The Mask Snail

Isognomostoma isognomostomos (Schröter 1784)


Mask snail (Isognomostoma isognomostomos).
Picture: Stefan Haller, (schneckenfoto.ch).
 

The mask snail, of all terrestrial snail species native to Europe, probably is the one with the longest name and the most astounding appearance. Though it looks like a mixture of a Helicid snail, a leaf snail and a whorl snail, it clearly belongs among the Helicid snails (Helicidae), like the copse snail (Arianta arbustorum) in the subfamily Ariantinae.

Description: An uncommon sight for a Helicid snail is the mask snail's shell covered with long, slightly curved hairs, which, similar to a cheese snail (Helicodontidae) or a leaf snail (Hygromiidae) will fall out from dead and weathered shells.

 
Mask snail (Isognomostoma isognomostomos).
1: Parietal lamella; 2: Palatal tooth; 3: Basal tooth; 4: Aper-
tural lip. Picture: Vollrath Wiese (Haus der Natur), Cismar.

The snails' shell II: Hairy shells.
Pfenninger, M. et al. (2005): "Why snails have hairs".

Especially conspicuous also is the half-moon-shaped aperture of the snail, whose apertural rim is widened to form a broad light brownish lip. The opening size of the aperture additionally is reduced by a basal (lower) and a palatal (lateral) tooth. Besides, the parietal (upper) part of the aperture displays a distinct whitish lamella. The shell navel (umbilicus) is almost entirely covered by the apertural lip.

Apart from its uncommon aperture form and the hairy shell surface, the mask snail is also special in producing an epiphragm during periods of dryness or cold. Like other snails, it then closes the aperture with a membrane of dried mucus, but in mask snails, this membrane is additionally reinforced with calcium carbonate.

Dimensions: H: 4 - 7 mm; W: 7 - 11 mm; N: 5. (Abbreviations).

Habitat and Distribution: Mask snails almost exclusively live in forests: In the leaf litter, under fallen logs and between rock rubble in humid mountain forests, also in woody gorges, where the snail mainly feeds on decaying plant matter and microbial fungi.

The mask snail is a typical character species of humid and wet forests rich in rock rubble. Only rarely, mask snails leave the forests to appear in half open habitats near rivers. Mask snails, on the other hand, not only are found in habitats close to nature: They also occupy habitats influenced by man, like quarries, freshly overgrown with primary forest. Mask snails prefer humidity, which is why they especially are found in shady places near springs, which is at the spring horizon near the foot of mountains, as well as near the crest of mountains, where it rains most.

 
Mask snail (Isognomostoma isognomostomos).
Picture: Stefan Haller, (schneckenfoto.ch).

The distribution area of Isognomostoma isognomostomos is alpine-carpathian: The species is found in the Alps and the Carpathians, but also in the German and Polish low mountain ranges, where it lives in altitudes of over 300 m MSL. In the Alps it climbs to altitudes of up to 1800 m MSL. In the east, the species' distribution area stretches as far as the Baltic countries. Isolated populations also live in East Prussia.

Threat Situation: The mask snail is endangered (EN) in Lower Saxony, where its distribution area has its northern limits with the Sünten and Teutoburg mountains. In Rhineland-Palatinate, the species is already classified as vulnerable (VU) (see also: IUCN Threat Categories).

Links

Francisco Welter-Schultes: Isognomostoma isognomostomos species homepage.
Mollusca.net: Weichtier des Jahres 2007: Maskenschnecke (Isognomostoma isognomostomos) (PDF). (In German)
Tierdoku.com: Maskenschnecke. (In German)
Fauna Europaea: Isognomostoma isognomostomos.
NABU Artenschutz: Feine Härchen als Aufprallschutz - Die Maskenschnecke. (In German)

Literature

Ehrmann, P. (1956): "Mollusken", in "Die Tierwelt Mitteleuropas", S. 143 f.
Fechter, R.; Falkner, G. (1990): "Weichtiere", S. 230.
Geyer, D. (1927): "Unsere Land- und Süßwassermuscheln", S. 90.
Pfleger, V. (1983): "Schnecken und Muscheln Europas", S. 150.


With pictures by Stefan Haller:
http://www.schneckenfoto.ch.