Spanish slug

The Spanish slug, also known by its scientific name Arion vulgaris, more commonly known in English-speaking countries under the incorrectly applied Latin name "Arion lusitanicus", is a species of air-breathing land slug, a terrestrial pulmonate gastropod mollusk in the family Arionidae, the roundback slugs.
They can lay up to 400 eggs in a single summer. These eggs often hatch within just 3.5–5 weeks.
This is a highly invasive slug species. It is often considered a pest, not only in areas where it has been accidentally introduced, but even in places where it is indigenous.
Taxonomy.
The validation of the specific name "Arion vulgaris" as available was proposed in 2018.
Distribution.
The native distribution of "Arion vulgaris" is not exactly known. Some sources have claimed it originates in northern Spain, western France and southern England, but provided no published references for these assumptions. In Britain the slug was first recorded in 1954, which does not indicate native distribution there. Reliable records from Spain have not been found although the Spanish term, "babosa" is found in a 1422 text. Moquin-Tandon described the species originally from France without exact indication of the locality, presumably western or southwestern France.
Non-indigenous distribution.
The non-indigenous distribution of "Arion vulgaris" includes the whole of western, central and northern Europe. Its distribution includes also various parts of Southern Europe and of Eastern Europe.
"Arion vulgaris" is also known from the USA since 1998.
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. 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 80–120 mm. The colour of the Spanish slug is brownish or reddish brown, or brightly orange. The tentacles are darker. Colouration varies, but not within the same population. Adults differ little from "Arion rufus", in contrast to the brightly coloured and banded juveniles that allow reliable determination of this species.
Reproductive system: The atrium is small. The oviduct is large and swollen, with the same diameter as atrium. The spermatheca is spherical, its diameter twice of oviduct. The long, anteriorly swollen oviductus with large longitudinal ligula inside is distinctive of "Arion vulgaris".
When disturbed, the slugs will emit copious amounts of mucus. When killed in boiling water (one of several methods used in the fight against the species as an invasive organism), the mucus will colour the water yellow.
Ecology.
"Arion vulgaris" inhabits cultivated habitats of any kind, as well as natural habitats such as river and lake margins, margins of forests, forests in valleys or moderately dry meadows. In Switzerland it has been found up to 1700 m altitude.
It is a serious horticultural pest in large parts of Europe. It is being dispersed mainly by soil used for horticultural purposes.
Hedgehogs and some birds, including many breeds of domestic ducks, feed on slugs, often voraciously enough to control their numbers effectively. Some amphibians and reptiles do so too. Large species of ground beetles and species of glowworm in the family Lampyridae feed on slugs, snails and their eggs. The Spanish slug does not appear to be subject to such constraints, and locally appears in extremely dense populations.
As an invasive species.
"Arion vulgaris" is considered among the 100 worst alien species in Europe in DAISIE European Invasive Alien Species Gateway, and this is the only land gastropod among them. "Arion vulgaris" is the worst slug pest in Europe and it has an important economical, ecological, health and social impact.
The local name of the slug in the regions it has invaded is typically a translation of "Spanish slug". In recent years, as its dominance has increased, it has been nicknamed "killer slug", 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. The German name "Spanische Wegschnecke" for the species translates "Spanish road slug", which could be misunderstood to indicate spread along roadside vegetation. "Wegschnecke" is the German artificial name invented for the genus "Arion", so all members of the genus carry this name.
The main reason behind problematic invasions of gardens by the Spanish slug is that it has adapted to a dry climate, where most eggs will dry out before hatching. The slug lays hundreds of eggs so that at least some may hatch. In the less dry regions of Northern Europe and Britain, the constraints of drought do not limit reproduction to the same degree. The species is also gregarious, allowing for mass occurrences. Like all pulmonate snails and slugs, it is a hermaphrodite and has the capacity to self-fertilize, meaning that one single slug can start an infestation. Poor control of exported produce has also been assumed to be a cause, as the main route for the slugs to spread was long time thought to have been through careless handling of imported produce from Spain and Portugal.
In recent years, it has been observed that the Spanish slug has bred with the indigenous black slug "Arion ater" to produce a more frost-resistant variety in the more northerly regions. Danish authorities maintain that the Spanish slug, black slug and red slug are in fact subspecies of the same 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 Alfred 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.
References.
This articles incorporates public domain text from the reference.