Arion obesoductus Reischütz, 1973 is a small Arion species that lives in undistrurbed forests. Its synonym includes Arion alpinus auct. nec Pollonera, 1887. It lives in Austria, Germany, Slovenia, Croatia, Italy, Switzerland and France. It is found in Alps and around Alps, but its distribution is more wide: also in Central Europe. The first records of this species from the Czech Republic were published in 2006 under the name Arion alpinus. It is not very common in the Czech Republic, but it was found in various areas there. It was neither found in Poland, nor Slovakia and Hungary.
It is often found on mushrooms and I have found it on mushrooms also. My record comes from the forest near the Pateřín village in the Central Moravia, Czech Republic. It is spruce forest (Picea abies) with beech (Fagus sylvatica). Arion obesoductus cooccur with slugs Arion fuscus, Malacolomax tenellus and Lehmannia marginata there. It is similar to Arion fuscus, but its foot is orange:
The orange foot of Arion obesodusctus (two specimen on the right) is easily recognizable even trough plastic translucent test-tube in comaprison of Arion fuscus (on the left). The length of the tube is 58 mm.
Dvořák L., Backeljau T., Reischütz P. L., Horsák M., Breugelmans K. & Jordaens K., 2006: Arion alpinus Pollonera, 1887 in the Czech Republic (Gastropoda: Arionidae). – Malacologica Bohemoslovaca, 5: 51–55. https://doi.org/10.5817/MaB2006-5-51
Horsák M., Juřičková L., Beran L., Čejka T. & Dvořák L. (2010). “Komentovaný seznam měkkýšů zjištěných ve volné přírodě České a Slovenské republiky. [Annotated list of mollusc species recorded outdoors in the Czech and Slovak Republics]. Malacologica Bohemoslovaca, Suppl. 1: 1-37. http://mollusca.sav.sk/pdf/9/Suppl-1-v2.pdf