Unravelling morphological overlap of the rock-dwelling snails Pyramidula saxatilis (Hartmann, 1842) and P. pusilla (Vallot, 1801)

Abstract

ABSTRACT Pyramidula saxatilis and P. pusilla are two Central European rock-dwelling snail species that frequently co-occur and show substantial overlap in overall shell morphology. The two species can be separated from each other by differences in mitochondrial and nuclear DNA (mtDNA and nDNA) sequences. Recent studies have not shown consistent differences between these species in shell shape but have suggested possible species-specific features in shell microstructure. We investigated this issue by studying variation in the microsculpture of the upper shell surface (based on five shells per population from a total of nine populations for each species) and by analysing the geometric morphometrics of shell shape (based on 51 individuals and 12 populations of P. saxatilis and 54 individuals and 14 populations of P. pusilla), with species identifications based on mtDNA and nDNA markers. While geometric morphometrics and canonical variance analysis did reveal some statistical differences in overall shell shape, these differences were too subtle to be consistently detected by the eye. However, the density and development of growth ridges on the upper shell surface of the two species were found to be statistically different, with P. saxatilis having denser and very regular ridges. As a final verification of the utility of shell miscrosculpture, we were able to separate these two species even in two mixed populations with highly overlapping shell phenotypes, the shell-based identifications being confirmed by cytochrome c oxidase subunit I sequence data for all of the collected individuals. We therefore recommend using shell microsculpture to distinguish these two species.