Desmoulin's Whorl Snail Vertigo moulinsiana (Dupuy) in Essex
There are also other records from Hertfordshire in the Lea Valley, further to the south.
In Britain, its distribution is centred on the rivers of the Hampshire Basin, the middle section of the
Thames basin and Norfolk (Kearney 1999), with Britain thought to hold the greatest number of
populations of this species in Europe (Killccn 2003). For this reason Britain has an especial
responsibility for its conservation. Throughout Europe it has been declining through wetland
drainage, changes in land management and scrub encroachment onto former fenland.
The aim of this survey was to advance one of the aims of the Essex BAP, which is to "undertake [a]
survey of former and likely sites to determine a true county distribution, by the year 2005". These
data might them be used to help maintain and enhance known populations through appropriate
management, site safeguards and consideration of water catchment changes.
Methodology
Survey work focussed on gathering revised data on the status of the snail at the three known locations
listed above and also on searching for the species at three new "pilot" sites. These three sites were
chosen on the grounds of their habitat type(s) present i.e. sedge-beds or similar fen vegetation,
geology i.e. fed by base-rich water and thirdly geography i.e. relatively close to current known
sites. Two of the pilot sites were situated in the river Stort catchment, where the three current
locations lie. These sites were Rushy Mead, an Essex Wildlife Trust Reserve just to the north of
Thorley Flood Pound and Parndon Mill, part of the Stort Valley Local Nature Reserve in Harlow,
downstream of the known sites. The third pilot site was adjacent to the River Cam to the west of
Saffron Walden in a catchment that drains northwards to Cambridge.
Standard methodologies recommend survey work during August when the snails tend to ascend tall
vegetation, wherefrom they can be beaten into white sheets. This current study was essentially a
springtime and early summer survey and relied on hand-searching through the low dense vegetation
and upper leaf litter layers close to the ground. This environment generally provides the high air
humidity that V. moulinsiana requires. During the search, small Vertigo specimens were collected
for determination to species level under a binocular microscope at a later date. Once draft
identification had been completed, a sample of specimens was sent to Dr Ian Killeen for independent
verification, to whom our thanks arc due. The larger or structurally more complex sites were sub-
divided into smaller units, with samples collected separately.
It should be noted that several of these sites are on private land, with no public access. EECOS
wishes to thank all the relevant landowners for permission to enter the sites in order to cany out
this survey work.
Site Descriptions
River Cam
The study area extended southwards from Adam Bridge to a small bridge adjacent to Gamage's
Lodge. Close to Adam Bridge are extensive Pond-sedge beds both sides of the river, with scattered
Yellow Flag-iris Iris pseudacorus, Reed Canary-grass Phalaris arundinacea and other herbs of fen
vegetation. Further south the sedge is confined to a narrower band immediately adjacent to the
river banks, although a species-rich side drain was also searched. Survey work ranged through the
extensive sedge beds to the north, but concentrated more on bankside or ditch bottom vegetation
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Essex Naturalist (New Series) 21 (2004)