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Anthropogenic climate change is causing observable changes in Antarctica and the Southern Ocean including increased air and ocean temperatures, glacial melt leading to sea-level rise and a reduction in salinity, and changes to freshwater water availability on land. These changes impact local Antarctic ecosystems and the Earth's climate system. The Antarctic has experienced significant past environmental change, including cycles of glaciation over the Quaternary Period (the past ~2.6 million years). Understanding Antarctica's paleoecosystems, and the corresponding paleoenvironments and climates that have shaped them, provides insight into present day ecosystem change, and importantly, helps constrain model projections of future change. Biological archives such as extant moss beds and peat profiles, biological proxies in lake and marine sediments, vertebrate animal colonies, and extant terrestrial and benthic marine invertebrates, complement other Antarctic paleoclimate archives by recording the nature and rate of past ecological change, the paleoenvironmental drivers of that change, and constrain current ecosystem and climate models. These archives provide invaluable information about terrestrial ice-free areas, a key location for Antarctic biodiversity, and the continental margin which is important for understanding ice sheet dynamics. Recent significant advances in analytical techniques (e.g., genomics, biogeochemical analyses) have led to new applications and greater power in elucidating the environmental records contained within biological archives. Paleoecological and paleoclimate discoveries derived from biological archives, and integration with existing data from other paleoclimate data sources, will significantly expand our understanding of past, present, and future ecological change, alongside climate change, in a unique, globally significant region.

The Antarctic marine environment, although rich in life, is predicted to experience rapid and significant effects from climate change. Despite a revolution in the approaches used to document biodiversity, less than one percent of Antarctic marine invertebrates are represented by DNA barcodes and we are at risk of losing biodiversity before discovery. The ease of sequencing mitochondrial DNA barcodes has promoted this relatively ‘universal’ species identification system across most metazoan phyla and barcode datasets are currently readily used for exploring questions of species-level taxonomy. Here we present the most well-sampled phylogeny of the direct-developing, Southern Ocean nudibranch mollusc, Doris kerguelenensis to date. This study sampled over 1000 new Doris kerguelenensis specimens spanning the Southern Ocean and sequenced the mitochondrial COI gene. Results of a maximum likelihood phylogeny and multiple subsequent species delimitation analyses identified 27 new species in this complex (now 59 in total). Using rarefaction techniques, we infer more species are yet to be discovered. Some species were only collected from southern South America or the sub-Antarctic islands, while at least four species were found spanning the Polar Front. This is contrary to dispersal predictions for species without a larval stage such as Doris kerguelenensis. Our work demonstrates the value of increasing geographic scope in sampling and highlights what could be lost given the current global biodiversity crisis.

The marine-based West Antarctic Ice Sheet (WAIS) is considered vulnerable to collapse under future climate trajectories and may even lie within the mitigated warming scenarios of 1.5–2 °C of the United Nations Paris Agreement. Knowledge of ice loss during similarly warm past climates, including the Last Interglacial period, when global sea levels were 5–10 m higher than today, and global average temperatures of 0.5–1.5 °C warmer, could resolve this uncertainty. Here we show, using a panel of genome-wide, single nucleotide polymorphisms of a circum-Antarctic octopus, persistent, historic signals of gene flow only possible with complete WAIS collapse. Our results provide the first empirical evidence that the tipping point of WAIS loss could be reached even under stringent climate mitigation scenarios.One-Sentence SummaryHistorical gene flow in marine animals indicate the West Antarctic ice sheet collapsed during the Last Interglacial period.

Numerous faunistic and ecological studies have been conducted throughout the Indo-Pacific Ocean to assess its biodiversity. Despite the abundance of research, studies on the species that inhabit the Indo-Pacific are still necessary due to its extent and high species richness. The major species richness of the genus Okenia Menke, 1830 (Nudibranchia, Goniodorididae) is found in the Indo-Pacific Ocean, including 38 of 60 valid species. Nevertheless, this number does not represent the real biodiversity, since at least 20 more species are already reported in field-guides as undescribed species belonging to this genus. The systematics of the genus Okenia are still unclear since it has been the subject of only a few and incomplete studies. In the present paper, we describe five new Okenia species from the coastlines of Japan, Mozambique and Australia: Okenia aurorapapillata sp. nov., Okenia elisae sp. nov., Okenia nakanoae sp. nov., Okenia siderata sp. nov. and Okenia tenuifibrata sp. nov. Moreover, anatomical details not previously described of Okenia atkinsonorum, Okenia barnardi, Okenia cf. echinata, Okenia hallucigenia, Okenia hiroi, Okenia japonica, Okenia pellucida, Okenia pilosa and Okenia rhinorma are provided. New partial sequences of standard markers (COI, 16S rRNA and H3) were obtained and a phylogenetic analysis that included all species with available data was performed. ZOOBANK urn:lsid:zoobank.org:pub:28AE2536-A264–4194–8AE3-C430620572E7

Despite strong historical biogeographical links between benthic faunal assemblages of the Magellan region of South America and the Antarctic Peninsula, very few studies have documented contemporary movement and gene flow in or out of the Southern Ocean, especially across the Antarctic Polar Front (APF). In fact, oceanographic barriers such as the APF and Antarctica's long geologic isolation have substantially separated the continents and facilitated the evolution of endemic marine taxa found within the Antarctic region. The Southern Ocean benthic sea slug complex

Sponges in the subfamily Phyllospongiinae are important components of coral reefs. However, significant taxonomic inconsistencies exist in this group due to the lack of useful morphological characters for species delineation. This study assesses the systematics of some common phyllospongiinids in the genera Carteriospongia, Phyllospongia and Strepsichordaia from tropical Australia and the Red Sea, by using a multigene approach that utilizes the Internal Transcribed Spacer 2, the complete ribosomal 18S rRNA and three 28s rRNA gene regions (D1‒D2, D3‒D5 and D6‒D8), which produced a phylogenetic framework in which complementary morphological taxonomic assessments were performed. Type specimens were included, where available, and six species clades were recovered, including the well-established Phyllospongia papyracea and Strepsichordaia lendenfeldi. Carteriospongia foliascens, the type species for the genus Carteriospongia, is transferred to the genus Phyllospongia, resulting in Carteriospongia becoming a synonym of Phyllospongia. Consequently, Carteriospongia flabellifera is removed from Carteriospongia and is reinstated to its original designation of Polyfibrospongia flabellifera. Two new species, Phyllospongia bergquistae sp. nov. and Polyfibrospongia kulit sp. nov., are described. With phyllospongiinid sponges increasingly used as models for assessing the effects of climate change and anthropogenic stressors, this study provides a reliable systematics framework for the accurate identification of common phyllospongiinids across the Indo-Pacific.

Throughout the Quaternary, the continental‐based Antarctic ice sheets expanded and contracted repeatedly. Evidence suggests that during glacial maxima, grounded ice eliminated most benthic (bottom‐dwelling) fauna across the Antarctic continental shelf. However, paleontological and molecular evidence indicates most extant Antarctica benthic taxa have persisted in situ throughout the Quaternary. Where and how the Antarctic benthic fauna survived throughout repeated glacial maxima remain mostly hypothesised. If understood, this would provide valuable insights into the ecology and evolution of Southern Ocean biota over geological timescales. Here we synthesised and appraised recent studies and presented an approach to demonstrate how genetic data can be effective in identifying where and how Antarctic benthic fauna survived glacial periods. We first examined the geological and ecological evidence for how glacial periods influenced past species demography in order to provide testable frameworks for future studies. We outlined past ice‐free areas from Antarctic ice sheet reconstructions that could serve as glacial refugia and discussed how benthic fauna with pelagic or non‐pelagic dispersal strategies moved into and out of glacial refugia. We also reviewed current molecular studies and collated proposed locations of Southern Ocean glacial refugia on the continental shelf around Antarctica, in the deep sea, and around sub‐Antarctic islands. Interestingly, the proposed glacial refugia based on molecular data generally do not correspond to the ice‐free areas identified by Antarctic ice sheet reconstructions. The potential biases in sampling and in the choice of molecular markers in current literature are discussed, along with the future directions for employing testable frameworks and genomic methods in Southern Ocean molecular studies. Continued data syntheses will elucidate greater understanding of where and how Southern Ocean benthic fauna persisted throughout glacial periods and provide insights into their resilience against climate changes in the future.

The cold waters of Antarctica are known to harbor a rich biodiversity. Our continuing interest in the chemical analysis of Antarctic invertebrates has resulted in the isolation of friomaramide (1), a new, highly modified hexapeptide, from the Antarctic sponge Inflatella coelosphaeroides. The structure of friomaramide was determined using spectroscopic methods and its configuration established by Marfey's method. Friomaramide, which bears the unusual permethylation of the amino acid backbone and is the longest polypeptide bearing a tryptenamine C-terminus, blocks >90% of Plasmodium falciparum liver-stage parasite development at 6.1 μM.

An anomalous El Nino-associated sea surface temperature stress event was predicted to affect tropical Australian reefs, including those in North Western Australia in the summer of 2015/2016. Thermal stress events are well known to result in widespread hard coral mortality events, but other symbiotic organisms such as soft corals, giant clams and sponges can also be affected. Here, we examine whether the 2016 thermal stress event deleteriously impacted coral reef communities in the remote Bonaparte Archipelago, central inshore Kimberley bioregion, North West Australia. Our results confirm the region experienced a thermal stress event of similar magnitude to other regional localities (i.e., southern Kimberley and Scott Reef), but contrary to those locations that experienced widespread bleaching events, we find no evidence to suggest widespread mortality events occurred among photosymbiotic organisms in the Bonaparte Archipelago. Photosymbiotic organisms in this region are assumed to be well adapted to fluctuating environmental conditions; however, in this instance, a greater magnitude of night-time cooling may have driven variability in regional susceptibility to thermal stress. The Bonaparte Archipelago is emerging as a globally significant ecological refuge for photosymbiotic benthic fauna that are threatened by cumulative anthropogenic and climate stressors in other parts of their distribution.

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

Sponges represent one of the most species-rich hosts for commensal barnacles yet host utilisation and diversity have not been thoroughly examined. This study investigated the diversity and phylogenetic relationships of sponge-inhabiting barnacles within a single, targeted host group, primarily from Western Australian waters. Specimens of the sponge order Dictyoceratida were surveyed and a total of 64 host morphospecies, representing four families, were identified as barnacle hosts during the study. Utilising molecular (COI, 12S) and morphological methods 42 molecular operational taxonomic units (MOTUs) of barnacles, representing Acasta, Archiacasta, Euacasta and Neoacasta were identified. Comparing inter- and intra-MOTU genetic distances showed a barcode gap between 2.5% and 5% for COI, but between 1% and 1.5% in the 12S dataset, thus demonstrating COI as a more reliable barcoding region. These sponge-inhabiting barnacles were demonstrated to show high levels of host specificity with the majority being found in a single sponge species (74%), a single genus (83%) or a single host family (93%). Phylogenetic relationships among the barnacles were reconstructed using mitochondrial (12S, COI) and nuclear (H3, 28S) markers. None of the barnacle genera were recovered as monophyletic. Euacasta was paraphyletic in relation to the remaining Acastinae genera, which were polyphyletic. Six well-supported clades of molecular operational taxonomic units, herein considered to represent species complexes, were recovered, but relationships between them were not well supported. These complexes showed differing patterns of host usage, though most were phylogenetically conserved with sister lineages typically occupying related hosts within the same genus or family of sponge. The results show that host specialists are predominant, and the dynamics of host usage have played a significant role in the evolutionary history of the Acastinae.

International audience; Members of the trochoidean genus Margarella (Calliostomatidae) are broadly distributed across Antarctic and sub-Antarctic ecosystems. Here we used novel mitochondrial and nuclear gene sequences to clarify species boundaries and phylogenetic relationships among seven nominal species distributed on either side of the Antarctic Polar Front (APF). Molecular reconstructions and species-delimitation analyses recognized only four species: M. antarctica (the Antarctic Peninsula), M. achilles (endemic to South Georgia), M. steineni (South Georgia and Crozet Island) and the morphologically variable M. violacea (=M. expansa, M. porcellana and M. pruinosa), with populations in southern South America, the Falkland/Malvinas, Crozet and Kerguelen Islands. Margarella violacea and M. achilles are sister species, closely related to M. steineni, with M. antarctica sister to all these. This taxonomy reflects contrasting biogeographic patterns on either side of the APF in the Southern Ocean. Populations of Margarella north of the APF (M. violacea) showed significant genetic variation but with many shared haplotypes between geographically distant populations. By contrast, populations south of the APF (M. antarctica, M. steineni and M. achilles) exhibited fewer haplotypes and comprised three distinct species, each occurring across a separate geographical range. We hypothesize that the biogeographical differences may be the consequence of the presence north of the APF of buoyant kelps – potential long-distance dispersal vectors for these vetigastropods with benthic-protected development – and their near-absence to the south. Finally, we suggest that the low levels of genetic diversity within higher-latitude Margarella reflect the impact of Quaternary glacial cycles that exterminated local populations during their maxima.

Tudivasum Rosenberg & Petit, 1987 is a morphologically distinct gastropod genus of low diversity. All but one species are known from Australia and they occur from the intertidal zone down to hundreds of metres on the continental shelf. These carnivorous gastropods are thought to have intracapsular development. The six currently recognized extant species are reviewed here and their geographical ranges clarified. Two new species, Tudivasum chaneyi n. sp. and T. ashmorense n. sp., are described from Ashmore Reef, Western Australia, and are characterized by differences in protoconch colour and shell sculpture. The third new species, T. westrale n. sp., is described from the mid-west coast of Western Australia, where it has long been misidentified as T. spinosum (H. Adams & A. Adams, 1864). We generated a molecular phylogeny based on mitochondrial DNA sequence data to test morphological species concepts and reconstruct relationships among four of the described species. High levels of divergence within one of the new species could indicate an additional cryptic species.

During the Last Glacial Maximum (LGM), global sea levels were 120-130 m lower than today, resulting in the emergence of most continental shelves and extirpation of subtidal organisms from these areas. During the interglacial periods, rapid inundation of shelf regions created a dynamic environment for coastal organisms, such as the charismatic leafy seadragon (Phycodurus eques, Syngnathidae), a brooder with low dispersal ability inhabiting kelp beds in temperate Australia. Reconstructions of the palaeoshoreline revealed that the increase of shallow areas since the LGM was not uniform across the species' range and we investigated the effects of these asymmetries on genetic diversity and structuring. Using targeted capture of 857 variable ultraconserved elements (UCEs, 2,845 single nucleotide polymorphisms) in 68 individuals, we found that the regionally different shelf topographies were paralleled by contrasting population genetic patterns. In the west, populations may not have persisted through sea-level lows because shallow seabed was very limited. Shallow genetic structure, weak expansion signals and a westward cline in genetic diversity indicate a postglacial recolonization of the western part of the range from a more eastern location following sea-level rise. In the east, shallow seabed persisted during the LGM and increased considerably after the flooding of large bays, which resulted in strong demographic expansions, deeper genetic structure and higher genetic diversity. This study suggests that postglacial flooding with rising sea levels produced locally variable signatures in colonizing populations.

The taxonomy and diversity of chromodorid nudibranchs (Mollusca: Gastropoda: Nudibranchia) are relatively well-studied, although molecular analyses have only been applied to test few species-level groups. Within Chomodorididae, the genus Goniobranchus Pease, 1866 contains many different groups based on external colouration, including the red-reticulate group. Previously, most of the red-reticulate group has been considered as either a single variable species, identified as either Goniobranchus reticulatus Pease, 1866 or Goniobranchus tinctorius (Ruppell & Leuckart, 1828), or as a species complex. In this study, red-reticulate nudibranchs were collected from different locations spanning the Indo-Pacific. From these specimens, DNA data were derived, consisting of mitochondrial cytochrome c oxidase subunit I (COI) and 16S ribosomal DNA (16S rDNA) sequences. COI data were analysed using a distance-based species delimitation approach (ABGD), and the combined dataset (COI+16S rDNA) was analysed using maximum likelihood (ML) and Bayesian inference (BI) methods. The analyses identified five species-level clades, but none of the clades’ morphotypes matched exactly with the original descriptions of G. tinctorius or G. reticulatus. The non-reticulate, spotted species Goniobranchus splendidus (Angas, 1864) falls inside this complex. The existence of multiple species-level clades in the Goniobranchus red-reticulate group suggests this is a group of closely related species, comprising at least five undescribed taxa.

Aim The Antarctic Circumpolar Current (ACC) connects benthic populations by transporting larvae around the continent, but also isolates faunas north and south of the Antarctic Convergence. We test circumpolar panmixia and dispersal across the Antarctic Convergence barrier in the benthic sea star Glabraster antarctica. Location The Southern Ocean and south Atlantic Ocean, with comprehensive sampling including the Magellanic region, Scotia Arc, Antarctic Peninsula, Ross Sea, and East Antarctica. Methods The cytochrome c oxidase subunit I (COI) gene (n = 285) and the internal transcribed spacer region 2 (ITS2; n = 33) were sequenced. We calculated haplotype networks for each genetic marker and estimated population connectivity and the geographic distribution of genetic structure using ΦST for COI data. Results Glabraster antarctica is a single circum-Antarctic species with instances of gene flow between distant locations. Despite the homogenizing potential of the ACC, population structure is high (ΦST = 0.5236), and some subpopulations are genetically isolated. Genetic breaks in the Magellanic region do not align with the Antarctic Convergence, in contrast with prior studies. Connectivity patterns in East Antarctic sites are not uniform, with some regional isolation and some surprising affinities to the distant Magellanic and Scotia Arc regions. Main conclusions Despite gene flow over extraordinary distances, there is strong phylogeographic structuring and genetic barriers evident between geographically proximate regions (e.g., Shag Rocks and South Georgia). Circumpolar panmixia is rejected, although some subpopulations show a circumpolar distribution. Stepping-stone dispersal occurs within the Scotia Arc but does not appear to facilitate connectivity across the Antarctic Convergence. The patterns of genetic connectivity in Antarctica are complex and should be considered in protected area planning for Antarctica.

Maintaining the accretion potential and three dimensional structure of coral reefs is a priority but reef-building scleractinian corals are highly threatened and retreating. Hence future reefs are predicted to be dominated by non-constructional taxa. Since the Late Triassic however, other non-scleractinian anthozoans such as Heliopora have contributed to tropical and subtropical reef-building. Heliopora is an ancient and highly conserved reef building octocoral genus within the monospecific Family Helioporidae, represented by a single extant species – H. coerulea, Pallas, 1766. Here we show integrated morphological, genomic and reproductive evidence to substantiate the existence of a second species within the genus Heliopora. Importantly, some individuals of the new species herein described as Heliopora hiberniana sp. nov. feature a white skeleton indicating that the most diagnostic and conserved Heliopora character (the blue skeleton) can be displaced. The new species is currently known only from offshore areas in north Western Australia, which is a part of the world where coral bleaching events have severely impacted the scleractinian community over the last two decades. Field observations indicate individuals of both H. coerulea and H. hiberniana sp. nov. were intact after the 2016 Scott Reef thermal stress event, and we discuss the possibility that bleaching resistant non-scleractinian reef builders such as Heliopora could provide new ecological opportunities for the reconfiguration of future reefs by filling empty niches and functional roles left open by the regression of scleractinian corals.

Predicting biotic resistance to highly invasive strains of “killer algae” (Caulerpa spp.) requires understanding the diversity and feeding preferences of native consumers, including sea slugs in family Oxynoidae. Past studies reported low algal host specificity for Oxynoe (6 spp.) and Lobiger (4 spp.), but these taxonomically challenging slugs may represent species complexes of unrecognized specialists that prefer different Caulerpa spp. Here, we assess global diversity of these genera by integrating gene sequences with morphological data from microscopic teeth and internal shells, the only hard parts in these soft-bodied invertebrates. Four delimitation methods applied to datasets comprising mtDNA and/or nuclear alleles yielded up to 16 species hypotheses for samples comprising five nominal taxa, including five highly divergent species in Lobiger and five in Oxynoe. Depending on the analysis, a further four to six species were recovered in the O. antillarum-viridis complex, a clade in which mitochondrial divergence was low and nuclear alleles were shared among lineages. Bayesian species delimitation using only morphological data supported most candidate species, however, and integrative analyses combining morphological and genetic data fully supported all complex members. Collectively, our findings double the recognized biodiversity in Oxynoidae, and illustrate the value of including data from traits that mediate fast-evolving ecological interactions during species delimitation. Preference for Caulerpa spp. and radular tooth characteristics covaried among newly delimited species, highlighting an unappreciated degree of host specialization and coevolution in these taxa that may help predict their role in containing outbreaks of invasive algae.

Repeat multibeam mapping of two slope-confined canyons on the northwest Australian margin provides new understanding of the processes that are active in shaping these environments. The Cape Range and Cloates canyons initiate on the mid to lower continental slope but are now known to be connected to the shelf via small channels and gullies. Both canyons have areas of steep walls, with evidence of slides, and large depressions on the canyon floors. These canyons were first mapped systematically with multibeam sonar in 2008 and were remapped in 2020 during a biodiversity survey that also collected high-resolution imagery and biological samples. Comparison of seabed features between these two time periods indicates active sliding, minor headwall retreat and continued excavation of deep depressions on the canyon floor. Significantly, intact blades of displaced seagrass imaged at various depths up to 4200 m throughout both canyons indicates that material sourced from the adjacent continental shelf is being transported through these canyon systems. Turbidity currents are actively modifying canyon walls and floor depressions, while also providing a sediment source that has resulted in minor accretion on the canyon floor. Sedimentation likely regulates benthic communities in these canyons, with imagery showing highest densities of sessile invertebrates in habitats protected from sedimentation (e.g. rock overhangs, cliff edges). Since steep canyon habitats are rare within these canyons, and support high benthic abundance, they likely represent biologically significant areas of the Gascoyne Marine Park. Repeat mapping provides an understanding of the dynamics of these canyons and a context for assessing and monitoring the stability of the seabed habitats within this marine reserve.

Sea levels at the end of this century are projected to be 0.26–0.98 m higher than today. The upper end of this range, and even higher estimates, cannot be ruled out because of major uncertainties in the dynamic response of polar ice sheets to a warming climate. Here, we propose an ecological genetics approach that can provide insight into the past stability and configuration of the West Antarctic Ice Sheet (WAIS). We propose independent testing of the hypothesis that a trans-Antarctic seaway occurred at the last interglacial. Examination of the genomic signatures of bottom-dwelling marine species using the latest methods can provide an independent window into the integrity of the WAIS more than 100,000 years ago. Periods of connectivity facilitated by trans-Antarctic seaways could be revealed by dating coalescent events recorded in DNA. These methods allow alternative scenarios to be tested against a fit to genomic data. Ideal candidate taxa for this work would need to possess a circumpolar distribution, a benthic habitat, and some level of genetic structure indicated by phylogeographical investigation. The purpose of this perspective piece is to set out an ecological genetics method to help resolve when the West Antarctic Ice Shelf last collapsed.

During a 2013 cruise in the Southern Ocean we collected specimens of the octocoral Plumarella delicatissima between 800 and 950 m depth. Five new furanocembranoid diterpenes, keikipukalides A-E (1-5), the known diterpene pukalide aldehyde (6), and the known norditerpenoid ineleganolide (7) were isolated from the coral. These Plumarella terpenes lack mammalian cytotoxicity, while 2-7 display activity against Leishmania donovani between 1.9 and 12 μM. Structure elucidation was facilitated by one- and two-dimensional NMR spectroscopy and mass spectrometry, and keikipukalides A and E were confirmed by X-ray crystallography.

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.