Arion occultus is a species

Species

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 of small air-breathing land slug

Slug

Slug is a common name that is normally applied to any gastropod mollusc that lacks a shell, has a very reduced shell, or has a small internal shell...

, a terrestrial

Terrestrial animal

Terrestrial animals are animals that live predominantly or entirely on land , as compared with aquatic animals, which live predominantly or entirely in the water , or amphibians, which rely on a combination of aquatic and terrestrial habitats...

 pulmonate gastropod mollusk in the family

Family (biology)

In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...

 Arionidae

Arionidae

Arionidae, common name the "roundback slugs" or "round back slugs" are a taxonomic family of air-breathing land slugs, terrestrial pulmonate gastropod mollusks in the superfamily Arionoidea.- Distribution :...

, the roundback slugs.

This recently-recognized species is part of the "Arion hortensis group" of species. It somewhat resembles Arion distinctus

Arion distinctus

Arion distinctus, common name the darkface arion, is species of air-breathing land slug in the family Arionidae, sometimes known as the roundback slugs...

in general appearance, and also resembles Arion alpinus

Arion alpinus

Arion alpinus Pollonera, 1887* Arion alpinus Pollonera, 1887, a synonym of Arion intermedius Normand, 1852* Arion alpinus auct. non Pollonera, 1887, a newly reinstated name by Manganelli et al. is Arion obesoductus Reischütz, 1973...

, but the internal anatomy of the reproductive system is different.

Distribution

This species occurs in:

• Ireland; appears to be introduced in Ireland, but its country of origin is not known.