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Annelida is one of three animal groups possessing segmentation and is central in considerations about the evolution of different character traits. It has even been proposed that the bilaterian ancestor resembled an annelid. However, a robust phylogeny of Annelida, especially with respect to the basal relationships, has been lacking. Our study based on transcriptomic data comprising 68,750–170,497 amino acid sites from 305 to 622 proteins resolves annelid relationships, including Chaetopteridae, Amphinomidae, Sipuncula, Oweniidae, and Magelonidae in the basal part of the tree. Myzostomida, which have been indicated to belong to the basal radiation as well, are now found deeply nested within Annelida as sister group to Errantia in most analyses. On the basis of our reconstruction of a robust annelid phylogeny, we show that the basal branching taxa include a huge variety of life styles such as tube dwelling and deposit feeding, endobenthic and burrowing, tubicolous and filter feeding, and errant and carnivorous forms. Ancestral character state reconstruction suggests that the ancestral annelid possessed a pair of either sensory or grooved palps, bicellular eyes, biramous parapodia bearing simple chaeta, and lacked nuchal organs. Because the oldest fossil of Annelida is reported for Sipuncula (520 Ma), we infer that the early diversification of annelids took place at least in the Lower Cambrian.

Coming to terms with worms The annelids, or segmented worms, comprise one of the largest and most diverse animal phyla, found everywhere from the soil to the ocean bed. Their evolutionary relationships are poorly understood, and a reconstruction of annelid evolutionary history would be welcome. To that end, Struck et al . present a new phylogenomic analysis of 34 annelid taxa. Notable features include a division of most annelids into the Sedentaria and the Errantia, reviving a neglected 150-year-old hypothesis that the annelids developed as two major groups or clades, one specialized for a sedentary way of life and the other for a more active lifestyle. The annelids, or ringed worms, comprise one of the largest and most diverse animal phyla, and are found everywhere from garden soil to the deep sea. Their precise phylogeny has always been sketchy, but now, a new phylogenomic analysis unravels annelid evolution. Notable features include a division of most annelids into the Errantia and Sedentaria — a restitution for two groups based on classical morphology — showing how many details of anatomy and life history bear on the evolution of this important animal group. Annelida, the ringed worms, is a highly diverse animal phylum that includes more than 15,000 described species and constitutes the dominant benthic macrofauna from the intertidal zone down to the deep sea. A robust annelid phylogeny would shape our understanding of animal body-plan evolution and shed light on the bilaterian ground pattern. Traditionally, Annelida has been split into two major groups: Clitellata (earthworms and leeches) and polychaetes (bristle worms), but recent evidence suggests that other taxa that were once considered to be separate phyla (Sipuncula, Echiura and Siboglinidae (also known as Pogonophora)) should be included in Annelida 1 , 2 , 3 , 4 . However, the deep-level evolutionary relationships of Annelida are still poorly understood, and a robust reconstruction of annelid evolutionary history is needed. Here we show that phylogenomic analyses of 34 annelid taxa, using 47,953 amino acid positions, recovered a well-supported phylogeny with strong support for major splits. Our results recover chaetopterids, myzostomids and sipunculids in the basal part of the tree, although the position of Myzostomida remains uncertain owing to its long branch. The remaining taxa are split into two clades: Errantia (which includes the model annelid Platynereis ), and Sedentaria (which includes Clitellata). Ancestral character trait reconstructions indicate that these clades show adaptation to either an errant or a sedentary lifestyle, with alteration of accompanying morphological traits such as peristaltic movement, parapodia and sensory perception. Finally, life history characters in Annelida seem to be phylogenetically informative.

The fossil record of the terminal Ediacaran Period is typified by the iconic index fossil Cloudina and its relatives. These tube-dwellers are presumed to be primitive metazoans, but resolving their phylogenetic identity has remained a point of contention. The root of the problem is a lack of diagnostic features; that is, phylogenetic interpretations have largely centered on the only available source of information—their external tubes. Here, using tomographic analyses of fossils from the Wood Canyon Formation (Nevada, USA), we report evidence of recognizable soft tissues within their external tubes. Although alternative interpretations are plausible, these internal cylindrical structures may be most appropriately interpreted as digestive tracts, which would be, to date, the earliest-known occurrence of such features in the fossil record. If this interpretation is correct, their nature as one-way through-guts not only provides evidence for establishing these fossils as definitive bilaterians but also has implications for the long-debated phylogenetic position of the broader cloudinomorphs. Cloudinomorphs were one of the few groups to survive from the Ediacaran into the Cambrian, but they are known only from their external tubes. Here, Schiffbauer et al. report soft-tissue preservation of cloudinomorphs; the internal structures are interpreted as guts characteristic of bilaterians.

As a result of their plastic body plan, the relationships of the annelid worms and even the taxonomic makeup of the phylum have long been contentious. Morphological cladistic analyses have typically recovered a monophyletic Polychaeta, with the simple-bodied forms assigned to an early-diverging clade or grade. This is in stark contrast to molecular trees, in which polychaetes are paraphyletic and include clitellates, echiurans and sipunculans. Cambrian stem group annelid body fossils are complex-bodied polychaetes that possess well-developed parapodia and paired head appendages (palps), suggesting that the root of annelids is misplaced in morphological trees. We present a reinvestigation of the morphology of key fossil taxa and include them in a comprehensive phylogenetic analysis of annelids. Analyses using probabilistic methods and both equal- and implied-weights parsimony recover paraphyletic polychaetes and support the conclusion that echiurans and clitellates are derived polychaetes. Morphological trees including fossils depict two main clades of crown-group annelids that are similar, but not identical, to Errantia and Sedentaria, the fundamental groupings in transcriptomic analyses. Removing fossils yields trees that are often less resolved and/or root the tree in greater conflict with molecular topologies. While there are many topological similarities between the analyses herein and recent phylogenomic hypotheses, differences include the exclusion of Sipuncula from Annelida and the taxa forming the deepest crown-group divergences.

Annelid disparity has resulted in morphological-based classifications that disagree with phylogenies based on Sanger sequencing and phylogenomic analyses. However, the data used for the latter studies came from various sources and technologies, involved poorly occupied matrices and lacked key lineages. Here, we generated a new Illumina-based data set to address annelid relationships from a fresh perspective, independent from previously generated data and with nearly fully occupied matrices. Our sampling reflects the span of annelid diversity, including two symbiotic annelid groups (Myzostomida and Spinther) and five meiofaunal groups once referred to as part of Archiannelida (three from Protodrilida, plus Dinophilus and Polygordius). As well as the placement of these unusual annelids, we sought to address the overall phylogeny of Annelida, and provide a new perspective for naming of major clades. Our results largely corroborate the phylogenomic results of Weigert et al. (2014; Illuminating the base of the annelid tree using transcriptomics. Mol Biol Evol. 31:1391-1401), with “Magelona + Owenia” and Chaetopteridae forming a grade with respect to all other annelids. Echiura and Sipuncula are supported as being annelid groups, with Sipuncula closest to amphinomids as sister group to Sedentaria and Errantia. We recovered the three Protodrilida terminals as sister clade to Phyllodocida and Eunicida (=clade Aciculata). We therefore place Protodrilida as part of Errantia. Polygordius was found to be sister group to the scaleworm terminal and the possibility that it is a simplified scaleworm clade, as has been shown for the former family Pisionidae, is discussed. Our results were equivocal with respect to Dinophilus, Myzostomida, and Spinther possibly owing to confounding long-branch effects.

Amblyosyllis is a worldwide distributed group of annelids mainly found in coastal environments. It is well known among the polychaete specialists mostly because of its notable beauty, showing bright colourful patterns and outstanding long and coiled appendices. Amblyosyllis is a monophyletic genus easy to identify due to its distinct diagnostic features; however, the species and their boundaries are, in most cases, not well defined. Herein, we provide an extensive sample of Amblyosyllis material (115 specimens) from several world geographic areas. We have studied the morphological features of each specimen and photographed them alive. Two mitochondrial DNA markers (COI and 16S) and one nuclear gene fragment (28S, D1 region) were sequenced. We performed phylogenetic analyses based on each DNA partition, as well as the combined data sets, obtaining congruent results. Species delimitation methods such as distance analyses, statistical parsimony networks and multi-rate Poisson tree processes were also applied. The combined results obtained from different methodologies and data sets are used to differentiate between, at least, 19 lineages compatible with the separately evolving meta-populations species concept. Four of these lineages are identified as nominal species, including the type species of Amblyosyllis, A. rhombeata. For three other lineages previously synonymized names are recovered, and seven lineages are described as new species. All of these species are described and supported by appropriate iconography. We recognize several morphological characters useful to identify species of Amblyosyllis, which in some cases should also be combined with molecular methods for species delineation. The genetic divergence in the genus is high, contrary to the morphological homogeneity observed. Two species show a wide geographical distribution, while the rest have a more restricted distribution. There are several examples of species with overlapping distribution patterns.

DNA-based identification through the use of metabarcoding has been proposed as the next step in the monitoring of biological communities, such as those assessed under the Water Framework Directive (WFD). Advances have been made in the field of metabarcoding, but challenges remain when using complex samples. Uneven biomass distributions, preferential amplification and reference database deficiencies can all lead to discrepancies between morphological and DNA-based taxa lists. The effects of different taxonomic groups on these issues remain understudied. By metabarcoding WFD monitoring samples, we analyzed six different taxonomic groups of freshwater organisms, both separately and combined. Identifications based on metabarcoding data were compared directly to morphological assessments performed under the WFD. The diversity of taxa for both morphological and DNA-based assessments was similar, although large differences were observed in some samples. The overlap between the two taxon lists was 56.8% on average across all taxa, and was highest for Crustacea, Heteroptera, and Coleoptera, and lowest for Annelida and Mollusca. Taxonomic sorting in six basic groups before DNA extraction and amplification improved taxon recovery by 46.5%. The impact on ecological quality ratio (EQR) scoring was considerable when replacing morphology with DNA-based identifications, but there was a high correlation when only replacing a single taxonomic group with molecular data. Different taxonomic groups provide their own challenges and benefits. Some groups might benefit from a more consistent and robust method of identification. Others present difficulties in molecular processing, due to uneven biomass distributions, large genetic diversity or shortcomings of the reference database. Sorting samples into basic taxonomic groups that require little taxonomic knowledge greatly improves the recovery of taxa with metabarcoding. Current standards for EQR monitoring may not be easily replaced completely with molecular strategies, but the effectiveness of molecular methods opens up the way for a paradigm shift in biomonitoring.

During a visit to polychaete-rearing facilities in the vicinity of Bay of Ca´diz (SW Iberian Peninsula, Atlantic Ocean), we sampled two populations of Marphysa (Annelida, Eunicidae) originally occurring at nearby intertidal soft bottoms, one being more than twice as long as the other at the same age. We analysed them using partial sequences of two mitochondrial genes, 16S rDNA and Cytochrome Oxidase I, and classical morphological observations. Our molecular results confirmed that the two populations corresponded to two different species, with PTP species delimitation values ranging from 0.973 (long-bodied species) to 0.999 (short-bodied species). Morphologically, the short-bodied species resembles the recently redescribed M. sanguinea (Montagu, 1813), but differs mainly in having some parapodia with two subacicular hooks (one bidentate and one unidentate) and three types of pectinate chaetae, Two isodont present all along the body, and one particularly large anodont asymmetric appearing only from mid-posterior parapodia. The long-bodied species resembles Marphysa aegypti Elgetany, El-Ghobashy, Ghoneim and Struck, 2018 both in size and in having very robust, unidentate subacicular hooks (single in most parapodia, two-both similar in size and form-in some posterior parapodia), but differs, among other features, in the maxillary formula, the number of acicula per parapodia and the number and shape of pectinate chaetae. Accordingly, we are here fully illustrating and formally describing the two Iberian populations as Marphysa gaditana sp. nov. (short-bodied) and Marphysa chirigota sp. nov. (long-bodied) and we are emending the description of M. aegypti based on our revision of the type material. Also, we discuss on the distribution of the species of the sanguinea-group and on the relevancy of taxonomically robust studies when dealing with species of commercial interest having the potential of being globally spread through human activities, as well as on the misunderstandings caused by the incorrect use of the \"cosmopolitan species\" concept.

The sibling species of Capitella capitata are globally known for their tolerance to disturbed habitats and the C. capitata complex is often used as an ecological indicator. A recent re-description proposed that C. capitata, originally described in Greenland is restricted to the Artic and Subarctic regions. Given their ecological relevance, we conducted a morphological and molecular analyses based on mtDNA sequences to investigate the diversity and distribution of the C. capitata complex along the Brazilian coast. Our morphological and molecular data were congruent and revealed the existence of four new species distinct from C. capitata, collected from the type locality. This study is the first characterization of the biodiversity and distribution of Capitella species made along the Brazilian coast and yielded a set of morphological characters corroborated by the mtDNA sequences for species identification. Our results increase the biodiversity of the genus along the Brazilian coast by describing four new species (Capitella aracaensis sp. n., Capitella biota sp. n., Capitella neoaciculata sp. n. and Capitella nonatoi sp. n.). One species was collected from only one sampling site, while the others are distributed along the coast.

Orthonectids are a group of highly simplified worm-like parasites that are placed within Lophotrochozoa by multigene mitochondrial and nuclear phylogenies. However, their exact position within Lophotrochozoa is uncertain due to the high rate of molecular evolution and putative long branch attraction artifacts. To examine the phylogenetic placement of orthonectids, we applied an alternative approach that takes into account rare evolutionary events (gene order rearrangements in mitochondrial DNA and individual changes in mitochondrial proteins) with an assessment of their probabilities based on a reference sequence database (RefSeq, NCBI). This approach strongly supports the branching of orthonectids among annelids, but does not conclusively resolve their position among the annelid taxa.