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The complex world of arthropods The evolutionary interrelationship of arthropods (jointed-legged animals) has long been a matter of dispute. A new phylogeny based on an analysis of over 41,000 base pairs of DNA from 75 species, including representatives of every major arthropod lineage, should ease the way towards a consensus on the matter. The data support the idea that insects are land-living crustaceans, that crustaceans comprise a diverse assemblage of at last three distinct arthropod types, and that myriapods (millipedes and centipedes) are the closest relatives of this great 'pancrustacean' group. The evolutionary interrelationships of arthropods has long been a matter of dispute. A new phylogeny applies an arsenal of techniques to more than 41,000 base pairs of DNA from 75 arthropod species. The results support the idea that insects are land–living crustaceans, that crustaceans comprise a diverse assemblage of at last three distinct arthropod types, and that myriapods (millipedes and centipedes) comprise the closest relatives of this great 'pancrustacean' group. The remarkable antiquity, diversity and ecological significance of arthropods have inspired numerous attempts to resolve their deep phylogenetic history, but the results of two decades of intensive molecular phylogenetics have been mixed 1 , 2 , 3 , 4 , 5 , 6 , 7 . The discovery that terrestrial insects (Hexapoda) are more closely related to aquatic Crustacea than to the terrestrial centipedes and millipedes 2 , 8 (Myriapoda) was an early, if exceptional, success. More typically, analyses based on limited samples of taxa and genes have generated results that are inconsistent, weakly supported and highly sensitive to analytical conditions 7 , 9 , 10 . Here we present strongly supported results from likelihood, Bayesian and parsimony analyses of over 41 kilobases of aligned DNA sequence from 62 single-copy nuclear protein-coding genes from 75 arthropod species. These species represent every major arthropod lineage, plus five species of tardigrades and onychophorans as outgroups. Our results strongly support Pancrustacea (Hexapoda plus Crustacea) but also strongly favour the traditional morphology-based Mandibulata 11 (Myriapoda plus Pancrustacea) over the molecule-based Paradoxopoda (Myriapoda plus Chelicerata) 2 , 5 , 12 . In addition to Hexapoda, Pancrustacea includes three major extant lineages of ‘crustaceans’, each spanning a significant range of morphological disparity. These are Oligostraca (ostracods, mystacocarids, branchiurans and pentastomids), Vericrustacea (malacostracans, thecostracans, copepods and branchiopods) and Xenocarida (cephalocarids and remipedes). Finally, within Pancrustacea we identify Xenocarida as the long-sought sister group to the Hexapoda, a result confirming that ‘crustaceans’ are not monophyletic. These results provide a statistically well-supported phylogenetic framework for the largest animal phylum and represent a step towards ending the often-heated, century-long debate on arthropod relationships.

• Premise of this study: Aquatic cyanolichens from the genus Peltigera section Hydrothyriae are subject to anthropogenic threats and, therefore, are considered endangered. In this study we addressed the phylogenetic placement of section Hydrothyriae within Peltigera. We delimited species within the section and identified their symbiotic cyanobacteria.• Methods: Species delimitation and population structure were explored using monophyly as a grouping criterion (RAxML) and Structurama based on three protein-coding genes in combination with two nuclear ribosomal loci. The 16S and rbcLX sequences for the cyanobionts were analyzed in the broad phylogenetic context of free-living and symbiotic cyanobacteria.• Key results: We confirm with high confidence the placement of section Hydrothyriae within the monophyletic genus Peltigera; however, its phylogenetic position within the genus remains unsettled. We recovered three distinct monophyletic groups corresponding to three species: P. hydrothyria, P. gowardii s.s., and P. aquatica Miadl. & Lendemer, the latter being formally introduced here. Each species was associated with an exclusive set of Nostoc haplotypes.• Conclusions: The ITS region alone provides sufficient genetic information to distinguish the three morphologically cryptic species within section Hydrothyriae. Section Hydrothyriae seems to be associated with a monophyletic lineage of Nostoc, that has not been found in symbiotic association with other members of Peltigera. Capsosira lowei should be transferred to the genus Nostoc. Potential threats to P. aquatica should be re-examined based on the recognition of two aquatic species in western North America.

This study attempts to resolve relationships among and within the four basal arthropod lineages (Pancrustacea, Myriapoda, Euchelicerata, Pycnogonida) and to assess the widespread expectation that remaining phylogenetic problems will yield to increasing amounts of sequence data. Sixty-eight regions of 62 protein-coding nuclear genes (approximately 41 kilobases (kb)/taxon) were sequenced for 12 taxonomically diverse arthropod taxa and a tardigrade outgroup. Parsimony, likelihood, and Bayesian analyses of total nucleotide data generally strongly supported the monophyly of each of the basal lineages represented by more than one species. Other relationships within the Arthropoda were also supported, with support levels depending on method of analysis and inclusion/exclusion of synonymous changes. Removing third codon positions, where the assumption of base compositional homogeneity was rejected, altered the results. Removing the final class of synonymous mutations—first codon positions encoding leucine and arginine, which were also compositionally heterogeneous—yielded a data set that was consistent with a hypothesis of base compositional homogeneity. Furthermore, under such a data-exclusion regime, all 68 gene regions individually were consistent with base compositional homogeneity. Restricting likelihood analyses to nonsynonymous change recovered trees with strong support for the basal lineages but not for other groups that were variably supported with more inclusive data sets. In a further effort to increase phylogenetic signal, three types of data exploration were undertaken. (1) Individual genes were ranked by their average rate of nonsynonymous change, and three rate categories were assigned—fast, intermediate, and slow. Then, bootstrap analysis of each gene was performed separately to see which taxonomic groups received strong support. Five taxonomic groups were strongly supported independently by two or more genes, and these genes mostly belonged to the slow or intermediate categories, whereas groups supported only by a single gene region tended to be from genes of the fast category, arguing that fast genes provide a less consistent signal. (2) A sensitivity analysis was performed in which increasing numbers of genes were excluded, beginning with the fastest. The number of strongly supported nodes increased up to a point and then decreased slightly. Recovery of Hexapoda required removal of fast genes. Support for Mandibulata (Pancrustacea + Myriapoda) also increased, at times to “strong” levels, with removal of the fastest genes. (3) Concordance selection was evaluated by clustering genes according to their ability to recover Pancrustacea, Euchelicerata, or Myriapoda and analyzing the three clusters separately. All clusters of genes recovered the three concordance clades but were at times inconsistent in the relationships recovered among and within these clades, a result that indicates that the a priori concordance criteria may bias phylogenetic signal in unexpected ways. In a further attempt to increase support of taxonomic relationships, sequence data from 49 additional taxa for three slow genes (i.e., EF-1α, EF-2, and Pol II) were combined with the various 13-taxon data sets. The 62-taxon analyses supported the results of the 13-taxon analyses and provided increased support for additional pancrustacean clades found in an earlier analysis including only EF-1α, EF-2, and Pol II.

Methane seeps provide a variety of ecosystem services, including the provision of complex habitat structures and high levels of primary production, which can act as trophic support to non-seep-endemic species in an otherwise food-limited environment. The discovery of hundreds of seeps on the US Atlantic margin, ranging in depth from ~50 to 1700m, provides the opportunity to assess depth-related differences in seep-associated communities. Here, we use photo transects to characterize the megafaunal communities at six seeps along the US Atlantic margin, comparing taxonomic richness and community structure (taxon-abundance patterns) at shallow (~400m) and deep (~1500m) seeps. We use molecular analysis to identify the mussel species present and stable isotope analysis to explore the trophic ecology of bathymodiolin mussels and red crabs (Chaceon quinquedens). Our results suggest a faunal boundary exists between shallow and deep seeps; depth, and the co-varying factor temperature, explained 72% of the variation observed in taxon-abundance patterns. All mussel samples were identified as Bathymodiolus childressi, extending the known dominance of B. childressi at seeps near Baltimore Canyon to seeps off New England. Stable isotope analyses suggest B. childressi relies predominantly, if not entirely, on methane-derived nutrition at both shallow and deep seeps. For red crab, the proportion of methane-derived carbon within muscle tissue is highly variable, contributing ~0% of nutrition for crabs sampled at Shallop East and West but ~30% and ~50% of nutrition for two individuals sampled at Chincoteague East. In addition to red crabs using seeps as a food resource, invertebrate larvae samples and observational data suggests Chincoteague East may act as a reproductive hotspot for red crabs. Fifteen mating pairs, three ovigerous females, and numerous zoea larvae (identified as belonging to C. quinquedens) were observed at or above Chincoteague East, providing what we believe is the first evidence that some seeps may act as a reproductive hotspot for a commercially valuable species. This study highlights two ways that seeps may support fishery productivity (i.e. providing trophic support and increasing reproductive success) and encourages future research exploring the connection between deep-sea chemosynthetic ecosystems and commercially valuable species.

In the past few decades, population genetics and phylogeographic studies have improved our knowledge of connectivity and population demography in marine environments. Studies of deep‐sea hydrothermal vent populations have identified barriers to gene flow, hybrid zones, and demographic events, such as historical population expansions and contractions. These deep‐sea studies, however, used few loci, which limit the amount of information they provided for coalescent analysis and thus our ability to confidently test complex population dynamics scenarios. In this study, we investigated population structure, demographic history, and gene flow directionality among four Western Pacific hydrothermal vent populations of the vent limpet Lepetodrilus aff. schrolli. These vent sites are located in the Manus and Lau back‐arc basins, currently of great interest for deep‐sea mineral extraction. A total of 42 loci were sequenced from each individual using high‐throughput amplicon sequencing. Amplicon sequences were analyzed using both genetic variant clustering methods and evolutionary coalescent approaches. Like most previously investigated vent species in the South Pacific, L. aff. schrolli showed no genetic structure within basins but significant differentiation between basins. We inferred significant directional gene flow from Manus Basin to Lau Basin, with low to no gene flow in the opposite direction. This study is one of the very few marine population studies using >10 loci for coalescent analysis and serves as a guide for future marine population studies. In this study, we use 42 gene amplicons to investigate the population structure, demographic history, and gene flow directionality among four Western Pacific hydrothermal vent populations of the vent limpet Lepetodrilus aff. schrolli. This species showed significant differentiation between basins as well as evidence of gene flow from Manus Basin to Lau Basin, with low to no gene flow in the opposite direction. This study serves a model for studying gene flow which can be used to predict the impact of environmental and/or human disturbance, such as mineral extraction.

1. Pink salmon Oncorhynchus gorbuscha was introduced from its native range in the Pacific to Northwest Russia several times since the 1950s. While this species has been regularly observed in rivers in northern Norway since then, there has been an upsurge in the numbers of odd-year O. gorbuscha individuals observed in rivers, particularly in northern Norway in recent years, and particularly in 2017 and 2019. 2. In the present pilot study, an assay was developed to detect O. gorbuscha from eDNA water samples. Positive control water samples were taken at two locations of the River Signaldalselva in northern Norway during the summer of 2019, when adults were spawning in the river. Samples showed positive detection of this species in the river, while negative control samples collected upstream migration barriers in central and southern Norway confirmed the absence of the target species. 3. These findings reveal that eDNA-based methods can be used to track the ongoing O. gorbuscha invasion of northern Europe and other regions where it might be or become invasive. ddPCR, eDNA, invasion, Norway, Oncorhynchus gorbuscha, pink salmon, qPCR

Both ornaments and weapons of sexual selection frequently exhibit prolific interspecific diversity of form. Yet, most studies of this diversity have focused on ornaments involved with female mate choice, rather than on the weapons of male competition. With few exceptions, the mechanisms of divergence in weapon morphology remain largely unexplored. Here, we characterize the evolutionary radiation of one type of weapon: beetle horns. We use partial sequences from four nuclear and three mitochondrial genes to develop a phylogenetic hypothesis for a worldwide sample of 48 species from the dung beetle genus Onthophagus (Coleoptera: Scarabaeidae). We then use these data to test for multiple evolutionary origins of horns and to characterize the evolutionary radiation of horns. Although our limited sampling of one of the world's most species‐rich genera almost certainly underestimates the number of evolutionary events, our phylogeny reveals prolific evolutionary lability of these exaggerated sexually selected weapons (more than 25 separate gains and losses of five different horn types). We discuss these results in the context of the natural history of these beetles and explore ways that sexual selection and ecology may have interacted to generate this extraordinary diversity of weapon morphology.

The white-clawed crayfish Austropotamobius pallipes has experienced extensive population declines in its native range in the last century and is now a threatened species. European legislation, therefore, requires A. pallipes populations to be monitored regularly and that Special Areas of Conservation (SACs) be designated in part of their range. These needs cannot be met without knowledge about the distribution of this species. However, common crayfish sampling approaches can be time consuming, expensive, or yield false negatives at low population densities. We, therefore, present an environmental DNA (eDNA) assay based on the mitochondrial cytochrome oxidase I (COI) gene to detect A. pallipes in water samples with species-specific primers, a minor groove binding (MGB) probe, and quantitative PCR. The results reflected the expected distribution of A. pallipes based on the known locations of this species in the catchment areas sampled. The results of this study indicate that eDNA is an effective tool for detecting A. pallipes in a lotic system and could, therefore, provide a valuable, non-invasive method for determining its distribution.

Both ornaments and weapons of sexual selection frequently exhibit prolific interspecific diversity of form. Yet, most studies of this diversity have focused on ornaments involved with female mate choice, rather than on the weapons of male competition. With few exceptions, the mechanisms of divergence in weapon morphology remain largely unexplored. Here, we characterize the evolutionary radiation of one type of weapon: beetle horns. We use partial sequences from four nuclear and three mitochondrial genes to develop a phylogenetic hypothesis for a worldwide sample of 48 species from the dung beetle genus Onthophagus (Coleoptera: Scarabaeidae). We then use these data to test for multiple evolutionary origins of horns and to characterize the evolutionary radiation of horns. Although our limited sampling of one of the world's most species-rich genera almost certainly underestimates the number of evolutionary events, our phylogeny reveals prolific evolutionary lability of these exaggerated sexually selected weapons (more than 25 separate gains and losses of five different horn types). We discuss these results in the context of the natural history of these beetles and explore ways that sexual selection and ecology may have interacted to generate this extraordinary diversity of weapon morphology.

Hyalinoecia artifex is an abundant species on the US Atlantic margin, although little is known about its behavior, ecology, or reproduction. We provide field notes and high-resolution in situ imagery of this species observed and collected in 2013 and 2015. Hyalinoecia artifex occurred primarily on soft sediment at a density of 10.9±0.95 individuals m⁻² (mean±standard error). Although they were found adjacent to mussel beds associated with methane seeps, worms were negatively associated with authigenic carbonate and other epibenthic megafauna in the study area. Quill worms moved across the sediment by extending their heads from the tubes and dragging the tubes forward. Stable isotope analysis indicated that H. artifex is a secondary consumer. Most individuals collected in 2015 were ripe, with mature oocytes up to 400 μm. Oocytes were yolky, yellow in color and positively buoyant; no brooding was observed.