). Our study recovered several undescribed taxa from different parts of the KKT and the adjacent abyssal plain, belonging to four families: Cylichnidae s. l., Scaphandridae, Aglajidae and Philinidae. The molecular phylogenetic and species delimitation analyses based on four gene markers (COI, 16S, H3, 28S) were conducted to determine species identity and phylogenetic relationships of several taxa. The morphological analysis included anatomical dissections and scanning electron microscopy. Herein, we focus on the family Philinidae as the most common cephalaspidean taxon in the sampling area but also provide general remarks on all the groups. All studied philinids from the KKT and adjacent abyssal plain belong to the genus Spiraphiline, which has been recorded from the northwestern Pacific for the first time. Three new species are described based on morphological and molecular data: Spiraphiline hadalis sp. nov., Spiraphiline kurilokamchatica sp. nov., both from the KKT, and Spiraphiline okhotensis sp. nov. from the western slope of the KKT and the Kuril Basin of the Sea of Okhotsk. The new subfamily Hermaniinae subfam. nov. with the genera Hermania and Spiniphiline is established; other philinid diversity is assigned to the subfamily Philininae. Preliminary data obtained for other families indicate a high cryptic diversity in the studied area, along with a very complex evolutionary history, especially in the families Aglajidae and Cylichnidae.

The position of scaphopods in molluscan phylogeny remains singularly contentious, with several sister relationships supported by morphological and phylogenomic data: Scaphopoda + Bivalvia (Diasoma), Scaphopoda + Cephalopoda (Variopoda), and Scaphopoda + Gastropoda. Nervous system architecture has contributed significant insights to reconstructing phylogeny in the Mollusca and other invertebrate groups, but a modern neurophylogenetic approach has not been applied to molluscs, hampered by a lack of clearly defined homologous characters that can be unequivocally compared across the radical body plan disparity among the living clades. We present the first three-dimensional reconstruction of the anterior nervous system of a scaphopod, Rhabdus rectius, using histological tomography. We also describe a new putative sensory organ, a paired and pigmented sensory mantle slit. This structure is restricted to our study species and not a general feature of scaphopods, but it forms an integral part of the description of the nervous system in R. rectius. It also highlights the potential utility of neuro-anatomical characters for multiple levels of phylogenetic inference beyond this study. This potential has not previously been exploited for the thorny problem of molluscan phylogeny. The neuroanatomy of scaphopods demonstrates a highly derived architecture that shares a number of key characters with the cephalopod nervous system, and supports a Scaphopoda + Cephalopoda grouping.

Invasion of the land has been a considerable challenge for aquatic organismal lineages, primarily because of desiccation and lack of physical support in air. Gastropod molluscs are among the most successful animals in this habitat shift because of their hard exoskeleton, whereas land slugs, regarded as descendants of land snails, gradually lost their shells in sheltered, moist environments. We present solid evidence of terrestrial invasion by a lineage of sea slugs; this invasion is represented by a newly discovered species, Aiteng marefugitus (Panpulmonata: Acochlidia), from a humid tropical rainforest on a small island in the Republic of Palau, western Pacific. The slug lineage seems to have invaded the forest habitat directly from the upper littoral zone in the Cenozoic.

The Splanchnotrophidae are a worldwide-distributed family of endoparasitic copepods, utilising shell-less opisthobranch gastropod hosts. Using scanning electron microscopy, we describe three new Ismaila Bergh, 1867 species infesting nudibranch hosts from southern Chile. Ismaila volatilis spec. nov. infests the proctonotid Janolus sp. and differs from all congeners by the size and number of dorsal bulges, the number of processes on the maxilla, and the thickness of thoracic appendages. Ismaila chaihuiensis spec. nov. was found in the doridoidean Diaulula punctuolata (D’Orbigny, 1837) and is diagnosed by a pore situated on a prominent bulge above the labrum. Ismaila genalis spec. nov. from a polycerid Holoplocamus papposus Odhner, 1926 host differs from its congeners in the size and form of the ventral bulges present on the head. These discoveries further broaden the range of splanchnotrophid host taxa; they are in line with earlier hypotheses of strict host specificity of Ismaila species and support Chile as a hotspot for Ismaila radiation. Herein, we present a key to the identification of all 14 Ismaila species. In Sulawesi (tropical Indo-West Pacific), Arthurius gibbosa spec. nov. infests the sacoglossan Elysia macnaei Marcus, 1982. The new species differs from both congeners by the short and stubby dorsal bulges. Uniquely among splanchnotrophids, long thoracic appendages are absent. Arthurius Huys, 2001 thus is the morphologically most divergent genus of the Splanchnotrophidae, but it is well characterized by the loss of several mouthparts and the preference for sacoglossan hosts. Currently, the distribution of the genus is limited to the Indo-West Pacific.

Species of Pleurobranchus (Mollusca: Gastropoda: Heterobranchia: Nudipleura: Pleurobranchidae) are commonly found worldwide, but there is a substantial amount of confusion regarding the ranges and identification of individual species. Difficulties in phylogenetic reconstruction and identification of pleurobranchids using morphological traits has resulted in complex classification schemes, with several species having disjunct ranges across physical and biogeographical barriers (including the tropical Indo-Pacific, the eastern Pacific, and the Atlantic). A sizeable number of species of Pleurobranchus has been described; however, many of these species are morphologically and biogeographically similar to others, and probably constitute synonyms. This paper provides a phylogenetic framework of classification for Pleurobranchus based on the mitochondrial genes cytochrome c oxidase I (COI) and 16S rDNA and the nuclear gene histone 3 (H3) using Bayesian and maximum likelihood approaches. Molecular phylogenies obtained recovered most of the well-established species of Pleurobranchus and some morphological characters were found to have taxonomic value for delimiting species in this group. Automatic barcode gap discovery (ABGD) analyses substantiated the distinctiveness of units/species recovered in the phylogenetic analyses, with some exceptions. Morphological descriptions for the 14 species recovered in the molecular phylogeny and discussions on the biogeography and colour variation are included. © 2015 The Linnean Society of London

The small volcanic island of Ascension is situated in the middle of the South Atlantic Ocean, more than 1500 km from the coast of Africa, its nearest continental area. To date, eight ‘opisthobranch’ species were reported from the island. As a result of a recent survey, 10 species were found. Seven species are new records from Ascension: Platydoris angustipes (Mörch, 1863), Diaulula sp., Dolabrifera dolabrifera (Rang, 1828), Aplysia parvula Guilding in Mörch, 1863 and Caliphylla mediterranea A. Costa, 1867, and two new species: Phidiana mimica sp. nov.; and Felimida atlantica sp. nov. Half of the species found have a wide geographical distribution, being not restricted to the Atlantic Ocean. However, traditional taxonomy based on few characters is probably masking complexes of species.

Molecular and morphological data from newly collected specimens and a review of the literature and type material indicate that the widespread tropical sea hare Dolabrifera dolabrifera is a species complex of five genetically distinct taxa. The name Dolabrifera dolabrifera is retained for a widespread species in the Indo-Pacific tropics. Dolabrifera nicaraguana is endemic to the eastern Pacific. Dolabrifera ascifera, D. virens and a new species described herein are restricted to the tropical Atlantic, with partially overlapping ranges in the Caribbean region and St. Helena. The temperate Pacific species Dolabrifera brazieri is also distinct and endemic to temperate southeastern Australia and New Zealand. These species of Dolabrifera constitute highly divergent lineages and most contain unique internal anatomical characteristics, particularly in the male reproductive morphology and shell shape, making them relatively easy to identify upon dissection. However, externally all these species are extremely variable in colour pattern and morphology and are virtually indistinguishable. This is particularly problematic for identification in the Atlantic Ocean where three species co-occur in the Caribbean region.

We provide a proof that the vanishing of $\ell^2$-Betti numbers of unimodular locally compact second countable groups is an invariant of coarse equivalence., Added Remark 13, to appear in Fundamenta Mathematicae

A combination of molecular, biogeographic and morphological data indicates that the eastern Pacific species Felimare ghiselini is a synonym of F. californiensis, also recorded from this region. However, a disjunct population from Peru, previously considered to be F. ghiselini, represents a distinct species, formally described herein as F. sechurana. Although previous work recognized that F. californiensis had a disjunct range with populations on the Pacific coast of North America and in the Gulf of California, the synonymization with F. ghiselini extends the range of this species into the southern part of the Gulf of California and also to Clipperton Island. Consequently, F. californiensis can now be stated to be common in the southern Gulf of California, minimizing concerns about the conservation status of this species. A latitudinal gradient in morphology is discussed. Speciation between F. californiensis and F. sechurana is hypothesized to be the result of dispersal of members of an ancestral population across the tropical eastern Pacific during the repeated periods of glaciation during the Pleistocene, followed by isolation. These results are consistent with a pattern of antitropical speciation that may have shaped biodiversity patterns across the eastern Pacific Ocean.

Sap-sucking sea slugs, Sacoglossa, are almost all suctorial herbivores feeding on siphonalean green algae. Ercolania is probably the most species-rich genus in the family Limapontiidae, containing at least 25 species, most of which feed on filamentous green algae of the order Cladophorales. Here we describe an ewErcolania species from the Andaman Sea coast of Thailand. The species is very small, generally less than 1 mm long in sexually mature animals. It is found exclusively on the leaves of the seagrass Halophila ovalis and accordingly we name it Ercolania halophilae n. sp. Anatomy and histology have been studied by serial sectioning and 3D-reconstruction. This is the first limapontioid species studied in full 3D-microanatomical detail. Ercolania halophilae shows several special features. A heart and pericardium are absent, as is also seen in a few other species of Sacoglossa. Salivary glands are very prominent and unequal on left and right sides. There is a system of highly vacuolated cells situated along the main digestive gland tubules. Those cells are apparently associated with the kidney and we assume that they are involved in excretion. Radular teeth are sabot-shaped and more similar in shape to those of the enigmatic Japanese Alderiopsis nigra than to any known species of Ercolania. The reproductive system is typical limapontiid, with a closed seminal receptacle, a penetrant penial stylet and a bilobed prostate gland. Branches of the albumen gland enter the cerata, as seen in some but not all species presently referred to Ercolania. We therefore place the species in Ercolania rather than the poorly defined Alderiopsis. We briefly compare limapontiid genera and show that revision is needed.

Molluscs are diverse and important enough to receive considerable scientific attention. We herein question recent opinions that the molluscan origin, deep inner relationships, and early evolution have been largely resolved. The traditional Testaria concept implied progressive evolution from worm-like aplacophoran to polyplacophoran and then conchiferan body plans; sole evidence for this came from morphocladistic analyses, which we conclude were confounded by homoplasy. The recently preferred Aculifera-Conchifera concept is supported by some but not all analyses using massive sequence data on rather small and uneven taxon sets. Recent results from mitogenomics indicate that gene-rearrangement events could cause sequence biases overriding potential phylogenetic signal. We discuss recent progress regarding multilocus marker analyses, particularly refining the neglected Serialia hypothesis with Monoplacophora sister to Polyplacophora. This third hypothesis for molluscan relationships is supported by an integr...

Our knowledge of the biodiversity and distribution patterns of benthic deep-sea faunas is still limited, with large parts of the world׳s abyss unexplored, lacking α-taxonomic data across oceans basins and especially of biogeographic transition zones between oceans. The Antarctic Polar Frontal Zone has been discussed as major biogeographic barrier hindering faunal exchange between Subantarctic and Antarctic provinces and conserving high rates of endemism in the Southern Ocean benthos. In the present study we report first, exploratory α-taxonomy on the malacofauna sampled by means of an epibenthic sledge from four bathyal respectively abyssal stations (2732–4327 m depth) in the vicinity of the Antarctic Polar Front during the SYSTCO II expedition (SYSTem COupling in the Southern Ocean, RV Polarstern cruise ANT XXVIII/3). We identified 58 distinct molluscan taxa based on external morphology (‘morphospecies’); of the 33 taxa successfully assigned to described species 94% were previously reported from the Southern Ocean, but 24% exhibit distribution ranges crossing the Polar Front. One North Atlantic scaphopod is reported for the first time in Antarctic waters. Our study supports that the Antarctic Polar Front does not serve as effective barrier preventing gene flow in deep-sea molluscs. The present dataset shows the general characteristics of deep-sea sampling: patchiness in distribution and a high degree of singletons. Overall molluscan abundances were generally low ranging between 3.60 and 24.65 ind./1000 m², but in comparison with equatorial and subtropic abyssal basins, gastropod species richness and abundance were reaching high values similar to high Antarctic stations. Comparison between high productivity and low productivity zones along the Polar Front suggests increased abundances and species richness in high productivity zones. Intensified sampling is needed, however, to outweigh stochastic errors and to evaluate the influence of carbon flux as driving factor to faunal composition and abundances of abyssal molluscs.

The Thecosomata are pelagic euopisthobranch pteropods that are important for marine food chains, but threatened by ocean acidification. Members of the suborder Euthecosomata are either torted snails with a coiled, sinistral shell (Limacinidae) or are straight-shelled with a symmetrical body and an unusual ventral mantle cavity (Orthoconcha). The classification and taxonomy of euthecosomes still depends on shells, but is being challenged by initial molecular data. There is a large body of morphoanatomical information dating from the beginning of the last century, and only biological (rather than soft-part anatomical) detail has been added since. For our initial study on pteropod morphoanatomy we have selected the potentially basal orthoconch genus Creseis Rang, 1828. Supplementing Meisenheimer’s (1905) monographic work, we redescribe the microanatomy of the Mediterranean C. clava (Rang, 1828) from serial semithin sections and present 3D-models of all major organ systems. In the absence of histological differences we interpret the head to be fused with the foot, forming the wings with reduced labial tentacles and rhinophores. The postpharyngeal nerve ring is strongly condensed, showing fused buccal ganglia and a short visceral loop with two discernable ganglia. The genital system is monaulic and hermaphroditic, with female glands and a potential allosperm receptacle of unclear homology. An open seminal groove connects with the frontal copulatory organ, which shows complex penial infoldings. We confirm the 1808 longitudinal rotation of the visceral organs relative to the condition in limacinids. As an alternative to ontogenetic detorsion, progenesis may have skipped torsion. Other obvious paedomorphoses such as single, basally forked tentacular nerves, suggest that heterochrony has been a driving force in thecosome evolution. Retaining the mantle cavity in a ventral position, Creseis has its large mantle gland—necessary for creating a mucus web for feeding—close to the mouth. Further comparative microanatomical data on orthoconchs and limacinids are needed to corroborate our assumptions of homology, and for reconstructing pteropod evolution once reliable molecular phylogenetic hypotheses are available.

One of the main difficulties in the taxonomy of heterobranch sea slugs is the interpretation of small morphological and body colour differences in a group of specimens, sympatric or allopatric, as variation of a single species or indicative of similar, but different, species. The aeolid Cratena peregrina is one of the most common and typical nudibranchs from the Mediterranean Sea and was recently informally recorded from Senegal, South Africa, India and in the western Atlantic. In the present work, we investigate the potential presence of C. peregrina on the coast of Brazil. Brazilian and Mediterranean specimens are compared through multiple approaches, including (1) a molecular phylogenetic analysis based on a mitochondrial and a nuclear marker (cytochrome c oxidase subunit I and H3, respectively); (2) performing population analyses such as haplotype networks via TCS and Birky’s coalescence-based K/u ratio; (3) automatic barcode gap discovery and (4) comparative morphological study. As a result of our integrative species delimitation approach, we conclude that the morphological and body colour differences observed between Mediterranean and Brazilian specimens are not due to intraspecific variation in C. peregrina and that C. peregrina is not present in Brazil. Instead, Brazilian specimens belong to a new species, C. minor n. sp., which is described herein. We use this case study to discuss currently available methods of species delimitation and their integrative application to heterobranch sea slugs.