[3][4] Further study of these morphospecies (typological species) revealed that their body colour may be influenced by diet, and that the putative genital differences do not consistently agree with identifications based on pigmentation and allozymes.

[5][6] In much of their range these slugs predominantly self-fertilise, leading to distinct genetic strains and thus generating associations between coloration and other phenotypic characters.

[7] Moreover, sequencing of mitochondrial DNA revealed that the inferred phylogeny did not match an arrangement in which the three morphologically recognised species form distinct clades.

[13] Before the taxonomic work of Lohmander in 1937[14] and Waldén in 1955,[15] and even for some years following, the different species of Carinarion were not distinguished and the name Arion circumscriptus was used to cover all forms.

[16][17] Like with other Arion species, the pneumostome lies in the anterior half of the mantle, and the back is round in cross-section rather than with a keel.

[18][19] However, the subgenus Carinarion is distinguished by a row of paler tubercles along the midline of the back, giving the impression of a keel; this is less prominent in adults.

Few individuals were found above ground over the dry summer months, but otherwise immature animals of a wide range of sizes were present throughout the year.

[26] Arion circumscriptus reproduces predominantly by self-fertilization, leading to inbred genetic strains, but the presence of rare heterozygotes demonstrates that mating does occasionally result in outcrossing.

[28] The species is widespread and common across North-West, Central and Eastern Europe, including southern Scandinavia (naturally to 62° in Sweden, further north still in gardens;[30] also in Iceland).