Desmoulin’s whorl snail (Vertigo moulinsiana), or Desi for short is a tiny Snail that is found on Cholsey Marsh. I am unsure of its status there but apparently, it is doing OK.
This tiny snail, not more than 3mm tall, is found in south and south eastern England around the edges of ponds, streams and rivers. In the warmer months, it lives on the leaves of tall wetland plants when it is easiest to see. Desmoulin's Whorl snail is a priority species on the UK Biodiversity Action Plan.
This species was named after the early 19th century French naturalist, Charles des Moulins.
Is the largest Vertigo species, with a shell height up to about 2.6 mm. It is restricted to calcareous wetlands, usually bordering lakes or rivers, or in fens. High humidity appears to be important in determining local distribution within sites. It normally lives on reed-grasses and sedges, such as reed sweet-grass Glyceria maxima and tussocks of greater pond-sedge Carex riparia and lesser pond-sedge C. acutiformis, where it feeds on the microflora, and in autumn it may ascend taller reeds and scrub. Like all Annex II Vertigo species, it is highly dependent on maintenance of existing local hydrological conditions.
In the United Kingdom, Desmoulin's whorl snail is listed as endangered. Its presence on the site of the planned Newbury bypass caused the building of that road to be postponed; the building works were able to go ahead once the snails had been moved to a new habitat nearby. It is reported to have since died out at the new site.
The distribution of this species is Atlantic (the part of the Palearctic area which is under the direct climatic influence of the Atlantic Ocean), and southern-European. This small snail occurs across Europe as far north as southern Sweden.
Within Western Europe, only the populations in England and Ireland are considered viable.
This species is mentioned in Annex II of the European Union's Habitats Directive.
Population numbers: No accurate figures exist. Though never a common species it is thought, its numbers have reduced recently.
Information gained from various internet sources.
Desmoulin's Whorl snail (c) Roger Key
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