According to Mayr , the initial step of any taxonomic work is to "sort that portion of the diversity of the individuals which is encountered into easily recognisable and internally homogeneous groups, and to find constant differences between such groups". In other words, there must be constant, apparent differences between biological entities in order to separate them into natural groups. This is true, no matter whether the taxonomic diagnosis is based on morphological, anatomical, molecular or other traits. In a second step, the so identified groups can be assigned to biological species, either already known to science or not, based on the degree of reproductive isolation to other such groups . Characters that are found to differ constantly among delimited biological species can then be used to re-identify them .
Traditionally, morphological traits were used for taxonomy. Recently, however, DNA-taxonomy [3, 4] has entered the field, often contradicting traditional views . Therefore, studies comparing traditional taxonomies with DNA-based results are needed , because species delimitation and eventual recognition is not only of interest for taxonomists and systematics. Species are the fundamental units in biogeography, ecology, macroevolution, biomonitoring and conservation biology [7–9]. An objective, rigorous taxonomic delimitation of species according to explicit criteria is therefore a necessary prerequisite for many studies in these disciplines. In this contribution, we compared the suitability and efficacy of shell morphology and sequence variation of a mitochondrial gene for taxonomic purposes in a freshwater snail taxon.
The genus Radix Montfort 1810, formerly included in Lymnaea, is part of the Lymnaeidae family (Basommatophora). It has a Palaearctic distribution, but the results of Remigio indicate a paraphyletic status of the Eurasian taxa on the one side and the mostly East Asian species on the other . The taxonomy and species determination is deemed difficult. Currently, five species, Radix ampla, R. auricularia, R. balthica, R. labiata and R. lagotis are recognised in North-Western Europe . The most recent taxonomic treatment, summarising previous work, states that species determination based on shell morphology is difficult, unreliable and should be supplemented by anatomical inspections. The latter are, however, also considered unreliable. Indeed, the indicated intraspecific variability of the putatively distinctive anatomical measurements largely overlaps among species  and therefore seems to be unsuitable for taxonomic distinction. The issue is further complicated by recent nomenclatorial revisions. The names R. peregra and R. ovata have lost their validity in favour of R. labiata and R. balthica, respectively , but are still used by some researchers (e.g.).
Species identification in Radix is not only of academic interest. The genus is e.g. involved in the transmission of parasitic diseases to humans [14, 15]. The study of these diseases is possibly impaired if the specific identity of the snail hosts implicated in larvae transmission cannot be unequivocally determined. Additionally, the presence or absence of certain Radix species is used to calculate an indicator of water quality in official assessments , which also requires their consistent and correct recognition. This highlights the need for reliable species identifications in this genus.
We compared the suitability of shell morphology and DNA-taxonomy to delimit Radix species by focussing on the following issues:
How many evolutionary lineages of Radix exist in North-Western Europe and do they correspond to biological species?
Does the shell variation of North-Western European Radix fall into separate, distinguishable units that correspond to the species descriptions in the taxonomic literature?
Is shell variation within and among Radix lineages species specific or influenced by the environment?