Euthyneuran gastropods represent one of the most diverse lineages in Mollusca (with over 30,000 species), play significant ecological roles in aquatic and terrestrial environments and affect many aspects of human life. However, our understanding of their evolutionary relationships remains incomplete due to missing data for key phylogenetic lineages. The present study integrates such a neglected, ancient snail family Ringiculidae into a molecular systematics of Euthyneura for the first time, and is supplemented by the first microanatomical data. Surprisingly, both molecular and morphological features present compelling evidence for the common ancestry of ringiculid snails with the highly dissimilar Nudipleura—the most species-rich and well-known taxon of sea slugs (nudibranchs and pleurobranchoids). A new taxon name Ringipleura is proposed here for these long-lost sisters, as one of three major euthyneuran clades with late Palaeozoic origins, along with Acteonacea (Acteonoidea + Rissoelloidea) and Tectipleura (Euopisthobranchia + Panpulmonata). The early Euthyneura are suggested to be at least temporary burrowers with a characteristic ‘bubble’ shell, hypertrophied foot and headshield as exemplified by many extant subtaxa with an infaunal mode of life, while the expansion of the mantle might have triggered the explosive Mesozoic radiation of the clade into diverse ecological niches.

BackgroundGastropods are among the most diverse animal clades, and have successfully colonized special habitats such as the marine sand interstitial. Specialized meiofaunal snails and slugs are tiny and worm-shaped. They combine regressive features – argued to be due to progenetic tendencies – with convergent adaptations. Microscopic size and concerted convergences make morphological examination non-trivial and hamper phylogenetic reconstructions. The enigmatic turbellarian-like Rhodopemorpha are a small group that has puzzled systematists for over a century. A preliminary molecular framework places the group far closer to the root of Heterobranchia – one of the major gastropod groups – than previously suggested. The poorly known meiofaunal Helminthope psammobionta Salvini-Plawen, 1991 from Bermuda is the most worm-shaped free-living gastropod and shows apparently aberrant aspects of anatomy. Its study may give important clues to understand the evolution of rhodopemorphs among basal heterobranchs versus their previously thought origin among ‘higher’ euthyneuran taxa.ResultsWe describe the 3D-microanatomy of H. psammobionta using three-dimensional digital reconstruction based on serial semithin histological sections. The new dataset expands upon the original description and corrects several aspects. Helminthope shows a set of typical adaptations and regressive characters present in other mesopsammic slugs (called ‘meiofaunal syndrome’ herein). The taxonomically important presence of five separate visceral loop ganglia is confirmed, but considerable further detail of the complex nervous system are corrected and revealed. The digestive and reproductive systems are simple and modified to the thread-like morphology of the animal; the anus is far posterior. There is no heart; the kidney resembles a protonephridium. Data on all organ systems are compiled and compared to Rhodope.ConclusionsHelminthope is related to Rhodope sharing unique apomorphies. We argue that the peculiar kidney, configuration of the visceral loop and simplicity or lack of other organs in Rhodopemorpha are results of progenesis. The posterior shift of the anus in Helminthope is interpreted as a peramorphy, i.e. hypertrophy of body length early in ontogeny. Our review of morphological and molecular evidence is consistent with an origin of Rhodopemorpha slugs among shelled ‘lower Heterobranchia’. Previously thought shared ‘diagnostic’ features such as five visceral ganglia are either plesiomorphic or convergent, while euthyneury and a double-rooted cerebral nerve likely evolved independently in Rhodopemorpha and Euthyneura.

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The evolution and diversification of euthyneuran slugs and snails was likely strongly influenced by habitat transitions from marine to terrestrial and limnic systems. Well-supported euthyneuran phylogenies with detailed morphological data can provide information on the historical, biological and ecological background in which these habitat shifts took place allowing for comparison across taxa. Acochlidian slugs are ‘basal pulmonates’ with uncertain relationships to other major panpulmonate clades. They present a unique evolutionary history with representatives in the marine mesopsammon, but also benthic lineages in brackish water, limnic habitats and (semi-)terrestrial environments. We present the first comprehensive molecular phylogeny on Acochlidia, based on a global sampling that covers nearly 85 % of the described species diversity, and additionally, nearly doubles known diversity by undescribed taxa. Our phylogenetic hypotheses are highly congruent with previous morphological analyses and confirm all included recognized families and genera. We further establish an ancestral area chronogram for Acochlidia, document changes in diversification rates in their evolution via the birth-death-shift-model and reconstruct the ancestral states for major ecological traits. Based on our data, Acochlidia originated from a marine, mesopsammic ancestor adapted to tropical waters, in the mid Mesozoic Jurassic. We found that the two major subclades present a remarkably different evolutionary history. The Microhedylacea are morphologically highly-adapted to the marine mesopsammon. They show a circum-tropical distribution with several independent shifts to temperate and temperate cold-habitats, but remained in stunning morphological and ecological stasis since the late Mesozoic. Their evolutionary specialization, which includes a remarkable and potentially irreversible ‘meiofaunal syndrome’, guaranteed long-term survival and locally high species densities but also resembles a dead-end road to morphological and ecological diversity. In contrast, the Hedylopsacea are characterized by morphological flexibility and ecologically by independent habitat shifts out of the marine mesopsammon, conquering (semi-)terrestrial and limnic habitats. Originating from interstitial ancestors with moderate adaptations to the mesopsammic world, they reestablished a benthic lifestyle and secondary ‘gigantism’ in body size. The major radiations and habitat shifts in hedylopsacean families occured in the central Indo-West Pacific in the Paleogene. In the Western Atlantic only one enigmatic representative is known probably presenting a relict of a former pan-Tethys distribution of the clade. This study on acochlidian phylogeny and biogeography adds another facet of the yet complex panpulmonate evolution and shows the various parallel pathways in which these snails and slugs invaded non-marine habitats. Given the complex evolutionary history of Acochlidia, which represent only a small group of Panpulmonata, this study highlights the need to gen...

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