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 mtDNA, nDNA, 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, CVAs 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.

Automated summary

Many molecular phylogenetic studies report the discovery of new cryptic species, but often these discoveries are not verified outside of DNA evidence or are only superficially analyzed. In this study, the researchers challenge the conclusions of a previous study on land snails, showing that the species can be visually distinguished through the use of mtDNA, nDNA, morphometric analyses, and shell features. The researchers propose an evolutionary explanation based on climate and thermal flux optimization. This study highlights the importance of microscopic shell features and provides a protocol for identifying visual identification criteria in taxonomic groups.