Molecular systematics of the Carinarion complex (Mollusca: Gastropoda: Pulmonata): a taxonomic riddle caused by a mixed breeding system

The original description of the slugs Arion (Carinarion) fasciatus, Arion (Carinarion) silvaticus and Arion (Carinarion) circumscriptus was based on subtle differences in body pigmentation and genital anatomy. However, body pigmentation in these slugs may be influenced by their diet, whereas the genital differences between the species could not be confirmed by subsequent multivariate morphometric analyses. Hence, the status of the three nominal morphospecies remains controversial, with electrophoretic studies based on albumen gland proteins and allozymes also providing conflicting results. These studies suggested that Carinarion species are difficult to reconcile with the biological species concept because there is evidence of interspecific hybridization in places where these predominantly self-fertilizing slugs apparently outcross. Therefore, in the present study, the three Carinarion species are evaluated under a phylogenetic species concept, using nucleotide sequences of the nuclear ribosomal internal transcribed spacer 1 (ITS-1) and the mitochondrial 16S rDNA. ITS-1 showed no species specific variation. However, 16S rDNA yielded five haplotype groups. Three of these grouped haplotypes by species, whereas the two others joined haplotypes of different species and included all haplotypes that were shared by species (22% of all haplotypes). Hence, the three nominal Carinarion species appear to be inconsistent with a phylogenetic species concept. The present data also confirmed that North American Carinarion populations are genetically impoverished and may be not sufficiently representative with respect to the taxonomy of Carinarion. In conclusion, we currently regard Carinarion as a single species-level taxon, whose taxonomically deceiving, correlated phenotypic and genetic intraspecific variation is caused by sustained self-fertilization.