Distribution and substrate preference of the invasive clam Corbicula fluminea in the river Rhine in the region of Basel (Switzerland, Germany, France)

Betr. Neozoen im Rhein, speziell die asiatische Körbchenmuschel The Asiatic clam Corbicula fluminea invaded the river Rhine in the Netherlands and Germany in the 1990s. It was first recorded in Switzerland (Basel) in 1995. We examined the distribution of the clam at 76 sites along the bank of the river Rhine, in three first order tributaries (Wiese, Birs, Ergolz) and in the Canal de Huningue in the region of Basel (Switzerland, Germany and France) in 2003. C. fluminea was found in the river Rhine and in the Canal de Huningue, which obtains water from the river Rhine. C. fluminea was recorded 22 km upstream of Basel, but not any further. This indicates a mean upstream spread of 2.4 km per year. It had not yet colonized any of the first order tributaries examined. The clam was most abundant on finegrained substrates (sand) with slowly flowing, shallow water. This finding was confirmed by a substrate choice experiment in the river Rhine. Our results show that the spread of C. fluminea in the river Rhine does not stop where cargo shipping ends. Passive dispersal by waterfowl and recreational boating may facilitate further upstream spread. C. fluminea might be less successful in colonizing rivers with rapid current such as first order tributaries. These are assumed to serve as refuges for native molluscs.