Zebra Mussels: A Warning and History

By Shaun Micklewright

Chris Mead, in his excellent book The State of the Nation's Birds, has suggested that the continuing increase in Tufted Duck Aythya fuligula numbers in Britain may be partly due to the spread of Zebra Mussels Dreissena polymorpha, an alien mollusc found in many watercourses throughout Britain. Intrigued by this suggestion, and more worryingly by a recent statement by Severn Trent Water highlighting the potential damage the mussel poses as a bio-fouling agent to raw water mains, I provide recorders with a brief history/identification guide for this fascinating and little known mollusc.

The Zebra Mussel Dreissena polymorpha is a native of the Black, Aral and Caspian Seas. During the eighteenth and nineteenth centuries it spread throughout Europe as many rivers became interconnected and commerce increased amongst countries. It was first recorded in Britain in 1824 at Surrey Commercial Docks (Greater London) (Marchant et al 1990) from where it has spread to nearly every major river and canal in Britain. In Worcestershire it has been sparsely recorded mainly in the rivers Severn and Avon with a few records from the river Stour.

The common name Zebra Mussel is derived from the pattern of zebra like stripes on the shell and the scientific name polymorpha, refers to the many morphs or forms that occur in the shell's colour pattern, including albino and solid black or brown.

Zebra Mussels have been reported to grow as large as 5cm long although most are rarely larger than 3cm. They live primarily in clusters on hard substrates in rivers, canals, lakes, and on industrial plant intake structures where they attach themselves firmly, making it difficult for either predators or high currents to pry them off. Where they are common they pose a great risk to water abstraction pipe work, and to other molluscs which they have been shown to smother and eventually kill.

A variety of animals prey upon them: fish, crayfish, and waterfowl have all been shown to consume them. The shell clusters can be extremely sharp so care should observed when collecting them.

Both Severn Trent Water and the Environment Agency are currently monitoring the mussel and any records would be most welcome. Please send your records to WBRC.

References

Picture based on that in Ellis AE. 1962. British Freshwater Bivalve Molluscs. Synopses of British Fauna. Linnean Society of London