Spanish Slug - Confusion Over Species

Confusion Over Species

The Spanish slug was identified as Arion lusitanicus when it was first reported as an invading species in France in 1956. This was most probably a case of misidentification. In slugs it is often difficult to establish good criteria for identifying species using external features or internal features, as colouration can be quite variable, and the rather plastic anatomy makes diagnostic anatomical features difficult to establish. Current consensus is that the true Arion lusitanicus is a microendemic species of the Serra da Arrábida mountains south of Lisbon, Portugal. A close analysis of slugs from the site in Portugal from where it was originally described by Jules François Mabille in 1868, showed that the true A. lusitanicus differed from the central European invader in its internal anatomy, the shape of the spermatophore and the number of chromosomes.

The invader was identified in 1997 as Arion vulgaris, which had been described by Moquin-Tandon in 1855 from an unrecorded site in France, presumably from western or southwestern France. The results of these studies were first published in a local journal in Spain. The news were not noticed outside Spain until they were mentioned in a widely dispersed atlas of British molluscs.

Still today we are confronted with the Central European 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 These reports are usually based on outdated information published in pre-1999 literature. Often this can be recognised by the name A. lusitanicus being used for the species, instead of A. vulgaris. Its common name "Spanish slug" was also based on the unsubstantiated assumption that the species would not only live in Portugal, but also in Spain. Portugal had less intensive economical trade connections with France. Actually Arion lusitanicus had never been reliably reported from Spain.