MolluscIreland - land and freshwater
  • Arion (Arion) ater (Linnaeus 1758) Large black slug
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Arion (Arion) ater
© Dr Roy Anderson
Arion (Arion) ater
© Dr Roy Anderson
Arion (Arion) ater
© Dr Roy Anderson

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Arion (Arion) ater
© Dr Roy Anderson

A large roundback slug varying in colour through browns to black, with a similarly coloured, dull foot fringe and exhibiting a strong rocking response when stimulated. Ubiquitous up to moderate altitudes. A jet black form occurs on peatlands.

Key characteristics

  • A large roundback slug coloured predominantly olive-, yellow-, reddish- to dark-brown or black with sole of more or less the same shade or lighter
  • Foot fringe lineolated and coloured a similar shade to the back
  • Young are a pale yellow to orange colour, sometimes with faint lateral bands, darkening as they mature
  • Animal rocks strongly from side to side when stroked firmly on the back
  • Mucus colourless or pale yellow
  • Self-fertilises; copulation extremely rare


90-140 mm.

World Distribution

Range uncertain because of difficulties in separating from related large roundback slugs. Probably occurs only in the British Isles, southern and western Scandinavia, Denmark and Schleswig-Holstein. Distribution type: Oceanic Temperate (71).

GBIF distribution map [open in new tab]

Irish Distribution

Very widespread, particularly in relatively undisturbed places, also in peatlands and to moderately high altitude.


  • Found with decreasing frequency in urban areas due to competition with other species; rarely in gardens
  • General in farmland, in hedge bottoms etc., also woodlands and wetland margins
  • On all kinds of peatland to moderate altitudes
  • Mating occurs in early summer, probably July in our area, but has rarely been observed as the predominant method of reproduction in Ireland is selfing (self-fertilisation)


Several other superficially similar species occur i.e. A. rufus, A. flagellus and A. vulgaris. The first two can be separated in the field with reasonable confidence but Arion vulgaris presents greater problems.

Red List status

  • Least concern (lc).

Wikipedia link

Wikipedia page for Arion ater

 Anderson, R., (2016). Arion (Arion) ater (Linnaeus 1758). [In] MolluscIreland. Accessed on 2024-03-02.