SURVEY FOR DESMOULIN’S WHORL SNAIL (VERTIGO MOULINSIANA)
Desmoulin’s Whorl Snail is listed under Annex II of the European Habitats Directive. It is a priority species in the UK Biodiversity Action Plan (HMSO, 1996) and is listed in the British Red Data Book (Bratton, 1991) as an RDB3 (Rare) species (Killeen, 2003).
Desmoulin’s Whorl Snail is largely restricted to old calcareous wetlands, usually adjacent to lowland rivers and lakes (Kerney, 1999). Most British records are from a band from east Dorset to north west Norfolk. Maps showing distribution of this snail in the British Isles are available (Kerney, 1999; Killeen, 2003).
Ecology of Desmoulin's Whorl Snail
Aquatonics Ltd have successfully surveyed for Desmoulin's Whorl Snail using a method based on the Level 2 survey technique detailed in the English Nature Report "Monitoring Desmoulin’s Whorl Snail" (Killeen and Moorkens, 2003). The main difference was that permanent marker posts were not installed and most samples were spot samples rather than on transects.
Monitoring Desmoulin's Whorl Snail
METHOD
For Figures 1 - 3 click here.
Suitable vegetation such as that shown in Figure 1 is assessed as a priority. If specimens are found in the most suitable habitats the survey can be extended to include less likely habitats and plant species.
Monitoring Desmoulin's Whorl Snail
GPS is used to record the position of each sampling site. The typical maximum height of the vegetation is measured to the nearest 10 cm and a photo taken. The dominant and sub-dominant plant species in the sample area are noted, using keys such as Stace (1991). The dampness of each site is assessed on a range of 1-5 (as recommended by (Killeen and Moorkens, 2003):
1. Dry. No visible moisture on ground surface
2. Damp. Ground visibly damp, but water does not rise under pressure
3. Wet. Water rises under light pressure
4. Very wet. Pools of standing water, generally less than 5 cm deep
5. Site under water. Entire sampling site in standing or flowing water over 5 cm deep
At each site a blue plastic sheet is laid on the ground at the base of a wall of vegetation. The vegetation from a strip 1 metre long and 0.5 m wide is bent over the sheet and vigorously shaken and beaten for 20 seconds by both surveyors to release the adhering snails. The material on the sheet is then carefully examined to determine whether any small molluscs, especially Desmoulin's Whorl Snail. It is also possible to see Desmoulin's Whorl Snail in situ if the stems of plants such as Carex spp are examined carefully (Figure 2).
Adult Desmoulin’s Whorl Snails can be identified in the field, using a magnification of 3-5x. Despite its small size (2.2 – 2.7 mm) it is the largest of the species of Vertigo (Cameron and Redfern, 1976, Killeen, 2003). Desmoulin’s Whorl Snails are dextrally coiled, adults are approximately 2.5 mm, and there are no regular striations on the upper whorl (Figure 3). Some specimens were brought back to the laboratory for a detailed examination to check that they had the characteristic four teeth in the aperture Figure 3).
Ecology of Desmoulin's Whorl Snail
REFERENCES
Bratton, JH (1991). British Red Data Books: 3. Invertebrates other than Insects. Joint Nature Conservation Committee. Peterborough.
Cameron, RAD and Redfern, M (1976). British Land Snails. Synopses of the British Fauna (New Series) Number 6. Linnean Society of London. Academic Press, London.
HMSO (1996). Biodiversity: the UK Steering Group Report. Volume 2, Action Plan. HMSO, London.
Kerney, M (1999). Atlas of the Land and Freshwater Molluscs of Britain and Ireland. Harley Books, Colchester.
Stace, C (1991). New Flora of the British Isles. Cambridge University Press, Cambridge.
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