Vertigo angustior – narrow-mouthed whorl snail
Vertigo angustior Jeffreys, 1830
Family: Vertiginidae
This tiny whorl snail occurs in undisturbed marshy places at the roots or on the stems of low vegetation. Because of its size it is easily overlooked and is consequently considered rare. There are less than a dozen localities known in the whole of Ireland. In Northern Ireland it is recorded from a small area on the Causeway coast in Antrim.
Its habitat requirements are thought to be fairly specific, swards in transition between marshland and drier meadowland being favoured although sward height does not seem critical to long term survival (Norris & Colville, 1974). The Causeway sites appear very densely vegetated and are ungrazed but do have a constant supply of freshwater from seepages. Recent discoveries of new sites in southern Britain have greatly extended its range there but, more importantly, have served to underline our current lack of understanding of the factors governing distribution.
The whorl snails have peculiar outgrowths called teeth in the aperture. These are not teeth in the sense of being chewing implements, but are shell outgrowths designed to prevent predators from entering the shell and killing the animal. The arrangement of teeth, or dentition, is used in separating one species from another.
In brief
- Known only from several bays along the coast at the Giant’s Causeway
- Favoured habitat: coastal turf with freshwater seepage; fenland
- Prefers some sun penetration to the ground but is apparently not adversely affected by lack of grazing; can be affected by excessive trampling or disturbance
- It can be seen throughout the year
- This is a UK Priority species
- Main threats are range fertilisation, overgrazing and drainage
- A Boreal relict, that is, commoner when the climate was sunnier and drier before 4,000 BP.
Species description
This is a tiny shell (about 2mm), mid-brown in colour, striate rather than glossy, and with a sinistral aperture (shell aperture on the left side rather than the right; Figure 1).
Life cycle
Present at all times of year, but population size may fluctuate according to breeding success. Generally abundant in the Causeway sites.
Similar species
Vertigo pusilla is the only other sinistral Vertigo in Ireland but has a different dentition and favours rocky places and dry woodland glades rather than marshland (Cameron, 2003). It also has a paler-coloured shell (Figure 2). Cameron (2003) should be consulted for differences between V. angustior and V. pusilla.
How to see this species
Within the land area of Northern Ireland it was recorded historically from a sheltered bay between Port Noffer and Port Moon, Giant's Causeway (Norris and Colville, 1974) and has been found abundantly as a subfossil in dune pockets at Portstewart in coastal County Londonderry (Standen, 1897; Welch, 1898). Holyoak (2005) re-discovered the Causeway colony at Port Noffer and found three other very similar sites nearby, at Port Ganny, Portnaboe and Port Moon.
These sites are ungrazed and comprise the overgrown margins of marine boulder beaches with marshy conditions caused by trickling freshwater seepages. Hands and knees observation of grass tussocks and flotsam close to the upper shore in these areas should reveal a tiny snail adhering to the base of stems or the underside of driftwood. It can be seen throughout the year. Relevant access permissions should always be sought prior to visiting any sites.
Current status
There are less than a dozen localities known in the whole of Ireland. In Northern Ireland it is recorded from a small area on the Causeway coast in Antrim. It is in a reasonably safe position within Northern Ireland. The occupied areas at the Giant’s Causeway are extensive and it appears abundant there. The sites are safeguarded by the World Heritage designation and by SAC, ASSI and NNR designations.
Why is this species a priority in Northern Ireland?
- Listed as a UK Priority species.
It is a rare species in Northern Ireland. It is important in a European context as it appears to be generally rare and is listed in the EU Habitats and Species Directive.
Threats/Causes of decline
Elsewhere in Ireland some sites have been threatened by coastal development such as golf courses and drainage schemes at fenland sites. The position at the Giant’s Causeway looks healthy, but a potential cause for concern would be eutrophication of the coastal seepages which maintain these sites. At Portballintrae and other places on the same coastline significant deterioration of water quality in seepages has been experienced in recent years with complete elimination of aquatic and semi-aquatic life.
Conservation of this species
Current action
There is a UK Species Action Plan which was published in 1995.
- The species is protected at the Giant’s Causeway by ASSI/SAC/NNR designations and the site is managed by the National Trust
- Implementation of the Northern Ireland Habitat Action Plan for Coastal Saltmarsh.
Proposed objectives/actions
- Perform targeted surveys to improve mapping coverage and knowledge of the species, in particular of seepage areas on the north Antrim coastline towards Ballycastle
- Monitor agricultural activity close to the Causeway and discourage any intensification of agricultural activity on land adjoining the Causeway.
What you can do
This is a difficult species to find, but if you encounter coastal seepages on the coast east of the Causeway, search for this species at the roots of vegetation. If you find whorl snails there, these could be other, more common, species so, note the find on an Ordnance Survey map and report the details to CEDaR, National Museums Northern Ireland, 153 Bangor Road, Cultra, Co. Down, BT18 0EU, Tel: 028 9039 5256, cedar.info [at] nmni.com or to roy.anderson [at] ntlworld.com.
Further information
Links
http://www.ukbap.org.uk/UKPlans.aspx?ID=626
Northern Ireland Habitat Action Plans
Literature
Cameron, R.A.D. (2003). Land snails in the British Isles. Field Studies Council, Aidgap Key OP79.
Holyoak, G.A. (2005). Vertigo angustior (Mollusca: Gastropoda) at the Giant’s Causeway, Co Antrim. Irish Naturalists’ Journal 28: 176.
Norris, A. and Colville, B. (1974). Notes on the occurrence of Vertigo angustior Jeffreys in Great Britain. J. Conch. Lond. 28: 141-154.
Ross, H.C.G. (1984). Catalogue of the land and freshwater Mollusca of the British Isles in the Ulster Museum. Ulster Museum, Belfast.
Standen (1897). The land Mollusca of Ballycastle and district, Co. Antrim. Irish Naturalist 6: 1-28.
Welch, R.J. (1898) Land-shell pockets on sand-dunes. Irish Naturalist 7: 72-82.
Text written by:
Dr Roy Anderson
iNaturalist: Species account : iNaturalist World Species Observations database