Molecular and morphological data reveal cryptic taxonomic diversity in the terrestrial slug complex Arion subfuscus/fuscus (Mollusca, Pulmonata, Arionidae) in continental north-west Europe

The importance and abundance of cryptic species among invertebrate taxa is well documented. Nowadays, taxonomic, phylogenetic and conservation biological studies frequently use molecular markers to delineate cryptic taxa. Such studies, however, often face the problem of the differential resolution of the molecular markers and techniques involved. This issue is explored in the present study of cryptic taxa within the terrestrial slug complex Arion subfuscus/fuscus in continental north-west Europe. To this end, morphological, allozyme and mitochondrial 16S rDNA sequence data have been jointly evaluated. Using allozyme data and gonad type, two distinct groups were consistently delineated, even under sympatric conditions. The 16S rDNA data strongly supported both those groups and even suggested the presence of three distinct taxa within one of them. However, in view of: (1) the allopatric distribution of three OTUs, (2) the lack of allozyme or morphological differentiation, and (3) the extremely high degree of intraspecific mtDNA variation reported in pulmonate gastropods, they are, for the time being, not regarded as valid species under the biological species concept. By means of 16S rDNA and allozyme data, the position of type and topotype material of A. subfuscus s.s. and A. fuscus relative to the newly defined OTUs was determined, thus clarifying the nomenclature of this species complex. Additionally, gonad type proved to be a useful character for distinguishing the two species in north-west Europe.