Shining Ram's-horn Snail (Segmentina nitida) - Species Action Plan

A truly aquatic species, it is small (4-5.5mm) with a translucent shell in the form of a spiral disc, shaped like a ram’s horn. The upper surface of the shell is strongly arched and the underside is almost flat. It lives in unpolluted, usually calcareous water in ponds and drains of grazing marshes. Associated with a rich variety of freshwater molluscs, including other rare species.
Ref 1/S15 Species Action Plan 15
Plan Author: English Nature
Plan Co-ordinator: English Nature
Review complete 31 December 1998

Action Plan Summary

Current Status

National Status

  • This snail can be found locally throughout Europe, as far north as southern Scandinavia. In Britain, it has shown a dramatic decline this century. It is now confined mainly to the Norfolk Broads and Pevensey Levels. The species is listed as endangered in the GB Red List. 

Norfolk Status

  • Found within the Broads and Breckland Natural Areas 
  • Occurs mostly in drainage ditches in marsh levels, in clean, hard water in densely vegetated places. At Thompson Common it lives uniquely in water-filled glacial hollows. Key populations are found within the River Waveney Valley

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Current factors affecting the habitat in Norfolk

  • Inappropriate ditch clearance. 
  • Eutrophication – nutrient enrichment from nitrates and phosphates. 
  • Conversion of grazing marshes to arable farming with associated water table lowering.

Current Action in Norfolk

None

Action Plan Objectives and Targets

National

  • Research the ecology of the species to understand why it is declining 
  • Identify and maintain all existing populations by the year 2000 
  • Enable existing populations to increase in size and spread in range 
  • Produce management advice by the year 2000. 

Norfolk

  • Identify and maintain all Broads populations and that at Thompson Common SSSI by the year 2000 
  • Enable existing populations to increase in size and spread in range

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Management Guidance

(This guidance is a general summary; for detailed information or advice consult the references or contacts below.)

It inhabits overgrown, late-succession ditches and foot drains, dominated by vegetation, often reed sweet grass (Glyceria maxima) choking the channel with floating grassy mats. Water levels can be very low, literally only a few centimetres deep. It is associated with filamentous algae and Lemna.

This snail thrives best in areas grazed by low numbers of livestock. Such grazing creates shallow depressions in which water and vegetation may reach higher temperatures.

It is important to maintain occupied ditches at the late successional (choked by vegetation) stage. This need can present problems when ditching operatives and even conservation advisers may feel that a ditch should be returned to open water! However, steps have to be taken to compensate for the damaging effects of eventual drying out. Creation of new features, such as shallow depressions on grazing marsh, is one option. In some situations it may be possible to excavate a new ditch, adjacent to an occupied one, in which to establish the snails prior to managing the choked ditch.

Over-frequent dredging, lowering of water levels and pollution are major causes of decline. Surviving populations are in areas of traditional grazing marsh where nitrogen and phosphate enrichment remains low.

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Key Contacts

  • General Information

Ian Killeen Email: ian@malacserv.demon.co.uk

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References

Environment Agency (1998). Snails. Environment Agency Species Awareness Leaflet No 2.

Jackson, M J and Howlett, D J (1999). Freshwater molluscs of the River Waveney grazing marshes. A survey carried out during the summer of 1997. Broads Authority report (BARS 18).

Killeen, I J and Willing, M J (1997). Survey of ditches in East Anglia for the freshwater snails Segmentina nitida and Anisus vorticulus. English Nature Reports No 220.

Killeen, I J and Willing, M J (1997). Survey of ditches in East Anglia and south-east England for the freshwater snails Segmentina nitida and Anisus vorticulus. English Nature Research Report 229. Peterborough: English Nature.

Willing, M (1997). Fresh and brackish-water molluscs: some conservation issues. British Wildlife 8(3): 151-159.

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