Depressed river mussel (Pseudanodonata complanata) - Species Action Plan

Like all mussels this animal has two ‘mirror image’ shells, hinged on one edge. At up to 80mm (3 inches) it is somewhat smaller than swan and duck mussels, and is distinguished by a ‘compressed’ shell. It lives inconspicuously in the bottom sediments of slow rivers.
Ref 1/S14 Species Action Plan 14
Plan Author: English Nature
Plan Co-ordinator: English Nature
Review completed January 2003

Action Plan Summary

Current Status

National Status

  • This mussel is seriously threatened throughout its European range. In the UK since 1950 it has been recorded from 63 ten km squares in England and Wales. However the species is easily overlooked, and may be more common than thought. The UK probably has the healthiest population in Europe, with the possible exception of Finland. 

Norfolk Status

  • Found within the Yare, Wensum and Waveney valleys, in the Broads and East Anglian Plain Natural Areas and watercourses in the west of the county, within the Fens Natural Area.

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Current factors affecting the habitat in Norfolk

The threats to the species are not fully known but are likely to include:

  • Water pollution 
  • Physical disturbance of river banks and channels 
  • Drought 
  • Collection of individuals for garden ponds and aquaria 

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Current Action in Norfolk

A survey of all likely habitats in Norfolk has been undertaken by the Ted Ellis Trust, commissioned by English Nature. A detailed Species Action Plan has been produced, including a thorough literature review.

In the Broads, the Broads Authority and the Environment Agency temporarily remove mussels before undertaking river dredging works. Further surveys are currently being undertaken by the Environment Agency. 

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Action Plan Objectives and Targets


  • Identify and maintain key populations by the year 2000 
  • Research the ecology and habitat preferences of this species 


  • Undertake surveys of the River Waveney 
  • Encourage appropriate habitat management for this species

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Management Guidance

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

Biggest threat is dredging (e.g. one dredging event wiped out an entire population of international importance in the Somerset Levels in 1999). Weed cutting can remove significant numbers. Corbicula is a likely threat in the Waveney. Reductions in fish populations will also impact heavily. Management plans are currently being produced for the Somerset Levels and River Arun by Cambridge University. The distribution in the Waveney is patchy, suggesting that a single localised event could have a massive impact on the population.

A degree of nutrient enrichment seems to be important, including relatively high abundances of algae.

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Key Contacts

  • General Information

Dr David Aldridge, Aquatic Ecology Group, Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ 
Tel: +44 (0)1223 334436 / Fax: +44 (0)1223 336676 / Email:

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Ellis, A E (1978). British Freshwater Bivalve Mollusca. 

Environment Agency (1998). River Mussels. Environment Agency Species Awareness Leaflet No 1.

Keys and identification notes: Synopses of the British Fauna No 11, published for the Linnean Society by Academic Press. ISBN – 0-12-236950-5.

Muller, S J (1999). Population genetics, ecology and waterway management in the conservation of Pseudoanodonta complanata (Rosmassler), MPhil Thesis University of Cambridge.

McIvor, A L (1999). Ecology and waterway management in the conservation of Pseudoanodonta complanata, MPhil Thesis University of Cambridge.

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

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