Spanish slug

The Spanish slug (''Arion vulgaris'', but also widely known 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. The life cycle is annual, with adults appearing in summer and dying off before winter.

Confusion over nomenclature

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 (e.g.). 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.

The misidentification was first recognised in 1997, and more widely publicised in an atlas of British molluscs. ''Arion vulgaris'' was proposed as a substitute name based on a drawing of the genitalia in an 1855 work by Alfred Moquin-Tandon. However, it is debatable whether the name applies to this drawing, so one temporary solution was to use the name ''Arion lusitanicus'' auct. non Mabille (i.e. "as used by authors other than Mabille"). Nevertheless, ''A. vulgaris'' has increasingly been used since, and this is the proposal that has been formally submitted to the International Commission on Zoological Nomenclature.

Still today, we are confronted with the invader being reported as having originally been introduced with vegetables from Spain. These reports are usually based on outdated information published in pre-1999 literature. 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. ''Arion vulgaris'' seems to be rare in Spain. Another name sometimes applied is the "Iberian slug".

Distribution

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,Kerney M. (1999). ''Atlas of the land and freshwater molluscs of Britain and Ireland''. pp. 1-264. Colchester. (Harley). 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.

Non-indigenous distribution

The non-indigenous distribution of ''Arion vulgaris'' includes almost the whole of Western and Central Europe and extends to various parts of Southern, Northern and Eastern Europe. It is now also recorded from Canada (2009) and Mexico. A 2017 report from the Asian part of Turkey was not based on dissection and is liable to be ''A. ater'' s.l. Reports of "''A. lusitanicus''" from Madeira may be copied from an 1895 report, in which case they need confirmation; similarly, a 1975 report from the Azores could be of the true ''A. lusitanicus''. Claims of its presence in Algeria also appear unsupported.

Chronological overview of expansion of ''Arion vulgaris'' in Europe:
* Great Britain – since 1954, later spread to Lincolnshire and Norfolk in 2012
* France – (expansion from native range) since 1955
* Switzerland – since at least 1956
* Italy – since 1965
* Bulgaria – since 1966
* Germany – since 1969

* Austria – since 1971
* Belgium – since 1973
* Sweden – since 1975
* Slovenia – since 1982–83
* Croatia – since 1982–83
* Ireland – since 1984
* Hungary – since 1985
* Poland – likely since 1987, certainly by 1993
* Norway – since 1988
* Netherlands – since 1989
* Spain – (possibly native; only known from north-east) since 1990
* Finland – since 1990; Valovirta I. (2001)
''Tehokkaan leviämisen mestari''
. accessed 27 September 2009. abundant in south
* Czech Republic – since 1991
* Denmark – since 1991
* Slovakia – since 1992
* Faroe Islands – since 1996
* Serbia – since 2002
* Montenegro – since 2002
* Iceland – since 2003
* Macedonia – since at least 2003
* Ukraine – since 2007, subsequently spread across almost whole country
* Lithuania – since 2008
* Estonia – since 2008
* Latvia – since 2009
* Russia – since 2009 (greenhouses), since 2019 (outdoors) (2018 record not confirmed anatomically)
* Romania – since 2012Păpureanu A.-M., Reise H. & Varga A. (2014). "First records of the invasive slug ''Arion lusitanicus'' auct. non Mabille (Gastropoda: Pulmonata: Arionidae) in Romania". ''Malacologica Bohemoslovaca'' 13: 6-11
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* Belarus – since 2020 (2018 record not confirmed anatomically)

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. Therefore, it has been suggested that this species be given top national quarantine significance in the USA.

British authorities were also concerned as of 2014 that it may become a major pest.

Description

The size of the adult slug is 60–140 mm. The colour ranges from yellow to black, but is most commonly brick-red, dirty orange, or brown. The tentacles are darker. Adult ''A. vulgaris'' may not differ in external appearance from ''Arion rufus

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