Red slug

File:Arion ater & rufus.jpg (Arion ater) and Red slug.]]

Slugs in the genus Arion have the pneumostome (respiratory pore) in the front part of the mantle and a round back without a keel. Arion rufus is one of the largest species in the genus, up to 150 mm extended. As an adult it lacks the lateral bands found in many smaller congeners, but in some forms the sides are paler than the back. The body colour is often reddish, sometimes vividly so, but orange, yellowish, brown and greyish forms are also usual, and some individuals may be black. The foot fringe is vertically striped and is often a different colour than the back. The head and tentacles are often darker than the rest of the body. The mucus is sticky and colourless or orange. Externally the species is not reliably distinguishable from several other species, including A. ater and A. vulgaris, and can be difficult to distinguish from less closely related species such as A. flagellus, especially in the Iberian Peninsula where further similar species exist.{{cite book |last1=Wiese |first1=V. |title=Die Landschnecken Deutschlands |date=2014 |publisher=Quelle & Meyer |location=Wiebelsheim |isbn=9783494015514}}{{cite book |last1=Rowson |first1=B. |last2=Turner |first2=J. |last3=Anderson |first3=R. |last4=Symondson |first4=B. |title=Slugs of Britain and Ireland: identification, understanding and control |date=2014 |publisher=FSC |location=Telford |isbn=9781908819130}}

Eggs are typically about 5 mm in diameter, slightly ovoid. They are opaque white, because of a calcareous layer in the shell. Juveniles often have lateral bands.

Adult A. rufus and A. ater are straightforward to distinguish from A. vulgaris and A. flagellus by dissection; in the former pair the oviduct is thinner because the ligula is positioned in the upper atrium, not the oviduct. The anatomical differences between A. rufus and A. ater are more subtle: the latter tends to have a larger lower atrium, and its ligula differs (e.g. bowl-shaped rather than C-shaped).

Arion rufus will often rock spectacularly from side to side when irritated, which is useful to distinguish it from A. flagellus and generally from A. vulgaris, but not from A. ater.

Linnaeus described A. rufus together with Arion ater in 1758.{{cite book |last1=Linnæus |first1=C. |title=Systema naturæ per regna tria naturæ, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata. |date=1758 |publisher=Salvius |location=Holmiæ |page=652 |doi=10.5962/bhl.title.542 |url=https://www.biodiversitylibrary.org/bibliography/542 }} His description consisted mainly of references to earlier published descriptions from around Europe.{{cite journal |last1=Regteren Altena |first1=C.O. van |title=Notes sur les limaces. 8. Sur l'état de deux espèces nominales du genre Arion |journal=Basteria |date=1963 |volume=27 |pages=1–6}} Recently a lectotype for A. rufus has been designated from amongst the specimens to which Linnaeus indirectly referred.{{cite journal |last1=Reise |first1=H. |last2=Schwarzer |first2=A.-K. |last3=Hutchinson |first3=J.M.C. |last4=Schlitt |first4=B. |title=Genital morphology differentiates three subspecies of the terrestrial slug Arion ater (Linnaeus, 1758) s.l. and reveals a continuum of intermediates with the invasive A. vulgaris Moquin-Tandon, 1855 |journal=Folia Malacologica |date=2020 |volume=28 |issue=1 |pages=1–34 |doi=10.12657/folmal.028.001|doi-access=free }} This is a non-surviving specimen amongst those that Martin Lister described from Almondbury in West Yorkshire, England.{{cite book |last1=Lister |first1=M. |title=Appendicis ad historiæ animalium Angliæ, tres tractatus, &c. Altera editiio, auctior & emendatior |date=1685 |publisher=Smith |location=Londini}}

Arion rufus can be externally indistinguishable from Arion ater.{{cite journal |last1=Boettger |first1=C.R. |title=Zur Kenntnis der großen Wegschnecken (Arion s. str.) Deutschlands |journal=Archiv für Molluskenkunde |date=1949 |volume=78 |pages=169–186}} There are anatomical differences between the taxa in their genitalia, but they hybridise,{{cite journal |last1=Evans |first1=N.J. |title=An investigation of the status of the terrestrial slugs Arion ater ater (L.) and Arion ater rufus (L.) (Mollusca, Gastropoda, Pulmonata) in Britain |journal=Zoologica Scripta |date=1986 |volume=15 |issue=4 |pages=313–322 |doi=10.1111/j.1463-6409.1986.tb00232.x|s2cid=84998531 }}{{cite journal |last1=Hatteland |first1=B.A. |last2=Solhøy |first2=T. |last3=Schander |first3=C. |last4=Skage |first4=M. |last5=Proschwitz |first5=T. von |last6=Noble |first6=L.R. |title=Introgression and differentiation of the invasive slug Arion vulgaris from native A. ater |journal=Malacologia |date=2015 |volume=58 |issue=1–2 |pages=303–321 |doi=10.4002/040.058.0210|s2cid=84108976 }} and so they have often been considered conspecific, particularly by British authors.{{cite book |last1=Kerney |first1=M. |title=Atlas of the land and freshwater molluscs of Britain and Ireland |date=1999 |publisher=Harley Books |location=Great Horkesley}} The appropriate name is then Arion ater rufus (i.e. a subspecies of A. ater), following the decision of Fleming ("the first reviser", in 1822) to give A. ater precedence over A. rufus.{{cite book |last1=Fleming |first1=J. |editor1-last=Napier |editor1-first=M. |title=Supplement to the fourth, fifth and sixth editions of the Encyclopædia Britannica with preliminary dissertations on the history of sciences. Vol. 5 . |date=1822 |publisher=Constable |location=Edinburgh |pages=567–584 |chapter=Mollusca}}

Differences in mitochondrial DNA and minor anatomical differences exist between populations of A. rufus found in the British Isles and the commonest form found on the European continent.{{cite book |last1=Chevallier |first1=H. |title=Les grands Arion de France (Mollusca, Pulmonata). Taxonomie, biogeographie, écologie, polymorphisme, croissance et cycle biologique |date=1974 |publisher=Ph. D. Thesis, Université de Paris}}{{cite journal |last1=Rowson |first1=B. |last2=Anderson |first2=R. |last3=Turner |first3=J.A. |last4=Symondson |first4=W.O.C. |title=The slugs of Britain and Ireland: undetected and undescribed species increase a well-studied, economically important fauna by more than 20% |journal=PLOS ONE |date=2014 |volume=9 |issue=3 |page=e91907 |doi=10.1371/journal.pone.0091907|pmid=24740519 |pmc=3989179 |doi-access=free }} These have been given the status of species or subspecies. Because of the designation of the lectotype, the name rufus now refers to the British form, which may be called Arion rufus, Arion rufus rufus or Arion ater rufus depending on which taxa are considered as species or subspecies. Garsault (1764){{cite book |last1=Garsault |first1=F. A. P. de |title=Les figures des plantes et animaux d'usage en medecine, décrits dans la Matiere Medicale de Mr. Geoffroy medecin, dessinés d'après nature par Mr. de Garsault, gravés par Mrs. Defehrt, Prevost, Duflos, Martinet &c. Niquet scrip. Vol. 5 |date=1764 |location=Paris |doi=10.5962/bhl.title.49481 |url=https://www.biodiversitylibrary.org/item/104537 }} appears to have been the first to describe the Continental form under a name other than rufus, so its name should be Arion ruber, Arion rufus ruber or Arion ater ruber. Quite possibly there are further forms of similar status from elsewhere in France and Spain.

Arion rufus is widespread in western Europe, including France, the Low Countries, Germany, western Poland, Switzerland and the southern parts of the British Isles.{{cite book |last1=Welter-Schultes |first1=F.W. |title=European non-marine molluscs, a guide for species identification |date=2012 |publisher=Planet Poster Editions |location=Göttingen}} In Scandinavia and northern Britain the closely related species A. ater occurs in its place, but A. rufus has invaded parts of Scandinavia over the last 200 years.{{cite journal |last1=Proschwitz |first1=T. von |title=Röda skogssnigeln Arion rufus (L.), en mellaneuropeisk art stadd i snabb spridning med människan i Sverige |journal=Fauna och Flora |date=1985 |volume=80 |pages=121–138}} It is also extending its range further eastward in Europe,{{cite journal |last1=Soroka |first1=M. |last2=Kozłowski |first2=J. |last3=Wiktor |first3=A. |last4=Kałuski |first4=T. |title=Distribution and genetic diversity of the terrestrial slugs Arion lusitanicus Mabille, 1868 and Arion rufus (Linnaeus, 1758) in Poland based on mitochondrial DNA |journal=Folia Biologica (Kraków) |date=2009 |volume=57 |issue=1–2 |pages=71–81|doi=10.3409/fb57_1-2.71-81 |pmid=19459464 }} and even in eastern Germany it is possibly an old introduction. Its status in Spain is uncertain because of the presence there of other similar species.{{cite journal |last1=Castillejo |first1=J. |last2=Rodríguez-Castro |first2=J. |last3=Iglesias-Piñeiro |first3=J. |title=J., J., J.Estudio comparativo de la anatomía y caracterización del ADN de los ariónidos descritos por Torres Mínguez (1925) en Cantabria (España): Arion cendreroi y A. fulvipes, y la de A. rufus y A. vulgaris (Gastropoda Pulmonata: Arionidae) |journal=Spira |date=2019 |volume=7 |pages=49–69}} Arion rufus is also known as an introduction in North America.{{cite journal |last1=Zemanova |first1=M.A. |last2=Broennimann |first2=O. |last3=Guisan |first3=A. |last4=Knop |first4=E. |last5=Heckel |first5=G. |title=Slimy invasion: climatic niche and current and future biogeography of Arion slug invaders |journal=Diversity and Distributions |date=2018 |volume=2018 |issue=11 |pages=1627–1640 |doi=10.1111/ddi.12789|doi-access=free }} A related form, reddish in coloration but perhaps more closely related to A. ater s.s., has recently turned up in Turkey, on both sides of the Bosphorus.{{cite journal |last1=Reise |first1=H. |last2=Arslangündoğdu |first2=Z. |last3=Schlitt |first3=B. |last4=Hutchinson |first4=J.M.C. |last5=Hızal |first5=E. |last6=Bacack |first6=E. |title=H. Reise, ZFirst records of the terrestrial slug Arion ater s.l. (Linnaeus, 1758) (Pulmonata: Arionidae) from Turkey |journal=Folia Malacologica |date=2018 |volume=26 |pages=213–220|doi=10.12657/folmal.026.024 |doi-access=free }}

It is familiar from gardens and parks as well as disturbed agricultural landscapes, but also from a diversity of natural habitats including woodland, meadows, margins of water bodies, coastal habitat, and moorland. However, in much of its range A. rufus has declined dramatically over the last decades due to replacement by the externally similar Arion vulgaris.{{cite journal |last1=Kappes |first1=H. |last2=Kobialka |first2=H. |title=Die Nacktschneckengesellschaften in NW-Deutschland (Gastropoda: Milacidae, Boettgerillidae, Limacidae, Agriolimacidae, Arionidae): ein Ergebnis der NRW-Kartierung |journal=Natur und Heimat |date=2009 |volume=69 |pages=73–94}}{{cite journal |last1=Zemanova |first1=M.A. |last2=Knop |first2=E. |last3=Heckel |first3=G. |title=Introgressive replacement of natives by invading Arion pest slugs |journal=Scientific Reports |date=2017 |volume=7 |issue=41908 |page=14908 |doi=10.1038/s41598-017-14619-y|pmid=29097725 |pmc=5668256 |doi-access=free }}

The species is adult in summer, although the timings vary by a month or so between localities. Typical would be for some individuals to reach adult size in May, but to mature only in July, with some surviving until October. The eggs hatch over a broad period from late autumn to spring and studies find great variation in growth rate. Adults are on average over 300 times the mass of hatchlings. The life cycle is predominantly annual; but possibly a few late hatchlings overwinter as juveniles and mature only 18 months or so after hatching.{{cite journal |last1=Hutchinson |first1=J.M.C. |last2=Reise |first2=H. |last3=Skujienė |first3=G. |title=Life cycles and adult sizes of five co-occurring species of Arion slugs |journal=Journal of Molluscan Studies |date=2017 |volume=83 |issue=1 |pages=88–105 |doi=10.1093/mollus/eyw042|doi-access=free }}

File:Limax maximus hunting 2010.jpg (Limax maximus) hunting Red slug.]]

At rest A. rufus contracts into a hemispherical shape. When disturbed, it performs a rocking motion, sometimes for many minutes. Although predominantly nocturnal, rain brings it out during daylight. It is an omnivore.

Arion rufus is hermaphrodite and during its mating spermatophores are swapped reciprocally. Mating typically starts with one slug following the other, periodically nibbling its tail. When the leading slug doubles back, they may form a wheel configuration circling clockwise, with each nibbling the other. This soon evolves into the yin-yang configuration (or the wheel step may be skipped), with each animal hooked around the other, their genital pores pressed together, the configuration no longer rotating. The genitalia take some minutes to engage and evert, the epiphallus (spermatophore producing organ) connecting to the bursa trunk of the other, mutually.{{cite journal |last1=Allgaier |first1=C. |title=How can two soft bodied animals be precisely connected? A miniature quick-connect system in the slugs, Arion lusitanicus and Arion rufus|journal=Journal of Morphology |date=2015 |volume=276 |issue=6 |pages=631–648 |doi=10.1002/jmor.20361|pmid=25532726 |s2cid=29079897 }} This may take several attempts and sometimes is unsuccessful. Then the genital atria evert, and swell rapidly (1–2 min) to form a large, white, spherical mass between the bodies. The slugs remain like this for 90 minutes or longer, during which time a spermatophore is manufactured, filled with sperm, and in part passed over to the partner, hidden by the enveloping atria. Eventually one partner starts to become active, the configuration consequently rotates, the atria contract, and the genitalia separate. The spermatophores, anchored in the recipient's bursa trunk, are thereby pulled out of the donor's epiphallus. As the genitalia fully retract the spermatophore is taken in.{{cite journal |last1=Dreijers |first1=E. |last2=Reise |first2=H. |last3=Hutchinson |first3=J.M.C. |title=Mating of the slugs Arion lusitanicus auct. non Mabille and A. rufus (L.): different genitalia and mating behaviours are incomplete barriers to interspecific sperm exchange |journal=Journal of Molluscan Studies |date=2013 |volume=79 |pages=51–63 |doi=10.1093/mollus/eys033|doi-access=free }}

Arion rufus 1650.JPG|Dark individual

Red slug (Arion rufus).JPG|Light individual

Rode wegslak (Arion rufus), 07-09-2024. (actm.).jpg|Copulation

Red Slug copulation.jpg|Mating

{{Reflist}}

{{Commons category|Arion rufus}}

• [http://eol.org/pages/403118/media Arion rufus] at Encyclopedia of Life (3 pages)
• [http://www.animalbase.uni-goettingen.de/zooweb/servlet/AnimalBase/home/species?id=1812 Arion rufus] at Animalbase taxonomy, short description, distribution, biology, status (threats), images

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Category:Arion (gastropod)

Category:Gastropods described in 1758

Category:Gastropods of Europe

Category:Taxa named by Carl Linnaeus