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Crystallization by particle attachment (CPA) of amorphous precursors has been demonstrated in modern biomineralized skeletons across a broad phylogenetic range of animals. Precisely the same precursors, hydrated (ACC-H₂O) and anhydrous calcium carbonate (ACC), have been observed spectromicroscopically in echinoderms, mollusks, and cnidarians, phyla drawn from the 3 major clades of eumetazoans. Scanning electron microscopy (SEM) here also shows evidence of CPA in tunicate chordates. This is surprising, as species in these clades have no common ancestor that formed a mineralized skeleton and appear to have evolved carbonate biomineralization independently millions of years after their late Neoproterozoic divergence. Here we correlate the occurrence of CPA from ACC precursor particles with nanoparticulate fabric and then use the latter to investigate the antiquity of the former. SEM images of early biominerals from Ediacaran and Cambrian shelly fossils show that these early calcifiers used attachment of ACC particles to form their biominerals. The convergent evolution of biomineral CPA may have been dictated by the same thermodynamics and kinetics as we observe today.

When molluscs got ahead The Mollusca are one of the most successful animal phyla — ubiquitous, varied in body plan and with a long fossil record. Their interrelationships have been a matter of debate, but phylogenomic methods are beginning to resolve the issue. A new study answers some questions about the base of the molluscan tree, showing that, contrary to the traditional view, bivalves and gastropods are members of sister taxa. This finding also raises the possibility that centralization of neural and sensory organs in the head region, and the development of protective shells, may have occurred on several occasions in the evolutionary history of the molluscs. Evolutionary relationships among the eight major lineages of Mollusca have remained unresolved despite their diversity and importance. Previous investigations of molluscan phylogeny, based primarily on nuclear ribosomal gene sequences 1 , 2 , 3 or morphological data 4 , have been unsuccessful at elucidating these relationships. Recently, phylogenomic studies using dozens to hundreds of genes have greatly improved our understanding of deep animal relationships 5 . However, limited genomic resources spanning molluscan diversity has prevented use of a phylogenomic approach. Here we use transcriptome and genome data from all major lineages (except Monoplacophora) and recover a well-supported topology for Mollusca. Our results strongly support the Aculifera hypothesis placing Polyplacophora (chitons) in a clade with a monophyletic Aplacophora (worm-like molluscs). Additionally, within Conchifera, a sister-taxon relationship between Gastropoda and Bivalvia is supported. This grouping has received little consideration and contains most (>95%) molluscan species. Thus we propose the node-based name Pleistomollusca. In light of these results, we examined the evolution of morphological characters and found support for advanced cephalization and shells as possibly having multiple origins within Mollusca.

Molluscs get together Phylogenomic methods are beginning to resolve one of the more tricky issues facing evolutionary biologists — making sense of the complicated interrelationships of the Mollusca. Casey Dunn and colleagues have managed to recover information from the crucial but hard-to-get-at Monoplacophora, a primitive group of deep-sea molluscs, revealing a shared ancestry with cephalopods. Together with the recent paper by Kocot et al . ( http://go.nature.com/g9trkt ), this study shows that the bivalves and gastropods form a single group. Molluscs (snails, octopuses, clams and their relatives) have a great disparity of body plans and, among the animals, only arthropods surpass them in species number. This diversity has made Mollusca one of the best-studied groups of animals, yet their evolutionary relationships remain poorly resolved 1 . Open questions have important implications for the origin of Mollusca and for morphological evolution within the group. These questions include whether the shell-less, vermiform aplacophoran molluscs diverged before the origin of the shelled molluscs (Conchifera) 2 , 3 , 4 or lost their shells secondarily. Monoplacophorans were not included in molecular studies until recently 5 , 6 , when it was proposed that they constitute a clade named Serialia together with Polyplacophora (chitons), reflecting the serial repetition of body organs in both groups 5 . Attempts to understand the early evolution of molluscs become even more complex when considering the large diversity of Cambrian fossils. These can have multiple dorsal shell plates and sclerites 7 , 8 , 9 , 10 or can be shell-less but with a typical molluscan radula and serially repeated gills 11 . To better resolve the relationships among molluscs, we generated transcriptome data for 15 species that, in combination with existing data, represent for the first time all major molluscan groups. We analysed multiple data sets containing up to 216,402 sites and 1,185 gene regions using multiple models and methods. Our results support the clade Aculifera, containing the three molluscan groups with spicules but without true shells, and they support the monophyly of Conchifera. Monoplacophora is not the sister group to other Conchifera but to Cephalopoda. Strong support is found for a clade that comprises Scaphopoda (tusk shells), Gastropoda and Bivalvia, with most analyses placing Scaphopoda and Gastropoda as sister groups. This well-resolved tree will constitute a framework for further studies of mollusc evolution, development and anatomy.

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.

Online citizen science projects have broadened options for accessing science and enabled different forms of participation in scientific research for adult and young volunteers. Yet, little is known regarding participation patterns among youth participants. Quantitative approaches were used to investigate the contribution of 183 young volunteers to citizen science on the iNaturalist platform and the participation behaviour that relates to their contribution. The participants accessed and used iNaturalist as part of one-day field-based events (bioblitzes) facilitated by museums. Compared to the observation behaviour of all iNaturalist users, as documented on the platform, the young volunteers observe fewer plants and birds, and more molluscs, arachnids and insects. The average daily contributions of young volunteers were found to be positively associated with a large proportion of active days on iNaturalist and a systematic contribution behaviour, yet negatively related to a long duration on the platform. This study enhances our understanding of young volunteers’ contributions to citizen science and provides insights for research on participation in online citizen science. Our findings have implications on how museums design the field-based events to encourage follow-up systematic participation and maintain active contribution.

Molluscs are the most diverse marine phylum and this high diversity has resulted in considerable taxonomic problems. Because the number of species in Canadian oceans remains uncertain, there is a need to incorporate molecular methods into species identifications. A 648 base pair segment of the cytochrome c oxidase subunit I gene has proven useful for the identification and discovery of species in many animal lineages. While the utility of DNA barcoding in molluscs has been demonstrated in other studies, this is the first effort to construct a DNA barcode registry for marine molluscs across such a large geographic area. This study examines patterns of DNA barcode variation in 227 species of Canadian marine molluscs. Intraspecific sequence divergences ranged from 0-26.4% and a barcode gap existed for most taxa. Eleven cases of relatively deep (>2%) intraspecific divergence were detected, suggesting the possible presence of overlooked species. Structural variation was detected in COI with indels found in 37 species, mostly bivalves. Some indels were present in divergent lineages, primarily in the region of the first external loop, suggesting certain areas are hotspots for change. Lastly, mean GC content varied substantially among orders (24.5%-46.5%), and showed a significant positive correlation with nearest neighbour distances. DNA barcoding is an effective tool for the identification of Canadian marine molluscs and for revealing possible cases of overlooked species. Some species with deep intraspecific divergence showed a biogeographic partition between lineages on the Atlantic, Arctic and Pacific coasts, suggesting the role of Pleistocene glaciations in the subdivision of their populations. Indels were prevalent in the barcode region of the COI gene in bivalves and gastropods. This study highlights the efficacy of DNA barcoding for providing insights into sequence variation across a broad taxonomic group on a large geographic scale.

Presence of a radula in Calvapilosa kroegeri confirms the molluscan affinity of sachitids, and the single shell plate reveals the ancestral condition for all crown molluscs and early evolution of the multi-plated body plan characteristic of Aculifera. Fossil reveals early mollusc morphology From the snails in your garden to giant squid that wrestle with sperm whales, the molluscs are one of the most disparate and successful animal phyla. But because they evolved rapidly in the Cambrian period, some 500 million years ago, there is still much debate about their early history—in particular, what the earliest molluscs looked like. A fossil from the Ordovician Fezouata formation in Morocco (known for relic Burgess-Shales-type animals) might shed light on the issue. The creature is a flattened slug-like animal with a distinct, single shell on its head, the rest of the body being covered with spines. The exciting part is that the animal has a radula, the distinctive rasping tongue that is a defining character for molluscs and is the reason slugs can demolish your lettuces so effectively. This phylogenetic analysis shows that the newly discovered creature groups with some other forms variously classified as molluscs, stem brachiopods or similar, but places it within the molluscs, at the base of the Aculifera (the multiple-shelled chitons plus the shell-free aplacophora), as opposed to the Conchifera (all other molluscs). This finding suggests that early molluscs had only a single shell. Exceptionally preserved fossils provide crucial insights into extinct body plans and organismal evolution 1 . Molluscs, one of the most disparate animal phyla, radiated rapidly during the early Cambrian period (approximately 535–520 million years ago (Ma)) 2 . The problematic fossil taxa Halkieria 3 and Orthrozanclus 4 (grouped in Sachitida) have been assigned variously to stem-group annelids, brachiopods 4 , 5 , stem-group molluscs 4 or stem-group aculiferans (Polyplacophora and Aplacophora) 6 , but their affinities have remained controversial owing to a lack of preserved diagnostic characters. Here we describe a new early sachitid, Calvapilosa kroegeri gen. et sp. nov. from the Fezouata biota of Morocco 7 , 8 (Early Ordovician epoch, around 478 Ma). The new taxon is characterized by the presence of a single large anterior shell plate and polystichous radula bearing a median tooth and several lateral and uncinal teeth in more than 125 rows. Its flattened body is covered by hollow spinose sclerites, and a smooth, ventral girdle flanks an extensive mantle cavity. Phylogenetic analyses resolve C. kroegeri as a stem-group aculiferan together with other single-plated forms such as Maikhanella ( Siphogonuchites ) and Orthrozanclus ; Halkieria is recovered closer to the aculiferan crown. These genera document the stepwise evolution of the aculiferan body plan from forms with a single, almost conchiferan-like shell through two-plated taxa such as Halkieria , to the eight-plated crown-group aculiferans. C. kroegeri therefore provides key evidence concerning the long debate about the crown molluscan affinities of sachitids. This new discovery strongly suggests that the possession of only a single calcareous shell plate and the presence of unmineralised sclerites are plesiomorphic (an ancestral trait) for the molluscan crown.

Background Histones are the basic packaging units of eukaryotic DNA and are essential for the dynamics of chromatin and the regulation of epigenetics. Canonical histones and their variants exhibit important functional differences in biological processes. However, little is known about the role of histone family members in molluscs, which are known for their ecological and morphological diversity. Results Core histone families of 28 molluscan species (12 bivalves, 8 gastropods, 6 cephalopods, 1 scaphopod and 1 polyplacophora) were systematically identified. The evolutionary conservation and lineage-specific innovations were discovered using phylogenomic and transcriptomic analyses. Cephalopods showed a striking expansion of canonical histone genes with brain-enriched expression patterns. Synteny analyses revealed conserved, collinear histone clusters unique to cephalopods. Histone variants, specially H2A and H3 paralogs, display conserved motifs potentially involved in nucleosome stability and lineage-specific residues involved in functional specialization. Developmental transcriptomics revealed the dynamic expression of histone variants in early embryogenesis and the gonads, suggesting that H2A and H3 variants are involved in chromatin remodeling, pluripotency maintenance and germline regulation. Macro-H2A was highly expressed during larval neurodevelopment and in sensory organs, suggesting important roles in neural plasticity. Conclusion This study represents the first comprehensive inventory and characterization of core histone genes in molluscs, and will facilitate understanding of the evolutionary patterns and functional properties of core histones in relation to neurogenesis of molluscs. These findings advance our understanding of chromatin evolution and its contribution to phenotypic innovation in non-model taxa.

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.

Mollusca is the second most species-rich animal phylum, but the pathways of early molluscan evolution have long been controversial 1 , 2 , 3 , 4 – 5 . Modern faunas retain only a fraction of the past forms in this hyperdiverse and long-lived group. Recent analyses 6 , 7 – 8 have consistently recovered a fundamental split into two sister clades, Conchifera (including gastropods, bivalves and cephalopods) and Aculifera 9 , comprising Polyplacophora (‘chitons’) and Aplacophora. Molluscan evolution in toto is characterized by plasticity in body-plan characters 10 , but historically aculiferans have been interpreted as more conservative 10 , 11 . The few completely preserved aculiferan or aculiferan-like fossils from the early Palaeozoic 12 , 13 , 14 , 15 , 16 , 17 , 18 – 19 have been largely regarded as transitional forms that inform questions of character polarity between the extant polyplacophoran and aplacophoran body forms 20 , 21 . The history of early aculiferans, and the morphological and ecological range that they occupied, remain inadequately sampled. Here we describe two new three-dimensionally preserved aculiferan species from the Silurian Herefordshire Lagerstätte 22 , 23 , which substantially extend the morphological and ecological range of the clade. Phylogenetic analyses indicate positions within a complex nexus of taxa and suggest reversals in the states of fundamental characters such as the presence of valves and the nature of the foot. In contrast to previous hypotheses of morphological conservatism, evolution in early aculiferans generated a profusion of unusual forms comparable to the diversification of other crown-group molluscs. Fossils of two new worm-like aculiferan species add to the diversity of this group, showing that evolution in early aculiferans generated unusual forms comparable to other crown-group molluscs.