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, Byelorussia, Ukraine, and 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) (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.