Arion distinctus

Arion distinctus is species of air-breathing land slug in the family Arionidae, sometimes known as the roundback slugs. It is a terrestrial pulmonate gastropod mollusc. Several vernacular names exist, but it is unclear if they are much in use: brown soil slug, common garden slug, darkface arion, Mabille's orange-soled slug, April slug. Arion hortensis was described by Férussac in 1819. Only in the 1970s did the amateur malacologist Stella Davies discover that in Britain that name had been applied to three distinct species. Later the name Arion distinctus was used for the one of these species that Davies had provisionally called 'form A'. Mabille's original 1868 description of A. distinctus included details (coloration, date of collection) that tended to indicate this species rather than Arion hortensis s.s. No type survived, so a neotype has been designated, collected from the same rough locality (near Sèvres, Paris). As in other members of the genus Arion, the pneumostome is in the anterior half of the mantle. Arion distinctus never gets bigger than about 4 cm extended. In contrast with members of the subgenus Carinarion (e.g. Arion circumscriptus), there is no dorsal line of pale tubercles suggesting a keel and the shape of the body in cross-section is a segment of a circle rather than a bell shape. Arion distinctus lacks the prickly tubercles of A. intermedius, and is larger and darker with prominent lateral bands. Arion subfuscus and A. fuscus can look similar to A. distinctus when preserved, but these species in life have bright orange mucus on the body and a pale sole, whereas in A. distinctus the sole appears yellow or orange from the sole mucus, but the body mucus is not coloured. However, reliable discrimination from other members of the subgenus Kobeltia is not always straightforward on external characters. In much of Western Europe the Kobeltia species most likely to be confused is A. hortensis; mixed populations often occur. Authorities differ in their advice on how reliably the two species can be separated using external characters when alive. Useful indications are that A. distinctus has dark tentacles without the red or violet tinge typical of A. hortensis, the general appearance of its back is yellower or browner than in A. hortensis, the row of turbercles directly above the sole is not as white as is typical of A. hortensis, and (least reliably) the lateral bands tend to be lower, running through the pneumostome rather than above. If dissection reveals a two-partite oviduct, one can be sure of A. distinctus, but in most regions a sizeable proportion of individuals have a three-partite oviduct like that of A. hortensis. The definitive character, distinguishing A. distinctus from all other Kobeltia species, is a conical structure inside the atrium that covers the entrance to the epiphallus, but it is not developed in juveniles. The structure is involved in receiving the partner's spermatophore. Arion distinctus occurs in a variety of moist habitats, including gardens, waste ground and woodland, but may be absent in harsh upland habitats. In Switzerland it occurs up to 2000 m. It is typically found amongst ground litter or sheltering under wood, stones and soil clods. Studies in agriculture and horticulture have often not distinguished A. distinctus and A. hortensis, but both species are considered to be economically significant pests.