The current consensus is that the true Arion lusitanicus is a species of the western part of the Iberian Peninsula.

[5][6] 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.

[9] Arion vulgaris was proposed as a substitute name[10] based on a drawing of the genitalia in an 1855 work by Alfred Moquin-Tandon.

The common name "Spanish slug" was further based on the unsubstantiated assumption that the species would not only live in Portugal, but also in Spain.

[23] Chronological overview of expansion of Arion vulgaris in Europe: This species has not yet become established in the USA, but it is considered to represent a potentially serious threat as a pest, an invasive species which could negatively affect agriculture, natural ecosystems, human health or commerce.

Adult A. vulgaris may not differ in external appearance from Arion rufus, and so reliable identification requires dissection to examine the genitalia.

[57][58] Whilst a slug can crawl several metres within a night,[58] long-distance dispersal is believed to be on vegetables, on horticultural seedlings, and on plant debris disposed of as waste.

[57] The species has an annual life cycle with mating starting in July and eggs first laid some weeks later in late summer.

[15] Arion vulgaris is the worst slug pest in Europe[15] and it has an important economic, ecological, and social impact.

In recent years, as its dominance has increased, it has been nicknamed "killer slug",[60] perhaps due to its tendency to eat dead or weaker individuals of the species, although its destructive impact on gardens may seem just as appropriate a reason for the name.

Long-distance transport of produce and garden plants has been assumed to be a common means of its rapid dispersal.

Besides causing economic damage, the arrival of A. vulgaris has often been associated with the disappearance within a few years of the similarly sized congener Arion ater s.l., at least in synanthropic habitats.

[65] However, genetic investigations have not shown that introgression of A. ater genes into A. vulgaris persist for long once the native species has disappeared.

[64] In the Swiss Alps, Arion ater rufus persists only at high elevations, and hybrids with A. vulgaris occur in a contact zone along the altitudinal gradient.

[63] Given the densities that A. vulgaris can attain, other ecological effects of its invasion on the native flora and fauna are to be expected.