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

Abstract

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.

The Phyloperiodic Approach Removes the “Cryptic Species” and Puts forward Multilevel Organismal Diversity

“…This is also due to the fact that any statements about the "morphologically indistinguishable species" have no biological basis since there cannot be two completely identical biological organisms, even twins [5,7,73], so "cryptic twin species" should not be excluded from that strict biological rule. There is growing dissatisfaction with the concept of "cryptic species" in various studies of disparate organismal groups, such as plants and animals [3,6,8,26,74,75], and more recently species for which morphological data can be provided have been highlighted as "non-cryptic" [76]. There are also a number of studies [77][78][79][80][81][82][83][84] in which the more neutral term "hidden diversity" has already been used, which does not overlap semantically with "cryptic species" in its original meaning: "well camouflaged on a substrate and in an environment" [7,9,[85][86][87].…”

“…The following guidelines have been proposed recently [7,53], and we follow them in this study with a little modification: (1) make a selection of a taxonomic group and specimens; (2) conduct a morphological study in a given group, e.g., SEM of previously used diagnostic characters, or application of other methods if available; (3) ensure that ontogenetic information is taken into account during taxonomic assessment (see more details in [29]); (4) perform a relevant bibliographic study of original sources, rather than simply referencing a taxonomic data base; (5) conduct a molecular study of taxonomically checked specimens with appropriate genetic markers; (6) compare the results of morphological (step 2) and taxonomic studies (steps 3, 4) with the molecular (step 5) results; (7) in case inconsistencies are found between commonly used diagnostic characters in a given group and results of molecular phylogeny, respective features should be presented in the phyloperiodic framework, which will enable their detailed comparison and further search for fine-scale differences for each of the closely related taxa; (8) in case difficult to distinguish variants (patterns) are present among the same parallel rows of the phyloperiodic tables, a relevant presentation of diagnostic features should be conducted in order to reveal fine-scale differences between related taxa; (9) a complete study at a given time and using available research possibilities for a given group of taxa should result in fine-scale taxonomic diagnoses for all closely related species in a given taxa/complex (including new taxa); and (10) test the established framework by further investigation of a given group with new materials and data.…”

“…Recently, a number of significant contradictions in the use of the term "cryptic species" have been clearly shown [1][2][3][4][5][6][7][8][9]. Despite this, the term "cryptic species" is still widely used in biodiversity studies [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28].…”

Delineating Paralaoma annabelli, a minute land snail impacted by the 2019–2020 wildfires in Australia

“…Considering the ambiguity of currently studied taxonomic characters in Paralaoma and Australian punctids in general, we recommend an integrative approach towards solving their taxonomy (Dayrat, 2005;Padial et al, 2010). Future studies should statistically reassess the shell-based hypotheses of all punctid taxa and complement them with molecular phylogenetics as has been done here and for other taxa (e.g., Horsáková et al, 2020;Horsáková et al, 2022). A geographically comprehensive sampling for these punctid taxa, including at type localities, will also be required to clarify the variation of characters within and between taxa at the species and genus level.…”