{{Short description|Species of gastropod}} {{Speciesbox | image = Arion subfuscus-Nl2.jpg | image_caption = An active individual from the Netherlands | image2 = Arion subfuscus.jpg | image2_caption = A contracted individual of ''Arion fuscus'' from the Czech Republic | status = LC | status_system = IUCN3.1 | status_ref = <ref name=IUCN>{{cite iucn |last1=Rowson |first1=B. |year=2017 |title=''Arion fuscus'' |volume=2017 |article-number=e.T171173A1322366 |doi=10.2305/IUCN.UK.2017-3.RLTS.T171173A1322366.en |access-date=4 May 2026}}</ref> | taxon = Arion fuscus | authority = (O.F. Müller, 1774) | synonyms = * ''Arion (Mesarion) fuscus'' <small>(O. F. Müller, 1774)</small> alternative representation * ''Limax fuscus'' <small>O. F. Müller, 1774</small> }}
'''''Arion fuscus''''', commonly known as the '''dusky arion''', is a species of small air-breathing land slug, a terrestrial pulmonate gastropod mollusk in the family Arionidae, the roundback slugs.<ref name="WoRMS">{{WRMS species|1049800|''Arion fuscus'' (O. F. Müller, 1774)|28 October 2025}}</ref>
''Arion subfuscus'' (Draparnaud, 1805) is considered by some authors to be a partial synonym for ''Arion fuscus''. ''Arion subfuscus'' (Draparnaud, 1805) and ''A. fuscus'' (Müller, 1774) have very similar morphology and have overlapping distributional ranges in northwestern Europe. ''Arion fuscus'' is widespread throughout Central, North and East Europe whereas ''A. subfuscus'' is restricted to West Europe. (For practical purposes the two are synonyms in Germany and in the Czech Republic.) The two taxa are referred to as the species complex ''Arion subfuscus''/''fuscus'' both members having several evolutionary lineages. They are only separable by alloenzyme analysis and gonad type.{{citation needed|date=May 2026}}
==Description== A 50–70 mm roundback slug. It is brown in colour, ranging from black-brown through olive-brown to orange and bright reddish-orange. Darker lateral bands are present. Dark pigment reaches under the pneumostome from back to front. The foot fringe is pale with thin lines. The sole is uniformly pale/translucent.The tentacles are pale or dark. It does not contract into a bell shape when stimulated. The dusky arion differs from other ''Arion'' species in its yellow-orange body mucus. Genitalia: Atrium about half diameter of spermatheca, spermatheca spherical, oviduct large and swollen like in ''Arion lusitanicus''.<ref>[http://www.animalbase.uni-goettingen.de/zooweb/servlet/AnimalBase/home/species?id=318 Animalbase (Welter-Schultes)]</ref>
==Distribution== This species occurs in European and Asian countries and islands including:
* Austria<ref>http://ipp.boku.ac.at/private/wf/Check_List_of_Austrian_Mollusca.html {{Webarchive|url=https://web.archive.org/web/20070504134236/http://ipp.boku.ac.at/private/wf/Check_List_of_Austrian_Mollusca.html |date=2007-05-04 }} Letzte Änderung am 24.10.2005. access 2 March 2007</ref> * Bulgaria * Czech Republic - least concern (LC)<ref>Juřičková L., Horsák M. & Beran L., 2001: Check-list of the molluscs (Mollusca) of the Czech Republic. Acta Soc. Zool. Bohem., 65: 25-40.</ref><ref>[https://mollusca.sav.sk/malacology/checklist.htm Check-list of the molluscs (Mollusca) of the Czech Republic<!-- Bot generated title -->]</ref> * Germany<ref>[https://www.mollbase.de/list/deunam.htm Deutsche Namen für einheimische Schnecken und Muscheln<!-- Bot generated title -->]</ref><ref>[https://www.mollbase.de/sh/arionidae/arion_subfuscus_atl.htm MollBase – Arion subfuscus – Braune Wegschnecke – Atlas 1991<!-- Bot generated title -->]</ref> * Netherlands<ref>{{in lang|nl}} [https://www.anemoon.org/anm/voorlopige-kaarten/kaarten-per-soort/landmollusken/wetenschappelijk/arion-subfuscus Arion subfuscus — Anemoon<!-- Bot generated title -->]</ref> * Poland * Slovakia * Great Britain * Ireland * China<ref name="Wiktor 2000">Wiktor A., De-niu C. & Wu M. (2000). "Stylommatophoran slugs of China (Gastropoda: Pulmonata) – prodromus". ''Folia Malacologica'' 8(1): 3-35.</ref>
In eastern North America, it is the most widely introduced non-native slug, with a range from Southern Canada to South Carolina, and west to Wisconsin.<ref name="carnegie">{{cite web |title=Arion subfuscus |url=https://carnegiemnh.org/mollusks/arion-subfuscus/ |website=carnegiemnh.org}}</ref>
==Ecology== This slug is crepuscular and most active at dusk.{{citation needed|date=May 2026}}
===Habitat=== This slug lives on the ground, under fallen logs, rocks and under bark in woodland areas, but it can also take advantage of hedges, gardens, pastures, open fields, drier jack pine stands<ref name="thesis">{{cite web |last1=Hall |first1=Olivia |title=THE STATUS OF INVASIVE DUSKY SLUG (ARION) SPECIES IN THE UPPER GREAT LAKES: A MOLECULAR APPROACH |url=https://commons.nmu.edu/theses/751 |website=All NMU Master's Theses |date=1 April 2023}}</ref>, yards, wetlands, forests and dunes. It is not as arboreal as the larger philomycid slugs.<ref name="carnegie"/> It's habitat varies by season, in fall it forages in moist plant matter, in winter it burrows into the soil, in Spring it returns to the surface, In summer it seeks shelter to avoid desiccation.<ref name="animaldiversity">{{cite web |title=Arion subfuscus {{!}} INFORMATION {{!}} Animal Diversity Web |url=https://animaldiversity.org/accounts/Arion_subfuscus/ |website=animaldiversity.org}}</ref>
===Diet=== This slug is omnivorous and detritivorous. It will eat animal excrement, dead insects, fungi, yellowing foliage, dying plants, dead or dying worms, and fruit.<ref name="animaldiversity"/><ref name="carnegie"/>
===Reproduction and life cycle=== Like most land gastropods, ''Arion fuscus'' is hermaphroditic and can both self fertilize and reproduce sexually depending on conditions. They generally live 8-12 months. It takes 4 weeks after fertilization to lay eggs, and they normally lay clusters of dozens of eggs at a time. ''Arion fuscus''/''subfuscus'' are semelparous and most adult arion subfuscus die shortly after reproduction but this can vary based on conditions. Generally eggs hatch sometime between autumn and winter, and adults die in summer. It is one of the species which may engage in apophallation when penises become stuck together.<ref name="animaldiversity"/>
===As an invasive species=== This species is the most widely introduced non-native slug in eastern North America, significantly outnumbering native species. Their flexible habitat requirements allow them to colonize more successfully than other slugs, and they may be able to hatch in autumn and become active in spring before other slugs who hatch in Spring. It may be a threat to the species calypso bulbosa and other early emerging native plant species in the Great Lakes region of the United States, and is the only slug active when it emerges. Further studies are necessary to confirm details.<ref name="thesis"/>
==References== {{Reflist}}
==Further reading== * Pinceel J., Joardens K., Van Houtte N., De Winter A.J. & Backeljau T. 2004: Molecular and morphological data reveal cryptic taxonomic diversity in the terrestrial slug complex Arion subfuscus/fuscus (Mollusca, Pulmonata, Arionidae) in continental north-west Europe. Biological Journal of the Linnean Society 83: 23–38. {{doi|10.1111/j.1095-8312.2004.00368.x}} * Pinceel J., Joardens K., Pfenninger M. & Backeljau T. 2005: Rangewide phylogeography of a terrestrial slug in Europe: evidence for Alpine refugia and rapid colonization after the Pleistocene glaciations. Molecular Ecology 14: 1133–1150. {{doi|10.1111/j.1365-294X.2005.02479.x}} * {{doi|10.1007/s10709-005-5816-3}} * "Activity and Ecological Distribution of the Slug, ''Arion subfuscus'' (Draparnaud) (Stylommatophora, Arionidae)". {{JSTOR|2424835}} * "Relations between forest management and slug assemblages (Gastropoda) of deciduous regrowth forests". {{doi|10.1016/j.foreco.2006.09.067}}
==External links== *[http://www.animalbase.uni-goettingen.de/zooweb/servlet/AnimalBase/home/species?id=318 ''Arion subfuscus''] at Animalbase taxonomy, short description, distribution, biology, status (threats), images *[https://eol.org/pages/403119/media ''Arion subfuscus''] at Encyclopedia of Life {{Commons category-inline|Arion subfuscus}}
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Category:Arion (gastropod) Category:Gastropods of Europe Category:Gastropods of Asia Category:Fauna of England Category:Gastropods described in 1774 Category:Taxa named by Otto Friedrich Müller
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