- Helix rotundatus O.F. Müller 1774: 29.
- Other synonyms in the European literature.
Identification. Shell subdiscoidal. Spire very low, slightly domed. Whorls c. 5½–6, rather closely coiled, rather slowly enlarging. Periphery shouldered, subangular, above middle of last whorl. Protoconch c. 1¼ whorls, smooth. Teleoconch with sharp, somewhat regular, slightly sinuous colabral riblets, weaker on base. Aperture transversely subovate, wider than high, edentulous. Lip not or scarcely thickened, simple, oblique and slightly sinuous from side. Umbilicus c. 1/3 of shell width, clearly showing all whorls within. Shell pale brown, grey-brown, or darker brown, with red-brown spots (actually colabral bands) in an alternating pattern (or rarely pale and without spots). Shell small, width to c. 4.5 mm (wider than high).
The animal is blue-black or grey, but paler on lower sides of foot, which is rounded in front and pointed at the back. The ocular tentacles are dark and almost cylindrical.
Comparison. Gonyodiscus rotundatus is readily recognized from all other land-snail species in BC by the alternating pattern of reddish-brown spots. However, care must be taken because shells rarely may be entirely unspotted and pale (Taylor 1914 in 1906–1914; iNaturalist). The whorls are more tightly coiled than either Discus species.
Habitat. In BC, this species is known to occur in unkempt gardens, parks, waste ground, and other generally weedy, unkempt, disturbed places.
Biology. Most reproduction takes place in July and August (Kuźnik-Kowalska 1999). Eggs are fully calcified, pearly-white, and 0.6–1.0 mm in diameter and 1–11 eggs are laid per clutch. German (1930) reported that eggs are laid in May through to September, hatching in 10–12 days; Kuźnik-Kowalska (1999) indicated that incubation was slightly longer, 14–36 days. This species is known to carry some eggs inside the umbilical cavity of the shell (Kuźnik-Kowalska & Pokryszko 2007). Following hatching, growth is apparently quick add a shell whorl each 33–35 days, until reaching maturity; snails reach maturity at 5½ whorls in their second or third year (or one year according to Germain 1930), and live for 2½–3½ years (Kuźnik-Kowalska 1999).
Geographic range. Introduced to BC, where it is known from several places in Vancouver (Forsyth et al. 2016) and Burnaby (iNaturalist). Elsewhere in Canada, introduced to Ontario (Toronto), Quebec (Montreal), New Brunswick (Saint John), Nova Scotia (Halifax), and Newfoundland (several places on the Avalon Peninsula) (Örstan 2012; Forsyth et al. 2016). Overall, this species is still very much localized in BC and Canada, and not widespread, as are many non-native land molluscs. In the USA, introduced to several states, including Washington, California, Michigan, Ohio, Pennsylvania, New York, Vermont, and Maine (Forsyth et al. 2016).
Southern Scandinavia and the British Isles, south through the Iberian Peninsula to Algeria (Welter-Schultes 2012); east to the Baltic countries, Belarus, Romania, Ukraine including Crimea (Sysoev and Schileyko 2009), south to Greece (Georgopoulou et al. 2025); Madeira (Seddon 2008), and the Azores (Backhuys 1975). Introduced elsewhere, including to Istanbul, Turkey (Örstan 2003) and South Africa (Herbert 2010).
Etymology. Gonyodiscus: gonia (Greek), angle + discus (Latin), a disc; the gender is masculine. Rotundatus (Latin), rounded.
Remarks. Until recently, this species was thought to belong to the genus Discus, but in their molecular analysis Salvador et al. (2023) found that it is not so closely related to Discus and proposed that Gonyodiscus be considered a full genus, not subgenus of Discus. Earlier, Umiński (1962) had noticed anatomical differences in the genitalia, with Discus having the penis retractor muscle attaching at the terminal end of the penis and the vas deferens with a lateral attachment on the penis; in Gonyodiscus, the penis retractor is attached laterally to the penis and the vas deferens attached terminally.
Some European authors (e.g. Falkner et al. 2002; Gargominy et al. 2011) have recognized at least two subspecies—the nominal subspecies and the Iberian G. rotundatus omalisma (Fagot, 1879), which has an even more flattened shell with a broader umbilicus (e.g. Gargominy & Ripken 2011). Although subspecies are not used here, BC snails seem to belong to the widespread, nominate subspecies, G. rotundatus rotundatus.
References
- 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. https://doi.org/10.22621/cfn.v130i3.1887
- Gargominy O, Ripken TEJ (2011) Une collection de référence pour la malacofaune terrestre de France. MalaCo Hors Série 1: 1–108.
- Germain L (1930) Mollusques terrestres et fluviatiles (première partie). Faune de France 21: 1-478.
- Georgopoulou E, Oraiopoulos N, Psirofonia P, Kollaros D (2025) Current status and distribution of alien land snails and slugs in Greece. Journal of Conchology 45: 679-694. https://doi.org/10.61733/jconch/4560
- Kuźnik-Kowalska E (1999) Life cycle and population dynamics of Discus rotundatus (O. F. Müller, 1774) (Gastropoda: Pulmonata: Endodontidae). Folia Malacologica 7: 5–17. https://doi.org/10.12657/folmal.007.001
- Kuźnik-Kowalska E, Pokryszko BM (2007) Incipient parental care in Discus—a plesiomorphic state of a truly endodontid character? Journal of Conchology 39: 467–468.
- Müller OF (1774) Vermium terrestrivm et fluviatilium, seu animalium infusorium, helminthicorum, et testaceorum, non marinorum, succincta historia. Volumen alterum: testacea. Heineck & Faber, Havniæ & Lipsiæ, xxxvi + 214 + [x] pp. https://doi.org/10.5962/bhl.title.46299
- 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.
- Salvador RB, Ravalo LGO, de Winter AJ (2023) Phylogenetic position of Canaridiscus and reestablishment of Gonyodiscus (Gastropoda, Discidae). Archiv für Molluskenkunde 152: 159–166. https://doi.org/10.1127/arch.moll/152/159-166
- Taylor JW (1906–1914) Monograph of the land & freshwater Mollusca of the British Isles. Zonitidæ. Endodontidæ. Helicidæ. Taylor Brothers, Leeds.
- Umiński T (1962) Revision of the Palearctic forms of the genus Discus Fitzinger, 1833 (Gastropoda, Endodontidae). Annales Zoologici 20: 299–333.