Gastropod Clades Research Articles

Many roads to success: alternative routes to building an economic shell in land snails.

Land snails exhibit an extraordinary variety of shell shapes. The way shells are constructed underlies biological and mechanical constraints that vary across gastropod clades. Here, we quantify shell geometry of the two largest groups, Stylommatophora and Cyclophoroidea, to assess the potential causes for variation in shell shape and its relative frequency. Based on micro-computed tomography scans, we estimate material efficiency through 2D and 3D generalizations of the isoperimetric ratio, quantifying the ratios between area and perimeter of whorl cross-sections (2D) and shell volume and surface (3D), respectively. We find that stylommatophorans optimize material usage through whorl overlap, which may have promoted the diversification of flat-shelled species. Cyclophoroids are bound to a circular cross-section because of their operculum; flat shells are comparatively rare. Both groups show similar solutions for tall shells, where local geometry has a smaller effect because of the double overlap between previous and current whorls. Our results suggest that material efficiency is a driving factor in the selection of shell geometry. Essentially, the evolutionary success of Stylommatophora likely roots in their higher flexibility to produce an economic shell.

On Paleozoic platycerate gastropods

The platycerate gastropods Orthonychia yutaroi Ebbestad, sp. nov. (Ordovician, Boda Limestone, Sweden), O. enorme (Silurian, Sweden, Gotland), O. parva (Pennsylvanian, Finis Shale Member, USA), and Orthonychia sp. (Mississippian, Imo Formation, USA) are studied including their protoconch morphology. Orthonychia yutaroi is the oldest known species in Orthonychia. Platycerates contain species with both, openly and tightly coiled protoconchs. This is the first report that tightly coiled protoconchs occur in Orthonychia. This and previously published observations blur the diagnostic difference between orders Cyrtoneritimorpha (openly coiled protoconch) and Cycloneritimorpha (tightly coiled protoconch). We suggest to treat Cyrtoneritimorpha and Cycloneritimorpha as synonyms of Neritimorpha. The monotypic Devonian genus Pragoserpulina is morphological so close to the Orthonychia species reported herein that synonymy of both genera seems to be possible (and thus of the families Pragoserpulinidae and Orthonychiidae). Protoconch morphology and dimensions suggest that the studied platycerate species had planktotrophic larval development. By contrast, two studied Carboniferous euomphaloid species (one with an openly and the other with tightly coiled protoconch) have paucispiral, large protoconchs indicating non-planktotrophic larval development. We assume that openly and tightly coiled protoconchs were present in various Paleozoic gastropod clades and that selection acted against the openly coiled protoconch morphology. It has previously been proposed that increasing predation pressure in the plankton was the reason for the demise of openly coiled protoconchs (Paleozoic plankton revolution). The presence of larval planktotrophy in platycerates excludes the possibility that they belong to extant basal gastropod clades such as Patellogastropoda, Cocculiniformia, and Vetigastropoda. However, a previously proposed close relationship to Neritimorpha is corroborated.

New mitogenomes in deep-water endemic Cocculinida and Neomphalida shed light on lineage-specific gene orders in major gastropod clades

Gastropoda is the most speciose class in Mollusca, the second largest animal phylum. The internal relationships of major gastropod groups remain largely unsettled, partly due to the insufficient data from key deep-water endemic lineages such as the subclass Neomphaliones. Neomphaliones currently includes two orders: Cocculinida, best known from sunken wood habitats, and Neomphalida, best known from hydrothermal vents and often referred to as the “hot vent clade.” Phylogenetic controversy has also been observed in this subclass across different studies, requesting additional investigations. Here, we assembled nine new mitogenomes from two Cocculinida and seven Neomphalida species and analyzed them with published gastropod mitogenomes, with a particular focus on Neomphaliones. The phylogenetic reconstruction of Gastropoda based on 13 mitochondrial protein-coding genes resulted in a topology largely congruent with previous reconstructions based on morphological characters. Furthermore, we recovered characteristic mitochondrial gene order arrangements of Cocculinida and Neomphalida compared to the hypothetical ancestral gastropod gene order, at a level similar to other subclass-level clades. Divergence time estimation showed that Cocculinida and Neomphalida diverged approximately 322.68 million years ago. In addition to characteristic gene order arrangements for the clade, Cocculinida mitogenomes also exhibit some minor rearrangements even among congeners. Within Neomphalida, our tree adds support to monophyletic Peltospiridae and Neomphalidae, with unique gene arrangement recovered for each family. Our results offer new insights into the rearrangement of mitogenomes in Gastropoda, providing another clue to the evolutionary history of gastropods.

Niche filtering, competition and species turnover in a metacommunity of freshwater molluscs

Metacommunity structure reflects the interplay of various processes, including niche filtering, extinction/colonization and interspecific interactions. Spatial patterns of species distributions are often analyzed to infer these processes. However, such inferences rely on often unrealistic equilibrium assumptions, and remain ambiguous, as different processes can produce similar patterns. Temporal data may improve these inferences. For example, stochastic species turnover may occur in local communities, while, on the long run, temporal changes are kept within limits set by locally available niches. Our objective is to explore how the joint analysis of spatial and temporal patterns can clarify the contribution of different processes to metacommunity structure. We recorded the occurrences of 21 freshwater mollusc species, and environmental data, in 250 sites over 17 successive years in a network of ponds in Guadeloupe (Lesser Antilles). We analyzed variation in α and β‐diversities in space and time, and used a joint‐species distribution mode to characterize species–environment and species–species relationships. Local communities showed pronounced temporal variation reflecting both imperfect species detection and true stochastic species turnover. On the long term however, local communities were largely controlled by niche filtering along two main environmental gradients, one driven by site connectivity, the other by hydrological stability and aquatic vegetation. Two gastropod clades, caenogastropods and pulmonates, showed contrasted spatio–temporal distributions resulting from different responses to these gradients, and these distributions seemed little altered by interspecific competition. Our study illustrates the benefit of using spatiotemporal metacommunity data to discern long‐term impacts of niche filtering and species interactions behind short‐term stochasticity.

Can fractal dimensions objectivize gastropod shell morphometrics? A case study from Lake Lugu (SW China).

Morphometrics are fundamental for the analysis of size and shape in fossils, particularly because soft parts or DNA are rarely preserved and hard parts such as shells are commonly the only source of information. Geometric morphometrics, that is, landmark analysis, is well established for the description of shape but it exhibits a couple of shortcomings resulting from subjective choices during landmarking (number and position of landmarks) and from difficulties in resolving shape at the level of micro‐sculpture.With the aid of high‐resolution 3D scanning technology and analyses of fractal dimensions, we test whether such shortcomings of linear and landmark morphometrics can be overcome. As a model group, we selected a clade of modern viviparid gastropods from Lake Lugu, with shells that show a high degree of sculptural variation. Linear and landmark analyses were applied to the same shells in order to establish the fractal dimensions. The genetic diversity of the gastropod clade was assessed.The genetic results suggest that the gastropod clade represents a single species. The results of all morphometric methods applied are in line with the genetic results, which is that no specific morphotype could be delimited. Apart from this overall agreement, landmark and fractal dimension analyses do not correspond to each other but represent data sets with different information. Generally, the fractal dimension values quantify the roughness of the shell surface, the resolution of the 3D scans determining the level. In our approach, we captured the micro‐sculpture but not the first‐order sculptural elements, which explains that fractal dimension and landmark data are not in phase.We can show that analyzing fractal dimensions of gastropod shells opens a window to more detailed information that can be considered in evolutionary and ecological contexts. We propose that using low‐resolution 3D scans may successfully substitute landmark analyses because it overcomes the subjective landmarking. Analyses of 3D scans with higher resolution than used in this study will provide surface roughness information at the mineralogical level. We suggest that fractal dimension analyses of a combination of differently resolved 3D models will significantly improve the quality of shell morphometrics.

Slit-band gastropods (Pleurotomariida) from the Upper Triassic St. Cassian Formation and their diversity dynamics in the Triassic.

The St. Cassian Formation, Italy, has yielded the most diverse marine invertebrate fauna known from the Triassic. A quarter of all described Triassic gastropod species has been reported from this formation. Most of the gastropod species from the St. Cassian Formation were erected in the 19th century and many of them are known only from their original figures and descriptions. The failure to study type specimens resulted in many erroneous identifications by subsequent authors. Here, we revise the slit band gastropods (Pleurotomariida) from the St. Cassian Formationone of the major groups present in this formation. A total of 77 nominate Pleurotomariida species belonging to 29 genera and 11 families are present in the St. Cassian Formation which comprises approximately 14% of the total nominate gastropod species of that formation. In addition, we revise several taxa that had been wrongly assigned to Pleurotomariida. As other gastropod clades, Pleurotomariida experienced a major extinction at the end-Permian mass extinction event. As in the Late Palaeozoic, their relative abundance in gastropod faunas continued to be 30% in some Anisian faunas but decreased to 510% afterwards. Their diversification at generic level became interrupted by an extinction event within the Carnian, probably by the Carnian Pluvial Event. As a result of their sluggish recovery compared to the other gastropod groups, their species diversity decreased from 26% during the Permian to 18% during the Triassic. Type specimens of the following genera are studied: Proteomphalus, Rhaphistomella, Temnotropis, Kittlidiscus, Stuorella, Schizogonium, Wortheniella, Bandelium, Lancedellia, Rinaldoella, Pseudowortheniella, Paleunema, Ampezzalina, Bandelastraea, Cheilotomona, Pseudoscalites, Delphinulopsis, and Cochlearia. Nine new pleurotomariidan genera are erected: Amplitomaria, Pseudoananias, Lineacingulum, Pressulasphaera, Cancellotomaria, Acutitomaria, Lineaetomaria, Nodocingulum, and Striacingulum. Eight new species are described: Schizogonium undae, Acutitomaria kustatscherae, Wortheniella klipsteini, Wortheniella paolofedelei, Rinaldoella tornata, Nodocingulum ernstkittli, Nodocingulum? turris, and Laubella subsulcata. Eoworthenia frydai is a new replacement name for Worthenia rarissima Barrande.

Molecular Characterization of Carbonic Anhydrase II (CA II) and Its Potential Involvement in Regulating Shell Formation in the Pacific Abalone, Haliotis discus hannai.

Carbonic anhydrases (CAs) are a family of metalloenzymes that can catalyze the reversible interconversion of CO2/HCO3–, ubiquitously present in both prokaryotes and eukaryotes. In the present study, a CA II (designated as HdhCA II) was sequenced and characterized from the mantle tissue of the Pacific abalone. The complete sequence of HdhCA II was 1,169 bp, encoding a polypeptide of 349 amino acids with a NH2-terminal signal peptide and a CA architectural domain. The predicted protein shared 98.57% and 68.59% sequence identities with CA II of Haliotis gigantea and Haliotis tuberculata, respectively. Two putative N-linked glycosylation motifs and two cysteine residues could potentially form intramolecular disulfide bond present in HdhCA II. The phylogenetic analysis indicated that HdhCA II was placed in a gastropod clade and robustly clustered with CA II of H. gigantea and H. tuberculata. The highest level of HdhCA II mRNA expression was detected in the shell forming mantle tissue. During ontogenesis, the mRNA of HdhCA II was detected in all stages, with larval shell formation stage showing the highest expression level. The in situ hybridization results detected the HdhCA II mRNA expression in the epithelial cells of the dorsal mantle pallial, an area known to express genes involved in the formation of a nacreous layer in the shell. This is the first report of HdhCA II in the Pacific abalone, and the results of this study indicate that this gene might play a role in the shell formation of abalone.

Assessing the systematics of Tylodinidae in the Mediterranean Sea and Eastern Atlantic Ocean: resurrectingTylodina rafinesquiiPhilippi, 1836 (Heterobranchia: Umbraculida)

ABSTRACTThe systematics of the gastropod clade Umbraculida, particularly the family Tylodinidae, has been a matter of debate. The Tylodinidae of the Mediterranean Sea are a case in point, with no comprehensive molecular assessment of diversity having been carried out to date. Several species and genera have been erected and synonymized in the course of the last two centuries and only a single species from each of the genera Tylodina and Anidolyta are considered to be present in these waters. In order to shed light on the controversial taxonomy of the group, we carried out both morpho-anatomical study and molecular analyses using fragments of two mitochondrial genes, cytochrome c oxidase subunit I and 16S rRNA, and the nuclear gene histone H3. Phylogenetic analyses and species delimitation tests clearly recovered two independent lineages of Tylodina from the Mediterranean and Eastern Atlantic coast, the type species T. perversa and the resurrected T. rafinesquii. We found clear differences in shell and radular morphology between both species, as well as differences in their habitat and food preferences. Interestingly, we found strong evidence that T. rafinesquii is sister to T. fungina from the Eastern Pacific rather than to the sympatric T. perversa. Furthermore, the new morphological data strongly encourage the suppression of the genus Anidolyta, which should be considered a junior synonym of Tylodina.

Hemocyanin of the caenogastropod Pomacea canaliculata exhibitsevolutionary differences among gastropod clades.

Structural knowledge of gastropod hemocyanins is scarce. To better understand their evolution and diversity we studied the hemocyanin of a caenogastropod, Pomacea canaliculata (PcH). Through a proteomic and genomic approach, we identified 4 PcH subunit isoforms, in contrast with other gastropods that usually have 2 or 3. Each isoform has the typical Keyhole limpet-type hemocyanin architecture, comprising a string of eight globular functional units (FUs). Correspondingly, genes are organized in eight FUs coding regions. All FUs in the 4 genes are encoded by more than one exon, a feature not found in non- caenogastropods. Transmission electron microscopy images of PcH showed a cylindrical structure organized in di, tri and tetra-decamers with an internal collar structure, being the di and tri-decameric cylinders the most abundant ones. PcH is N-glycosylated with high mannose and hybrid-type structures, and complex-type N-linked glycans, with absence of sialic acid. Terminal β-N-GlcNAc residues and nonreducing terminal α-GalNAc are also present. The molecule lacks O-linked glycosylation but presents the T-antigen (Gal-β1,3-GalNAc). Using an anti-PcH polyclonal antibody, no cross-immunoreactivity was observed against other gastropod hemocyanins, highlighting the presence of clade-specific structural differences among gastropod hemocyanins. This is, to the best of our knowledge, the first gene structure study of a Caenogastropoda hemocyanin.

Are assortative mating and genital divergence driven by reinforcement?

The evolution of assortative mating is a key part of the speciation process. Stronger assortment, or greater divergence in mating traits, between species pairs with overlapping ranges is commonly observed, but possible causes of this pattern of reproductive character displacement are difficult to distinguish. We use a multidisciplinary approach to provide a rare example where it is possible to distinguish among hypotheses concerning the evolution of reproductive character displacement. We build on an earlier comparative analysis that illustrated a strong pattern of greater divergence in penis form between pairs of sister species with overlapping ranges than between allopatric sister‐species pairs, in a large clade of marine gastropods (Littorinidae). We investigate both assortative mating and divergence in male genitalia in one of the sister‐species pairs, discriminating among three contrasting processes each of which can generate a pattern of reproductive character displacement: reinforcement, reproductive interference and the Templeton effect. We demonstrate reproductive character displacement in assortative mating, but not in genital form between this pair of sister species and use demographic models to distinguish among the different processes. Our results support a model with no gene flow since secondary contact and thus favor reproductive interference as the cause of reproductive character displacement for mate choice, rather than reinforcement. High gene flow within species argues against the Templeton effect. Secondary contact appears to have had little impact on genital divergence.

Hemocyanin genes as indicators of habitat shifts in Panpulmonata?

Hemocyanin is the primary respiratory protein for the majority of the Mollusca and therefore directly interfaces with the physiological requirements of each species and the environments to which they are adapted. Hemocyanin is therefore likely to have been evolutionarily imprinted by significant habitat shifts. In the gastropod clade Panpulmonata (>30,000 species) major realm transitions have occurred multiple times independently and may have contributed to the diversification of this group. Yet, little is known about the adaptive changes linked to these habitat shifts. In order to gain deeper insight into the evolution of panpulmonate hemocyanins and to infer possible impacts associated with those scenarios, we have assembled and analysed hemocyanin isoforms from 4 panpulmonate species: (i) Helix pomatia, (ii) Cantareus aspersus (both Helicidae, Stylommatophora), (iii) Arion vulgaris (Arionidae, Stylommatophora) and (iv) Lymnaea stagnalis (Lymnaeidae, Hygrophila). Additionally, we describe a new hemocyanin isoform within the genome of the euopisthobranch Aplysia californica. Using these newly acquired hemocyanin data, we performed a phylogenetic analysis that reveals independent duplication events of hemocyanin within lineages that correlate with significant habitat shifts.

Differential response to Cadmium exposure by expression of a two and a three-domain metallothionein isoform in the land winkle Pomatias elegans: Valuating the marine heritage of a land snail

Through evolution, marine snails have adapted several times independently to terrestrial life. A prime example for such transitions is the adaptation to terrestrial conditions in members of the gastropod clade of Littorinoidea (Caenogastropoda). Some species of this lineage like the periwinkle (Littorina littorea), live in intertidal habitats, where they are intermittently exposed to semi-terrestrial conditions. Pomatias elegans is a close relative of Littorina littorea that has successfully colonized terrestrial habitats. Evolutionary transitions from marine to terrestrial conditions have often been fostered in marine ancestors by acquisition of physiological pre-adaptations to terrestrial life. Such pre-adaptations are based, among others, on the optimization of a wide repertoire of stress resistance mechanisms, such as the expression of metal inactivating metallothioneins (MTs).The objective of our study was to explore the Cd handling strategy in the terrestrial snail Pomatias elegans in comparison to that observed previously in Littorina littorea. After Cd exposure, the metal is accumulated mainly in the midgut gland of Pomatias elegans, in a similar way as in its marine relative. Upon Cd exposure, Pomatias elegans expresses Cd-specific MTs, as also described from Littorina littorea. In contrast to the latter species, however, the detoxification of Cd in Pomatias elegans is mediated by two different MT isoforms, one two-domain and one three-domain MT. Although the MT proteins of both species are homologous and clearly originate from one common ancestor, the three-domain MT isoform of Pomatias elegans has evolved independently from the three-domain MT of its marine counterpart, probably by addition of a third domain to the pre-existing two-domain MT. Obviously, the occurrence of homologous MT structures in both species is a hereditary character, whereas the differentiation into two distinct MT isoforms with different upregulation capacities in Pomatias elegans is an adaptive feature that probably emerged upon transition to life on land.

Life in the arena: infaunal gastropods and the late Phanerozoic expansion of marine ecosystems into sand

AbstractMarine ecosystems have expanded into the infaunal realm below the surface of soft sediments throughout the Phanerozoic eon. During the Palaeozoic era, this expansion largely involved sedentary animals living in permanent resting places. Active sand‐burrowing animals colonized the infaunal environment later, but when this happened and when specialization for infaunal life evolved remain open questions. Here, phylogenetic evidence, fossil occurrences and previously established criteria for recognizing the sand‐burrowing habit in marine gastropods are used to determine how many gastropod clades became infaunal and when the transitions from surface‐dwelling to infaunal life occurred. At least 20, and as many as 35, clades (all but one of post‐Palaeozoic age) contain actively infaunal species. The overwhelming majority (15 of 20 clades) became infaunal during the Cenozoic, and clades with hundreds of infaunal species in the living fauna diversified beginning in the Early Miocene. The repeated evolution of, and specialization to, the sand‐burrowing habit by gastropods and other animals was enabled by increased habitat availability and higher marine productivity, and was necessitated by intensifying predation. As a result, the infaunal realm was transformed from a marine refuge to an integrated part of the marine biosphere in which high performance in locomotion and defence has become the norm.

Insights into the Systematics, Phylogeny, and Evolution of Cladobranchia (Gastropoda: Heterobranchia).

Abstract: Cladobranchia is a diverse clade of nudibranch gastropods, with ∼1,000 coastal (benthic) and off shore (pelagic) marine species. Their diversity may be tied to dietary specialization and the evolution of remarkable adaptations that are rare among animals, such as the sequestration of cnidarian nematocysts. Due to the deep divergences within Cladobranchia, and other higher-order taxa within Mollusca, phylogenetic analyses based on morphology or a few genes have been unable to resolve relationships within these clades with high support. The introduction of high-throughput sequencing technologies, however, has provided the opportunity to collect large genomic data sets, which have proven useful in resolving relationships within molluscs, as in other metazoan groups. As sequencing costs decrease, it has become easier to collect ever larger data sets in an attempt to resolve relationships within more recently diverged groups, such as Cladobranchia. The phyly and relationships of the three main groups...

The varix: evolution, distribution, and phylogenetic clumping of a repeated gastropod innovation

Author(s): Webster, NB; Vermeij, GJ | Abstract: A recurrent theme in evolution is the repeated, independent origin of broadly adaptive, architecturally and functionally similar traits and structures. One such is the varix, a shell-sculpture innovation in gastropods. This periodic shell thickening functions mainly to defend the animal against shell crushing and peeling predators. Varices can be highly elaborate, forming broad wings or spines, and are often aligned in synchronous patterns. Here we define the different types of varices, explore their function and morphological variation, document the recent and fossil distribution of varicate taxa, and discuss emergent patterns of evolution. We conservatively found 41 separate origins of varices, which were concentrated in the more derived gastropod clades and generally arose since the mid-Mesozoic. Varices are more prevalent among marine, warm, and shallow waters, where predation is intense, on high-spired shells and in clades with collabral ribs. Diversification rates were correlated in a few cases with the presence of varices, especially in the Muricidae and Tonnoidea, but more than half of the origins are represented by three or fewer genera. Varices arose many times in many forms, but generally in a phylogenetically clumped manner (more frequently in particular higher taxa), a pattern common to many adaptations.

The limpet form in gastropods: evolution, distribution, and implications for the comparative study of history

The limpet form – a cap-shaped or slipper-shaped univalved shell – convergently evolved in many gastropod lineages, but questions remain about when, how often, and under which circumstances it originated. Except for some predation-resistant limpets in shallow-water marine environments, limpets are not well adapted to intense competition and predation, leading to the prediction that they originated in refugial habitats where exposure to predators and competitors is low. A survey of fossil and living limpets indicates that the limpet form evolved independently in at least 54 lineages, with particularly frequent origins in early-diverging gastropod clades, as well as in Neritimorpha and Heterobranchia. There are at least 14 origins in freshwater and 10 in the deep sea, with known times ranging from the Cambrian to the Neogene. Shallow-water limpets are most diverse at mid-latitudes; predation-resistant taxa are rare in cold water and absent in freshwater. These patterns contrast with the mainly Late Cretaceous and Caenozoic warm-water origins of features such as the labral tooth, enveloped shell, varices, and burrowing-enhancing sculpture that confer defensive and competitive benefits on molluscs.

The Mitochondrial Genomes of the Nudibranch Mollusks, Melibe leonina and Tritonia diomedea, and Their Impact on Gastropod Phylogeny.

The phylogenetic relationships among certain groups of gastropods have remained unresolved in recent studies, especially in the diverse subclass Opisthobranchia, where nudibranchs have been poorly represented. Here we present the complete mitochondrial genomes of Melibe leonina and Tritonia diomedea (more recently named T. tetraquetra), two nudibranchs from the unrepresented Cladobranchia group, and report on the resulting phylogenetic analyses. Both genomes coded for the typical thirteen protein-coding genes, twenty-two transfer RNAs, and two ribosomal RNAs seen in other species. The twelve-nucleotide deletion previously reported for the cytochrome oxidase 1 gene in several other Melibe species was further clarified as three separate deletion events. These deletions were not present in any opisthobranchs examined in our study, including the newly sequenced M. leonina or T. diomedea, suggesting that these previously reported deletions may represent more recently divergent taxa. Analysis of the secondary structures for all twenty-two tRNAs of both M. leonina and T. diomedea indicated truncated d arms for the two serine tRNAs, as seen in some other heterobranchs. In addition, the serine 1 tRNA in T. diomedea contained an anticodon not yet reported in any other gastropod. For phylogenetic analysis, we used the thirteen protein-coding genes from the mitochondrial genomes of M. leonina, T. diomedea, and seventy-one other gastropods. Phylogenetic analyses were performed for both the class Gastropoda and the subclass Opisthobranchia. Both Bayesian and maximum likelihood analyses resulted in similar tree topologies. In the Opisthobranchia, the five orders represented in our study were monophyletic (Anaspidea, Cephalaspidea, Notaspidea, Nudibranchia, Sacoglossa). In Gastropoda, two of the three traditional subclasses, Opisthobranchia and Pulmonata, were not monophyletic. In contrast, four of the more recently named gastropod clades (Vetigastropoda, Neritimorpha, Caenogastropoda, and Heterobranchia) were all monophyletic, and thus appear to be better classifications for this diverse group.

A new phylogeny of the Cephalaspidea (Gastropoda: Heterobranchia) based on expanded taxon sampling and gene markers

The Cephalaspidea is a diverse marine clade of euthyneuran gastropods with many groups still known largely from shells or scant anatomical data. The definition of the group and the relationships between members has been hampered by the difficulty of establishing sound synapomorphies, but the advent of molecular phylogenetics is helping to change significantly this situation. Yet, because of limited taxon sampling and few genetic markers employed in previous studies, many questions about the sister relationships and monophyletic status of several families remained open.In this study 109 species of Cephalaspidea were included covering 100% of traditional family-level diversity (12 families) and 50% of all genera (33 genera). Bayesian and maximum likelihood phylogenetics analyses based on two mitochondrial (COI, 16S rRNA) and two nuclear gene markers (28S rRNA and Histone-3) were used to infer the relationships of Cephalaspidea. The monophyly of the Cephalaspidea was confirmed. The families Cylichnidae, Diaphanidae, Haminoeidae, Philinidae, and Retusidae were found non-monophyletic. This result suggests that the family level taxonomy of the Cephalaspidea warrants a profound revision and several new family and genus names are required to reflect the new phylogenetic hypothesis presented here. We propose a new classification of the Cephalaspidea including five new families (Alacuppidae, Colinatydidae, Colpodaspididae, Mnestiidae, Philinorbidae) and one new genus (Alacuppa). Two family names (Acteocinidae, Laonidae) and two genera (Laona, Philinorbis) are reinstated as valid. An additional lineage with family rank (Philinidae “Clade 4”) was unravelled, but no genus and species names are available to reflect the phylogeny and formal description will take place elsewhere.

Phylogenomic analyses of deep gastropod relationships reject Orthogastropoda.

Gastropods are a highly diverse clade of molluscs that includes many familiar animals, such as limpets, snails, slugs and sea slugs. It is one of the most abundant groups of animals in the sea and the only molluscan lineage that has successfully colonized land. Yet the relationships among and within its constituent clades have remained in flux for over a century of morphological, anatomical and molecular study. Here, we re-evaluate gastropod phylogenetic relationships by collecting new transcriptome data for 40 species and analysing them in combination with publicly available genomes and transcriptomes. Our datasets include all five main gastropod clades: Patellogastropoda, Vetigastropoda, Neritimorpha, Caenogastropoda and Heterobranchia. We use two different methods to assign orthology, subsample each of these matrices into three increasingly dense subsets, and analyse all six of these supermatrices with two different models of molecular evolution. All 12 analyses yield the same unrooted network connecting the five major gastropod lineages. This reduces deep gastropod phylogeny to three alternative rooting hypotheses. These results reject the prevalent hypothesis of gastropod phylogeny, Orthogastropoda. Our dated tree is congruent with a possible end-Permian recovery of some gastropod clades, namely Caenogastropoda and some Heterobranchia subclades.

Larval ecology and morphology in fossil gastropods

AbstractThe shell of marine gastropods conserves and reflects early ontogeny, including embryonic and larval stages, to a high degree when compared with other marine invertebrates. Planktotrophic larval development is indicated by a small embryonic shell (size is also related to systematic placement) with little yolk followed by a multiwhorled shell formed by a free‐swimming veliger larva. Basal gastropod clades (e.g. Vetigastropoda) lack planktotrophic larval development. The great majority of Late Palaeozoic and Mesozoic ‘derived’ marine gastropods (Neritimorpha, Caenogastropoda and Heterobranchia) with known protoconch had planktotrophic larval development. Dimensions of internal moulds of protoconchs suggest that planktotrophic larval development was largely absent in the Cambrian and evolved at the Cambrian–Ordovician transition, mainly due to increasing benthic predation. The evolution of planktotrophic larval development offered advantages and opportunities such as more effective dispersal, enhanced gene flow between populations and prevention of inbreeding. Early gastropod larval shells were openly coiled and weakly sculptured. During the Mid‐ and Late Palaeozoic, modern tightly coiled larval shells (commonly with strong sculpture) evolved due to increasing predation pressure in the plankton. The presence of numerous Late Palaeozoic and Triassic gastropod species with planktotrophic larval development suggests sufficient primary production although direct evidence for phytoplankton is scarce in this period. Contrary to previous suggestions, it seems unlikely that the end‐Permian mass extinction selected against species with planktotrophic larval development. The molluscan classes with highest species diversity (Gastropoda and Bivalvia) are those which may have planktotrophic larval development. Extremely high diversity in such groups as Caenogastropoda or eulamellibranch bivalves is the result of high phylogenetic activity and is associated with the presence of planktotrophic veliger larvae in many members of these groups, although causality has not been shown yet. A new gastropod species and genus, Anachronistella peterwagneri, is described from the Late Triassic Cassian Formation; it is the first known Triassic gastropod with an openly coiled larval shell.