Taxonomic Significance of Ribosomal ITS-1 Sequence Markers in Self-fertilizing Land Snails of Cochlicopa (Stylommatophora, Cochlicopidae)

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.

When is a "cryptic" species not a cryptic species: A consideration from the Holarctic micro-landsnail genus Euconulus (Gastropoda: Stylommatophora)

Naive use of molecular data may lead to ambiguous conclusions, especially within the context of "cryptic" species. Here, we integrated molecular and morphometric data to evaluate phylogenetic relationships in the widespread terrestrial micro-snail genus, Euconulus. We analyzed mitochondrial (16S + COII) and nuclear (ITS1 + ITS2) sequence across 94 populations from Europe, Asia and North America within the nominate species E. alderi, E. fulvus and E. polygyratus, and used the southeastern USA E. chersinus, E. dentatus, and E. trochulus as comparative outgroups. Phylogeny was reconstructed using four different reconstruction methods to identify robust, well-supported topological features. We then performed discriminant analysis on shell measurements between these genetically-identified species-level clades. These analyses provided evidence for a biologically valid North American "cryptic" species within E. alderi. However, while highly supported polyphyletic structure was also observed within E. fulvus, disagreement in placement of individuals between mtDNA and nDNA clades, lack of morphological differences, and presence of potential hybrids imply that these lineages do not rise to the threshold as biologically valid cryptic species, and rather appear to simply represent a complex of geographically structured populations within a single species. These results caution that entering into a cryptic species hypothesis should not be undertaken lightly, and should be optimally supported along multiple lines of evidence. Generally, post-hoc analyses of macro-scale features should be conducted to attempt identification of previously ignored diagnostic traits. If such traits cannot be found, i.e. in the case of potentially "fully cryptic" species, additional criteria should be met to propound a cryptic species hypothesis, including the agreement in tree topology among both mtDNA and nDNA, and little (or no) evidence of hybridization based on a critical analysis of sequence chromatograms. Even when the above conditions are satisfied, it only implies that the cryptic species hypothesis is plausible, but should optimally be subjected to further careful examination.