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Body size is a fundamental characteristic of animals that impacts every aspect of their biology from anatomical complexity to ecology. In Mollusca, Solenogastres has been considered important to understanding the group’s early evolution as most morphology-based phylogenetic reconstructions placed it as an early branching molluscan lineage. Under this scenario, molluscs were thought to have evolved from a small, turbellarian-like ancestor and small (i.e., macrofaunal) body size was inferred to be plesiomorphic for Solenogastres. More recently, phylogenomic studies have shown that aplacophorans (Solenogastres + Caudofoveata) form a clade with chitons (Polyplacophora), which is sister to all other molluscs, suggesting a relatively large-bodied (i.e., megafaunal) ancestor for Mollusca. Meanwhile, recent investigations into aplacophoran phylogeny have called the assumption that the last common ancestor of Solenogastres was small-bodied into question, but sampling of meiofaunal species was limited, biasing these studies towards large-bodied taxa and leaving fundamental questions about solenogaster body size evolution unanswered. Here, we supplemented available data with transcriptomes from eight diverse meiofaunal species of Solenogastres and conducted phylogenomic analyses on datasets of up to 949 genes. Maximum likelihood analyses support the meiofaunal family Meiomeniidae as the sister group to all other solenogasters, congruent with earlier ideas of a small-bodied ancestor of Solenogastres. In contrast, Bayesian Inference analyses support the large-bodied family Amphimeniidae as the sister group to all other solenogasters. Investigation of phylogenetic signal by comparing site-wise likelihood scores for the two competing hypotheses support the Meiomeniidae-first topology. In light of these results, we performed ancestral character state reconstruction to explore the implications of both hypotheses on understanding of Solenogaster evolution and review previous hypotheses about body size evolution and its potential consequences for solenogaster biology. Both hypotheses imply that body size evolution has been highly dynamic over the course of solenogaster evolution and that their relatively static body plan has successfully allowed for evolutionary transitions between meio-, macro- and megafaunal size ranges. 

Abstract Schizocardium karankawa  sp. nov. has been collected from subtidal muds of the Laguna Madre, Texas, and the Mississippi coast, Gulf of Mexico. The Texas population is reproductive from early February to mid-April. Gametes are liberated by a small incision in a gonad. Oocyte germinal vesicle breakdown is increased in the presence of sperm, and the highest fertilization success was in the artificial seawater Jamarin U. Manually dechorionated embryos develop normally. Development was asynchronous via a tornaria larva, metamorphosis and maintained to the juvenile worm 6 gill-pore stage. Phalloidin-labeled late-stage tornaria revealed retractor muscles that connect the pericardial sac with the apical tuft anteriorly, the oesophagus ventrally, and muscle cells of the early mesocoels. The muscle development of early juvenile worms began with dorso-lateral trunk muscles, lateral trunk bands, and sphincters around the gill pores and anus. Adult worms are characterized by a stomochord that bifurcates anteriorly into paired vermiform processes, gill bars that extend almost the entire dorsal to ventral branchial region resulting in a narrow ventral hypobranchial ridge, and an elaborate epibranchial organ with six zones of discrete cell types. The trunk has up to three rows of liver sacs, and lateral gonads. The acorn worm evo-devo model species  Saccoglossus kowalevskii ,  Ptychodera flava , and  Schizocardium   californicum  are phylogenetically distant with disparate life histories. S. karnakawa  from  S.   californicum  are phylogenetically close, and differences between them that become apparent as adult worms include the number of gill pores and hepatic sacs, and elaborations of the heart–kidney–stomochord complex. An important challenge for evolutionary developmental biology is to form links from phylogenetically distant and large-scale differences to phylogenetically close and small-scale differences. This description of the embryology, development, and adult morphology of S. karankawa permits investigations into how acorn worm development evolves at fine scales. 

Bryozoans are mostly sessile aquatic colonial invertebrates belonging to the clade Lophotrochozoa, which unites many protostome bilaterian phyla such as molluscs, annelids and brachiopods. While Hox and ParaHox genes have been extensively studied in various lophotrochozoan lineages, investigations on Hox and ParaHox gene complements in bryozoans are scarce. Herein, we present the most comprehensive survey of Hox and ParaHox gene complements in bryozoans using four genomes and 35 transcriptomes representing all bryozoan clades: Cheilostomata, Ctenostomata, Cyclostomata and Phylactolaemata. Using similarity searches, phylogenetic analyses and detailed manual curation, we have identified five Hox genes in bryozoans (pb, Dfd, Lox5, Lox4 and Post2) and one ParaHox gene (Cdx). Interestingly, we observed lineage-specific duplication of certain Hox and ParaHox genes (Dfd, Lox5 and Cdx) in some bryozoan lineages. The bryozoan Hox cluster does not retain the ancestral lophotrochozoan condition but appears relatively simple (includes only five genes) and broken into two genomic regions, characterized by the loss and duplication of serval genes. Importantly, bryozoans share the lack of two Hox genes (Post1 and Scr) with their proposed sister-taxon, Phoronida, which suggests that those genes were missing in the most common ancestor of bryozoans and phoronids. 

Many molluscan genomes have been published to date, however only three are from representatives of the subphylum Aculifera (Polyplacophora, Caudofoveata, and Solenogastres), the sister taxon to all other molluscs. Currently, genomic resources are completely lacking for Solenogastres. This gap in knowledge hinders comparative and evolutionary studies. Here, we sequenced the genomes of the solenogaster aplacophoransEpimenia babaiSalvini-Plawen, 1997 andNeomenia megatrapezataSalvini-Plawen & Paar-Gausch, 2004 using a hybrid approach combining Oxford Nanopore and Illumina reads. ForE. babai, we produced a 628 Mbp haploid assembly (N50 = 413 Kbp, L50 = 370) that is rather complete with a BUSCO completeness score of 90.1% (82.0% single, 8.1% duplicated, 6.0% fragmented, and 3.9% missing). ForN. megatrapezata, we produced a 412 Mbp haploid assembly (N50 = 132 Kbp, L50 = 881) that is also rather complete with a BUSCO completeness score of 85.1% (81.7% single, 3.4% duplicated, 8.1% fragmented, and 6.8% missing). Our annotation pipeline predicted 25,393 gene models forE. babaiwith a BUSCO score of 92.4% (80.5% single, 11.9% duplicated, 4.9% fragmented, and 2.7% missing) and 22,463 gene models forN. megatrapezatawith a BUSCO score of 90.2% (81.0% single, 9.2% duplicated, 4.7% fragmented, and 5.1% missing). Phylogenomic analysis recovered Solenogastres as the sister taxon to Polyplacophora and Aculifera as the sister taxon to all other sampled molluscs with maximal support. These represent the first whole-genome resources for Solenogastres and will be valuable for future studies investigating this understudied group and molluscan evolution as a whole. 

The methane seeps on the Pacific margin of Costa Rica support extensive animal diversity and offer insights into deep-sea biogeography. During five expeditions between 2009 and 2019, we conducted intensive faunal sampling via 63 submersible dives to 11 localities at depths of 300–3600 m. Based on these expeditions and published literature, we compiled voucher specimens, images, and 274 newly published DNA sequences to present a taxonomic inventory of macrofaunal and megafaunal diversity with a focus on invertebrates. In total 488 morphospecies were identified, representing the highest number of distinct morphospecies published from a single seep or vent region to date. Of these, 131 are described species, at least 58 are undescribed species, and the remainder include some degree of taxonomic uncertainty, likely representing additional undescribed species. Of the described species, 38 are known only from the Costa Rica seeps and their vicinity. Fifteen range extensions are also reported for species known from Mexico, the Galápagos seamounts, Chile, and the western Pacific; as well as 16 new depth records and three new seep records for species known to occur at vents or organic falls. No single evolutionary narrative explains the patterns of biodiversity at these seeps, as even morphologically indistinguishable species can show different biogeographic affinities, biogeographic ranges, or depth ranges. The value of careful molecular taxonomy and comprehensive specimen-based regional inventories is emphasized for biodiversity research and monitoring. 

Solenogastres and Caudofoveata (Aplacophora) remain some of the least known molluscs, despite ubiquity in the marine environment and importance in understanding molluscan evolution. The use of new morphological techniques and development of DNA barcode libraries have helped make specimen identification easier. However, for solenogasters, using histology for identification and adequate description of species remains necessary in most cases. This, together with the facts that knowledge about solenogaster species distributions is biased and that most species were described from one or very few individuals, explains why many open questions about the actual distribution, intra- and interspecific variability, etc., remain. We performed an integrative taxonomic study of eight specimens of solenogasters from the South China Sea (West Pacific Ocean) thatresulted in the identification of four new species of Proneomeniidae. Species identification and description following the established diagnostic characters were straightforward. However, phylogenetic analysis of molecular data obtained from these specimens and other members of Proneomeniidae indicate that the family is polyphyletic. We recovered representatives of two other families, Epimeniidae (Epimenia) and Strophomeniidae (Anamenia), nested within Proneomeniidae with strong support. Ancestral character state reconstruction indicates that characters commonly used in solenogaster taxonomy, such as the radula and foregut glands, may be more evolutionarily labile in this group than previously known. Therefore our work fills knowledge gaps regarding the diversity and distribution of members of this family but raises important questions about solenogaster taxonomy and systematics that should be further assessed with additional markers and broader taxon sampling. ZooBank: urn:lsid:zoobank.org:pub:BCADACD6-9AD0-442A-AD64-031BA8D88599 

The almost simultaneous emergence of major animal phyla during the early Cambrian shaped modern animal biodiversity. Reconstructing evolutionary relationships among such closely spaced branches in the animal tree of life has proven to be a major challenge, hindering understanding of early animal evolution and the fossil record. This is particularly true in the species-rich and highly varied Mollusca where dramatic inconsistency among paleontological, morphological, and molecular evidence has led to a long-standing debate about the group’s phylogeny and the nature of dozens of enigmatic fossil taxa. A critical step needed to overcome this issue is to supplement available genomic data, which is plentiful for well-studied lineages, with genomes from rare but key lineages, such as Scaphopoda. Here, by presenting chromosome-level genomes from both extant scaphopod orders and leveraging complete genomes spanning Mollusca, we provide strong support for Scaphopoda as the sister taxon of Bivalvia, revitalizing the morphology-based Diasoma hypothesis originally proposed 50 years ago. Our molecular clock analysis confidently dates the split between Bivalvia and Scaphopoda at ~520 Ma, prompting a reinterpretation of controversial laterally compressed Early Cambrian fossils, includingAnabarella,Watsonella,andMellopegma,as stem diasomes. Moreover, we show that incongruence in the phylogenetic placement of Scaphopoda in previous phylogenomic studies was due to ancient incomplete lineage sorting (ILS) that occurred during the rapid radiation of Conchifera. Our findings highlight the need to consider ILS as a potential source of error in deep phylogeny reconstruction, especially in the context of the unique nature of the Cambrian Explosion. 

Euphrosinidae (Amphinomida) is a clade of generally small, short but stout annelids characterized by long, calcareous chaetae that may be distally forked or ringent. Little is known about the diversity of Euphrosinidae from the North Atlantic and the phylogeny of the group has received little attention. Here, we examined 59 specimens of Euphrosinidae (primarily from the IceAGE I and II cruises) and sequenced fragments of the mitochondrial 16S rDNA and nuclear 28S rDNA genes to improve understanding of euphrosinid diversity in the North Atlantic and gain insights into euphrosinid phylogeny. Maximum likelihood analysis of 28S + 16S recovered Euphrosine as a ‘basal’ paraphyletic grade; a clade containing E. armadillo (plus other unidentified specimens) was sister to Euphrosinopsis + Euphrosinella while a clade containing E. aurantiaca and E. foliosa (plus three unidentified species) was recovered sister to all other sampled Euphrosinidae species. Species delimitation analyses based on 16S sequences identified between 14 and 11 species of Euphrosinidae with as many as ten distinct species from the North Atlantic. The IceAGE material investigated includes one new species of Euphrosinopsis and at least one new species of Euphrosinella. Unfortunately, because most of this material was preserved in ethanol, we were unable to characterize key features needed for adequate species descriptions. Additionally, PCR contaminants from presumed gut contents suggest that some euphrosinids eat other annelids, namely Cirratulidae and Syllidae.