The Spanish slug (Arion vulgaris, but formerly widely referred to as Arion lusitanicus owing to a misidentification) is an air-breathing land slug, a terrestrial pulmonate gastropod mollusk in the family Arionidae, the roundback slugs. Other vernacular names are Lusitanian slug, Iberian slug, and killer slug.
It is a large, conspicuous slug, which has spread across much of Europe since the 1950s and now reached North America. It may attain high densities and be a serious horticultural and agricultural pest, and is considered an invasive species. The life cycle is annual, with adults appearing in summer and dying off before winter.
The Spanish slug was identified as Arion lusitanicus when it was first reported as an invading species in France in 1956, and hence it is sometimes called the Lusitanian slug. This was a case of misidentification. In slugs, it is often impossible to find external characters that distinguish closely related species using external features, as colouration can be quite variable, and the rather plastic anatomy makes diagnostic anatomical features difficult to establish. The current consensus is that the true Arion lusitanicus is a species of the western part of the Iberian Peninsula. Examination of slugs from the Serra da Arrábida mountains in Portugal from where it was originally described by Jules François Mabille in 1868 showed that the true A. lusitanicus differed from the invader in its internal anatomy, the shape of the spermatophore and the number of chromosomes. Commission on Zoological Nomenclature.
The native distribution of Arion vulgaris is not exactly known. Genetic evidence (the higher incidence of rare alleles) suggests an origin in France or Spain, contrary to earlier genetic analyses that did not adequately sample these regions. In Britain the slug was first recorded in 1954, which is not an indication of it being native there. It is presumed that the specimen illustrated in Moquin-Tandon's original 1855 description was from France.
