Discus rotundatus

Discus (Gonyodiscus) rotundatus (O. F. Müller, 1774)
Spotted Disc

  • Helix rotundatus O. F. Müller 1774: 29.
  • Other synonyms in Europe.
Discus rotundatus; Queen Elizabeth Park, Vancouver, BC (RGF 10.187.3658), width 5.2 mm.

Identification. Shell subdiscoidal. Spire very low, quite flat but slightly domed. Whorls ca 5½–6, rather closely coiled, rather slowly enlarging. Periphery subangular, above middle of last whorl. Teleoconch with sharp, somewhat regular, slightly sinuous colabral riblets, weaker on base. Aperture subovate, wider than high, edentulous. Lip not thickened, simple, or scarcely thickened. Umbilicus ca 1/3 of shell width. Shell pale brown or grey-brown with red-brown spots in an alternating pattern. Shell small, width to ca 4.5 mm (wider than high).

Discus rotundatus, the Rotund Disc, McLarens beach, Saint John, new brunswick (45°14'15.2"n, 66°06'4.2"w); collected by RGF, 24 June 2015, NBM 009144.
Discus rotundatus; McLarens Beach, Saint John, New Brunswick (NBM 009144).

Among the species of Discus in Canada, D. rotundatus is recognized by the alternating pattern of reddish-brown spots and tighter coiling of the whorls. Anguispira alternata is a larger snail with fewer whorls at the same size.

Animal blue-black or grey, paler on sides of foot; ocular tentacles dark and almost cylindrical.

Habitat. In unkempt gardens, parks, waste ground, and other generally weedy, unkempt, disturbed places.

Global range. Southern Scandinavia and the British Isles, south through the Iberian Peninsula to Algeria; east to the Baltic countries, Byelorus, and Ukraine including Crimea (Sysoev & Schileyko 2009); Madeira (Seddon 2008) and the Azores (Backhuys 1975). Introduced to Istanbul, Turkey (Örstan 2003), South Africa (Herbert 2010). Introduced to several states in the USA, including Washington, Michigan, Ohio, Pennsylvania, New York, Vermont, and Maine (Forsyth et al. 2016).

Canadian range. Introduced. British Columbia (Vancouver), Ontario (Toronto), Quebec (Montreal), New Brunswick (Saint John), Nova Scotia (Halifax), and Newfoundland (several places on the Avalon Peninsula) (Örstan 2012; Forsyth & Williston 2012; Forsyth et al. 2016). Overall, still very much localized, not spreading far from where introduced.

Etymology. Latin, rotundatus, “rounded”.

Remarks. Some European authors (e.g. Falkner et al. 2002, Gargominy et al. 2011) recognize at least two subspecies—the nominal subspecies and the Iberian D. rotundatus omalisma (Fagot, 1879). Although subspecies are not used here, Canadian populations likely belong to the widespread, nominotypical subspecies, D. rotundatus rotundatus.

References

  • Backhuys W (1975) Zoogeography and taxonomy of the land and freshwater molluscs of the Azores. Backhuys & Meesters, Amsterdam, xii + 350 pp.
  • Falkner G, Ripken TEJ, Falkner M (2002) Mollusques continentaux de France. Liste de Référence annotée et Bibliographie. Patrimoines naturels 52: 1–350 pp.
  • Forsyth RG, Williston P (2012) Terrestrial snails from an urban park in Vancouver, British Columbia. The Festivus 44: 77–80.
  • Forsyth RG, Maunder JE, McAlpine DF, Noseworthy RG (2016) Distributional status of an introduced land snail Discus rotundatus (Rotund Disc, Mollusca: Discidae) in Canada. The Canadian Field-Naturalist 130: 235–246.
  • Gargominy O, Prie V, Bichain J-M, Cucherat X, Fontaine B (2011) Liste de référence annotée des mollusques continentaux de France. MalaCo 7: 307–382.
  • Herbert D (2010) The introduced terrestrial Mollusca of South Africa. SANBI biodiversity series 15. South African National Biodiversity Institute, Pretoria, vi + 108 pp.
  • Müller OF (1774) Vermium terrestrium et fluviatilium, seu animalium Infusorium, Helminthicorum, et Testaceorum, non marinorum, succincta historia. Volumen alterum: Testacea.Heineck & Faber, Havniæ & Lipsia, xxxvi + 214 + [x] pp.
  • Örstan A (2003) The first record of Discus rotundatus from Turkey. Triton 7: 27.
  • Örstan A (2012) The first record of the European land snail Discus rotundatus (Müller, 1774) from Montreal, Canada (Discidae: Pulmonata). Check List 8: 537–539.
  • Seddon MB (2008) The landsnails of Madeira. An illustrated compendium of the land snails and slugs of the Madeiran archipelago. Studies in Biodiversity and Systematics of Terrestrial Organisms from the National Museum of Wales, Biotir Reports 2: viii + 204 pp.
  • Sysoev A, Schileyko A (2009) Land snails and slugs of Russia and adjacent countries. Pensoft, Sofia/Moscow, 212 pp. + 142 pls.