The Asian Clam in Britain

The Society's Annual Award for 2003 was given to Dr Müller for his D Phil thesis to the University of Cambridge on "Ecology and impacts of the non-indigenous Asian clam Corbicula fluminea (Müller, 1774) in Britain". Dr Muller's thesis integrates a variety of approaches. It provides a malacological example in the wider biological sphere of conservation and invasion biology. The work suggests that it is not the heavy consumers of food resources which will out-compete other (native) clams, but those which can grow on the poorer resources.

The Asian clam has spread from Asia to North & South America and Europe, colonising hundreds of kilometres of waterways within a few decades. It causes substantial problems by biofouling power station cooling waters, drainage and irrigation channels and gravel extraction and has been linked to wide-ranging ecological changes.

Mitochondrial sequences of clams in Britain are identical to those in mainland North America and northern Europe populations. The British population was likely founded from Dutch or north German stock. It has spread through nearly 60 km of waterways in the Broads since first reported in 1998. It occurs at high densities (>4000 m²) in some sites and grows to reproductive size in 12 months. In the River Chet it attains a length of 45 mm - larger than elsewhere in Europe. However it is unable to cope with the organically enriched silty sediments of much of the Broads and is presently confined to downstream slightly brackish stretches of all major rivers, not yet reaching upstream where most native bivalves occur. Dispersal in the Broads occurs through saltation and diffusion dispersal: boats play only a minor role in ongoing dispersal.

The clams impact negatively on growth and condition of threatened Unionid mussels where they co-exist (the Broads is the primary site for Europe's most threatened unionid, the depressed river mussel Pseudanodonta complanata). The clams also accelerate decomposition of organic material in surface deposits.

Native predators have a weak impact because they do not consume the abundant small clams. Fish are strongly if indirectly implicated, and remove many of the larger individuals. Mechanical dredging was ineffective in reducing clam densities.

GIS mapping, relating to calcium concentration, pH, and critical lower temperatures for survival and reproduction, suggests that the clam could colonise large parts of southern Britain, including important industrial waterways, although limited in the west and in Wales and in the Pennines. The UK is predicted to suffer millions of pounds of industrial damage, particularly in drinking water abstraction and power generation. Predicted climatic changes will increase the range greatly by 2080.

Bill Bailey