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• Zonitoides (Zonitoides) excavatus (Alder 1830) Hollowed glass snail

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© Dr Roy Anderson

© Dr Roy Anderson

Shell tightly coiled within a moderately raised spire. Whorls fairly convex with deep sutures. The specific name indicates the very wide (hollowed out) umbilicus. Surface horn-coloured to translucent greenish, glossy with coarse growth lines. The animal is variable, from pale grey to nearly black. In acidic woods in areas of base-poor rocks.

Key characteristics

• Characterised by its very deep and broad umbilicus, rather like that of Discus rotundatus
• Shell a depressed cone of about 5½ whorls.
• Whorls fairly convex with deep sutures
• Surface very glossy, brown or translucent greenish
• Animal usually dark but may be pale (see first photo)

Size

5.3-6 mm.

World Distribution

Restricted to coastal regions on the periphery of the North Sea and to the British Isles. Distribution type: Oceanic Temperate (71).

GBIF distribution map [open in new tab]

Irish Distribution

The only obligate calcifuge (‘lime-hating’) land mollusc in Ireland and absent from the central plain and from eastern counties except for two very isolated colonies in Mourne Park and Rostrevor Forest, Co. Down and a few localities in the Wicklow Hills. Widespead but uncommon in the west, favouring areas of acidic bedrock.

Ecology

• Essentially a woodland species living in acid birch or oak litter
• Adapted to base-deficient conditions including lagg woodland of raised bogs where its companions may include Discus rotundatus, Columella aspera and Oxychilus alliarius
• Strangely absent from lagg woods in eastern counties

Red List status

• Vulnerable (VU).

Wikipedia link

Wikipedia page for Zonitoides excavatus