Glass Snails (Vitrinidae)

Larger glass snail (Phenacolimax major).
Ear shaped glass snail (Eucobresia diaphana). Source: KERNEY.

Eucobresia nivalis, an Alpine species of glass snail.
Picture: Helmut Nisters.

Glass snails are a family of terrestrial snails, whose glassy translucent shells bears a clear similarity to slugs. Numerous European species of glass snails live on humid habitats on the ground, up to high altitudes in the Alps.

The form of glass snail shells differs especially in regards to the size of the apertural whorl compared to the other whorls of the shell. The shell of a globular pellucid glass snail (Vitrina pellucida), for example, has got three shell whorls, which is about as many as it can be among glass snails.

Besides, the apertural whorl of a pellucid glass snail is not as much extended as that of an ear shaped glass snail (Eucobresia diaphana). The spire of an E. diaphana shell contains at least one half whorl less than one of V. pellucida, which is why it is called an ear shaped shell (the extreme of which can be found among sea-living abalones, Haliotidae).

Apart from the shell, also the mantle may be developed in different ways: Looking at a living glass snail, one may find that especially among species of Eucobresia and Semilimax (Literally this means half-Limax – Limax is a slug genus) there is a dorsal mantle shield partly covering the shell.

	Ear-shaped glass snail (Eucobresia diaphana). Pictures: By friendly courtesy of Andreas Heidl.

A lobe of the mantle also covers the shell spire, so that in some species the shell may almost entirely be overgrown by the mantle, a clear tendency towards a slug. The colour of the mantle as well as of the remainder of the snail's body varies with its environment, so specimens of Eucobresia diaphana glass snails from high altitudes in the Alps are coloured rock grey, in contrary to their blacker relatives from the lowlands.

The further evolution of glass snails can clearly be envisioned, observing glass snail species from the Azores (the Plutonia genus), whose shell has been completely overgrown by the mantle.

During the evolution of terrestrial snails, slug groups have evolved several times independently from each other. The development strategy of shell reduction is called vitrinization, after the Vitrinidae family, which is but one path of this strategy.

Systematically, glass snails are placed near the partly carnivorous glossy snails (Zonitidae), exclusively carnivorous Daudebardiid half slugs (Daudebardiidae) and keel slugs (Milacidae).

Complete species list for the United Kingdom and Ireland:
According to Clecom (January 2008)

Vitrinidae Fitzinger, 1833

Vitrininae Fitzinger, 1833

Semilimax Stabile, 1859

Semilimax pyrenaicus (A. Férussac, 1821) *)

Vitrina Draparnaud, 1801

Vitrina pellucida (O.F. Müller, 1774)

Plutoniinae T. Cockerell, 1893

Phenacolimax Stabile, 1859

Phenacolimax major (A. Férussac, 1807) **)

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*) Ireland only.
**) United Kingdom only.