No abstract

BackgroundA robust phylogenetic hypothesis of euthyneuran gastropods, as a basis to reconstructing their evolutionary history, is still hindered by several groups of aberrant, more or less worm-like slugs with unclear phylogenetic relationships. As a traditional "order" in the Opisthobranchia, the Acochlidia have a long history of controversial placements, among others influenced by convergent adaptation to the mainly meiofaunal habitats. The present study includes six out of seven acochlidian families in a comprehensive euthyneuran taxon sampling with special focus on minute, aberrant slugs. Since there is no fossil record of tiny, shell-less gastropods, a molecular clock was used to estimate divergence times within Euthyneura.ResultsOur multi-locus molecular study confirms Acochlidia in a pulmonate relationship, as sister to Eupulmonata. Previous hypotheses of opisthobranch relations, or of a common origin with other meiofaunal Euthyneura, are clearly rejected. The enigmatic amphibious and insectivorous Aitengidae incerta sedis clusters within Acochlidia, as sister to meiofaunal and brackish Pseudunelidae and limnic Acochlidiidae. Euthyneura, Opisthobranchia and Pulmonata as traditionally defined are non-monophyletic. A relaxed molecular clock approach indicates a late Palaeozoic diversification of Euthyneura and a Mesozoic origin of the major euthyneuran diversity, including Acochlidia.ConclusionsThe present study shows that the inclusion of small, enigmatic groups is necessary to solve deep-level phylogenetic relationships, and underlines that "pulmonate" and "opisthobranch" phylogeny, respectively, cannot be solved independently from each other. Our phylogenetic hypothesis requires reinvestigation of the traditional classification of Euthyneura: morphological synapomorphies of the traditionally defined Pulmonata and Opisthobranchia are evaluated in light of the presented phylogeny, and a redefinition of major groups is proposed. It is demonstrated that the invasion of the meiofaunal habitat has occurred several times independently in various euthyneuran taxa, leading to convergent adaptations previously misinterpreted as synapomorphies. The inclusion of Acochlidia extends the structural and biological diversity in pulmonates, presenting a remarkable flexibility concerning habitat choice.

1 Kano, Y. (2008). Vetigastropod phylogeny and a new concept of Seguenzioidea: independent evolution of copulatory organs in the deep-sea habitats. -Zoologica Scripta, 37, 1-21. Bayesian and maximum-likelihood phylogenies of Vetigastropoda (Mollusca: Gastropoda) were reconstructed by separate and combined analyses of one mitochondrial (cytochrome oxidase I, COI) and two nuclear (histone H3 and 18S rRNA) gene sequences, with an emphasis on dense taxonomic sampling. More than 70 vetigastropod species belonging to 13 families and 25 subfamilies constituted a robust clade against the two outgroup clades Neomphalina and Cocculinoidea. The phylogenetically controversial family Seguenziidae appeared as a derived Vetigastropoda and constituted a highly supported clade with eucycline and cataegine trochids, and three skeneimorphs (Adeuomphalus, Ventsia and Xyloskenea). These taxa herein treated as the superfamily Seguenzioidea are morphologically very diverse and grouped only by the combination of symplesiomorphies in the shell, radular and head-foot characters. Anatomical peculiarities of Seguenziidae, including the presence of the penis and seminal receptacle, are all apomorphic conditions independently derived from those in higher gastropod clades, as a consequence of the small size and in response to deep-sea habitats, where sperm storage seems to be especially beneficial with low numerical density of individuals and limited periodic cues for gametogenesis. Indeed, internal or semi-internal fertilization has been evolved at least six times in Vetigastropoda, essentially in deep-sea lineages, with weak phylogenetic constraints. Other new vetigastropod clades with high support values include: Turbinidae + Tegulinae (Trochidae) + Skeneidae s.s., Clypeosectidae + Lepetodrilidae, Anatominae (Scissurellidae) + Bathyxylophila (Skeneidae) and Lepetodriloidea + Scissurellidae + Bathyxylophila.

Rubyspira, a new genus of deep-sea snails (Gastropoda: Abyssochrysoidea) with two living species, derives its nutrition from decomposing whalebones. Molecular phylogenetic and morphological evidence places the new genus in an exclusively deep-sea assemblage that includes several close relatives previously known as fossils associated with Cretaceous cold seeps, plesiosaur bones, and Eocene whalebones. The ability to exploit a variety of marine reducing environments may have contributed to the evolutionary longevity of this gastropod lineage.

No abstract

Recent expeditions have revealed high levels of biodiversity in the tropical deep-sea, yet little is known about the age or origin of this biodiversity, and large-scale molecular studies are still few in number. In this study, we had access to the largest number of solariellid gastropods ever collected for molecular studies, including many rare and unusual taxa. We used a Bayesian chronogram of these deep-sea gastropods (1) to test the hypothesis that deep-water communities arose onshore, (2) to determine whether Antarctica acted as a source of diversity for deep-water communities elsewhere and (3) to determine how factors like global climate change have affected evolution on the continental slope. We show that although fossil data suggest that solariellid gastropods likely arose in a shallow, tropical environment, interpretation of the molecular data is equivocal with respect to the origin of the group. On the other hand, the molecular data clearly show that Antarctic species sampled represent a recent invasion, rather than a relictual ancestral lineage. We also show that an abrupt period of global warming during the Palaeocene Eocene Thermal Maximum (PETM) leaves no molecular record of change in diversification rate in solariellids and that the group radiated before the PETM. Conversely, there is a substantial, although not significant increase in the rate of diversification of a major clade approximately 33.7 Mya, coinciding with a period of global cooling at the Eocene–Oligocene transition. Increased nutrients made available by contemporaneous changes to erosion, ocean circulation, tectonic events and upwelling may explain increased diversification, suggesting that food availability may have been a factor limiting exploitation of deep-sea habitats. Tectonic events that shaped diversification in reef-associated taxa and deep-water squat lobsters in central Indo-West Pacific were also probably important in the evolution of solariellids during the Oligo-Miocene.

Molluscs are a diverse animal phylum with a formidable fossil record. Although there is little doubt about the monophyly of the eight extant classes, relationships between these groups are controversial. We analysed a comprehensive multilocus molecular data set for molluscs, the first to include multiple species from all classes, including five monoplacophorans in both extant families. Our analyses of five markers resolve two major clades: the first includes gastropods and bivalves sister to Serialia (monoplacophorans and chitons), and the second comprises scaphopods sister to aplacophorans and cephalopods. Traditional groupings such as Testaria, Aculifera, and Conchifera are rejected by our data with significant Approximately Unbiased (AU) test values. A new molecular clock indicates that molluscs had a terminal Precambrian origin with rapid divergence of all eight extant classes in the Cambrian. The recovery of Serialia as a derived, Late Cambrian clade is potentially in line with the stratigraphic chronology of morphologically heterogeneous early mollusc fossils. Serialia is in conflict with traditional molluscan classifications and recent phylogenomic data. Yet our hypothesis, as others from molecular data, implies frequent molluscan shell and body transformations by heterochronic shifts in development and multiple convergent adaptations, leading to the variable shells and body plans in extant lineages.

The nucleotide sequences of the complete or nearly complete mitochondrial (mt) genomes of seven vetigastropods were determined: Angaria neglecta (Angarioidea), Phasianella solida (Phasianelloidea), Granata lyrata (Seguenzioidea), Tegula lividomaculata and Bolma rugosa (Trochoidea), Diodora graeca (Fissurelloidea) and Lepetodrilus schrolli (Lepetodriloidea). While the mt genomes of the superfamilies Angarioidea, Phasianelloidea, Seguenzioidea and Trochoidea conform generally to the ancestral gene order of Vetigastropoda and Gastropoda, those of the superfamilies Fissurelloidea and Lepetodriloidea have suffered important rearrangements. The gene order of the mtDNA of Chrysomallon squamiferum, a representative of Neomphalina, was also analysed since it has been proposed to be closely related to Vetigastropoda, and showed a distinct arrangement. The reconstructed phylogenies recovered Neomphalina as a distinct gastropod lineage that is the sister group (only with moderate bootstrap support) of a clade including Vetigastropoda and Neritimorpha + Caeno‐gastropoda while the relative position of Heterobranchia and Patellogastropoda in the gastropod tree could not be determined definitively due to their long branches. Within the monophyletic Vetigastropoda, the superfamily Fissurelloidea was recovered as the sister group of two lineages, one including Lepetodriloidea as the sister group of Seguenzioidea + Halitoidea, the other including Phasianelloidea, Angarioidea and Trochoidea without resolved relationships. The long branches of Fissurelloidea were found to introduce significant tree instability in phylogenetic reconstruction. The new phylogeny supports that the loss of the right pallial gill occurred multiple times in vetigastropod evolution as previously suggested and that Phasianelloidea, Angarioidea and Trochoidea radiated from a common asymmetric (single‐gilled) ancestor that lived in the middle Palaeozoic.