Anisus vorticulus by Paul Sterry
This tiny freshwater snail is both a national and international rarity and Carlton Marshes is an important UK site for this species. With a shell diameter of 3.5-6mm it’s really difficult to spot and also tricky to identify as it looks similar to the very common Whirlpool Ram’s-horn snail. This little snail has very exacting requirements and, in England, it’s only found on grazing marshes in well-vegetated open ditches fed by clean, slightly calcareous water.
Over the last 60 years land use changes have had a major impact on the distribution of this snail. Over-frequent mechanised ditch clearance and the conversion of grazing marshes to arable farming, together with the associated lowering of water tables and enrichment of ditch waters by artificial fertilisers, have all taken their toll.
As a consequence, the UK populations of the snail have been reduced to a handful of places where conditions remained suitable. The Waveney Valley is one of the few places where some pockets of suitable good habitat remain to support strong populations of this rare snail. To ensure the snail thrives, the ditches at Carlton & Oulton Marshes are managed in a way which also favours many other rare aquatic invertebrates, including dragonflies and other snails. The ditches are only cleaned out when necessary and then only in sections, so that the snails can recolonise cleared sections from the uncleared stretches.
Other Trust reserves in the area, such as Castle Marshes, also support this species and additional land has recently been acquired by the Trust to extend Carlton Marshes. On these new sites, ditches have been restored to help bring back a rich variety of plants and invertebrates and in time it is hoped that the Little Whirlpool Ram’s-horn will also colonise these new areas. Future survey work will monitor how the snails are doing, hopefully finding them in the newly restored ditches. Working with neighbouring landowners it should be possible to ensure that in the coming years more suitable habitat is created or restored so that these very rare snails become just a little less rare.
Anisus vorticulus is a UK Biodiversity Action Plan priority species and the only British non-marine snail which is a European Protected Species. Dr Martin Willing of the Conchological Society of Great Britain and Ireland has been researching this species for many years. To find out more about the Society and its conservation work click here www.conchsoc.org. If you would like to know more about Anisus vorticulus please email Dr Willing.