Disentangling true shape differences and experimenter bias: are dextral and sinistral snail shells exact mirror images?

Population effects of chiral snail shell development relate handedness to health and disease

The spiral patterns of snail shells exhibit chirality, or “handedness.” These patterns often heavily favor the dextral (right-handed, or clockwise) over the sinistral (left-handed, clockwise) phenotype. While the developmental pathways resulting in each enantiomorph (non-superimposable mirror image form) have been studied extensively, there has been limited investigation into how the emphasis on one spiral direction over the other may confer survival benefit. This perspective essay proposes that developmental events determining cell cleavage robustness, mating compatibility, and predator evasion can influence the distribution of dextral and sinistral snails. The connection between chirality and survivability has broader implications for exploring the role of handedness in diseases and their treatments.

Stay in shape: Assessing the adaptive potential of shell morphology and its sensitivity to temperature in the invasive New Zealand mud snail Potamopyrgus antipodarum through phenotypic plasticity and natural selection in Europe

Climate change may force organisms to adapt genetically or plastically to new environmental conditions. Invasive species show remarkable potential for rapid adaptation. The ovoviviparous New Zealand mud snail (NZMS), Potamopyrgus antipodarum, has successfully established across Europe with two clonally reproducing mitochondrial lineages since its arrival in the first half of the 19th century. Its remarkable variation in shell morphology was shown to be fitness relevant. We investigated the effects of temperature on shell morphology across 11 populations from Germany and the Iberian Peninsula in a common garden across three temperatures. We analyzed size and shape using geometric morphometrics. For both, we compared reaction norms and estimated heritabilities. For size, the interaction of temperature and haplotype explained about 50% of the total variance. We also observed more genotype by environment interactions indicating a higher degree of population differentiation than in shape. Across the three temperatures, size followed the expectations of the temperature‐size rule, with individuals growing larger in cold environments. Changes in shape may have compensated for changes in size affecting space for brooding embryos. Heritability estimates were relatively high. As indicated by the very low coefficients of variation for clonal repeatability (CV_A), they can probably not be compared in absolute terms. However, they showed some sensitivity to temperature, in haplotype t more so than in z, which was only found in Portugal. The low CV_A values indicate that genetic variation among European populations is still restricted with a low potential to react to selection. A considerable fraction of the genetic variation was due to differences between the clonal lineages. The NZMS has apparently not been long enough in Europe to accumulate significant genetic variation relevant for morphological adaptation. As temperature is obviously not the sole factor influencing shell morphology, their interaction will probably not be a factor limiting population persistence under a warming climate in Europe.

Deciphering “cryptic” nature of European rock-dwelling Pyramidula snails (Gastropoda: Stylommatophora)

Many molecular phylogenetic studies conclude by reporting discoveries of new “cryptic” species. However, these putative biological entities are typically left unverified outside of the dna evidence or subjected to only superficial post-hoc analyses. Minute land snails of the Western Palearctic Pyramidula represent one of such examples being considered a cryptic species complex based on previously conducted molecular phylogeny. Several species appear indistinguishable due to noticeable shell tendency towards either high-spired (Pyramidula rupestris and P. jaenensis) or low-spired (P. saxatilis and P. pusilla) morphotype. Here, we challenge this conclusion by using mt DNA, n DNA, morphometric analyses and qualitative shell features, and seek for a potential evolutionary mechanism behind the conchological similarities. Through an empirical integration of multiple data types we document that the studied taxa can be visually distinguished. Unlike isolated shell measurements, cva s based on traditional morphometrics and geometric morphometrics have power to separate all species from each other, except for P. saxatilis and P. pusilla. However, only a use of previously overlooked shell surface microsculpture makes it possible to identify individuals of all species. Considering tight associations between shell measurements and climate, we propose an evolutionary explanation based on optimization of thermal flux under different climatic selection pressures. Our study brings the awareness towards microscopic shell features, and outlines a general protocol to identify robust visual identification criteria in taxonomic groups containing cryptic (and non-cryptic) members. It also exemplifies an integration of various data types for macroscale species identification, which we believe should follow any discovery of putatively cryptic species.

Internet 'shellebrity' reflects on origin of rare mirror-image snails

While animal bodies are typically bilaterally symmetric on the outside, the internal organs nearly always show an invariant left-right (LR) asymmetry. In comparison, snails are both internally and externally LR asymmetric, outwardly obvious in the shell coiling direction, or chirality. Although some species of snail are naturally variable for chirality, sinistral individuals occur very rarely in most species. The developmental and genetic basis of these rare mirror-imaged individuals remains mysterious. To resolve this issue, the finding of a 'one in a million' sinistral garden snail called 'Jeremy' was used to recruit citizen scientists to find further sinistral snails. These snails were then bred together to understand whether their occurrence is due an inherited condition. The combined evidence shows that rare sinistral garden snails are not usually produced due to a major effect maternal Mendelian locus. Instead, they are likely mainly produced by a developmental accident. This finding has relevance to understanding the common factors that define cellular and organismal LR asymmetry, and the origin of rare reversed individuals in other animal groups that exhibit nearly invariant LR asymmetry.

Adaptive phenotypic plasticity in a clonal invader

Organisms featuring wide trait variability and occurring in a wide range of habitats, such as the ovoviviparous New Zealand freshwater snail Potamopyrgus antipodarum, are ideal models to study adaptation. Since the mid‐19th century, P. antipodarum, characterized by extremely variable shell morphology, has successfully invaded aquatic areas on four continents. Because these obligately and wholly asexual invasive populations harbor low genetic diversity compared to mixed sexual/asexual populations in the native range, we hypothesized that (1) this phenotypic variation in the invasive range might be adaptive with respect to colonization of novel habitats, and (2) that at least some of the variation might be caused by phenotypic plasticity. We surveyed 425 snails from 21 localities across northwest Europe to attempt to disentangle genetic and environmental effects on shell morphology. We analyzed brood size as proxy for fitness and shell geometric morphometrics, while controlling for genetic background. Our survey revealed 10 SNP genotypes nested into two mtDNA haplotypes and indicated that mainly lineage drove variation in shell shape but not size. Physicochemical parameters affected both shell shape and size and the interaction of these traits with brood size. In particular, stronger stream flow rates were associated with larger shells. Our measurements of brood size suggested that relatively larger slender snails with relatively large apertures were better adapted to strong flow than counterparts with broader shells and relatively small apertures. In conclusion, the apparent potential to modify shell morphology plays likely a key role in the invasive success of P. antipodarum; the two main components of shell morphology, namely shape and size, being differentially controlled, the former mainly genetically and the latter predominantly by phenotypic plasticity.

Adaptive responses and invasion: the role of plasticity and evolution in snail shell morphology

Invasive species often exhibit either evolved or plastic adaptations in response to spatially varying environmental conditions. We investigated whether evolved or plastic adaptation was driving variation in shell morphology among invasive populations of the New Zealand mud snail (Potamopyrgus antipodarum) in the western United States. We found that invasive populations exhibit considerable shell shape variation and inhabit a variety of flow velocity habitats. We investigated the importance of evolution and plasticity by examining variation in shell morphological traits 1) between the parental and F₁ generations for each population and 2) among populations of the first lab generation (F₁) in a common garden, full-sib design using Canonical Variate Analyses (CVA). We compared the F₁ generation to the parental lineages and found significant differences in overall shell shape indicating a plastic response. However, when examining differences among the F₁ populations, we found that they maintained among-population shell shape differences, indicating a genetic response. The F₁ generation exhibited a smaller shell morph more suited to the low-flow common garden environment within a single generation. Our results suggest that phenotypic plasticity in conjunction with evolution may be driving variation in shell morphology of this widespread invasive snail.

Radiating on oceanic islands: patterns and processes of speciation in the land snail genus Theba (Risso 1826)

Island radiations have played a major role in shaping our current understanding of allopatric, sympatric and parapatric speciation. However, the fact that species divergence correlates with island size emphasizes the importance of geographic isolation (allopatry) in speciation. Based on molecular and morphological data, we investigated the diversification of the land snail genus Theba on the two Canary Islands of Lanzarote and Fuerteventura. Due to the geological history of both islands, this study system provides ideal conditions to investigate the interplay of biogeography, dispersal ability and differentiation in generating species diversity. Our analyses demonstrated extensive cryptic diversification of Theba on these islands, probably driven mainly by non-adaptive allopatric differentiation and secondary gene flow. In a few cases, we observed a complete absence of gene flow among sympatrically distributed forms suggesting an advanced stage of speciation. On the Jandía peninsula genome scans suggested genotype-environment associations and potentially adaptive diversification of two closely related Theba species to different ecological environments. We found support for the idea that genetic differentiation was enhanced by divergent selection in different environments. The diversification of Theba on both islands is therefore best explained by a mixture of non-adaptive and adaptive speciation, promoted by ecological and geomorphological factors.

Phylogenetic reconstruction and shell evolution of the Diplommatinidae (Gastropoda: Caenogastropoda)

The fascinating and often unlikely shell shapes in the terrestrial micromollusc family Diplommatinidae (Gastropoda: Caenogastropoda) provide a particularly attractive set of multiple morphological traits to investigate evolutionary patterns of shape variation. Here, a molecular phylogenetic reconstruction, based on five genes and 2700 bp, was undertaken for this family, integrated with ancestral state reconstruction and phylogenetic PCA of discrete and quantitative traits, respectively. We found strong support for the Diplommatininae as a monophyletic group, separating the Cochlostomatidae into a separate family. Five main clades appear within the Diplommatininae, corresponding with both coiling direction and biogeographic patterns. A Belau clade (A) with highly diverse (but always sinistral) morphology comprised Hungerfordia, Palaina, and some Diplommatina. Arinia (dextral) and Opisthostoma (sinistroid) are sister groups in clade B. Clade C and D solely contain sinistral Diplommatina that are robust and little ornamented (clade C) or slender and sculptured (clade D). Clade E is dextral but biogeographically diverse with species from all sampled regions save the Caroline Islands. Adelopoma, Diplommatina, Palaina, and Hungerfordia require revision to allow taxonomy to reflect phylogeny, whereas Opisthostoma is clearly monophyletic. Ancestral state reconstruction suggests a sinistral origin for the Diplommatinidae, with three reversals to dextrality.