Zebra mussels (Dreissena polymorpha) in Ireland, AFLP-fingerprinting and boat traffic both indicate an origin from Britain

1. The zebra mussel (Dreissena polymorpha ) is an aquatic nuisance species that invaded Ireland around 1994. We studied the invasion of the zebra mussel combining field surveys and genetic studies, to determine the origin of invasion and the vector of introduction. 2. Field surveys showed that live zebra mussels, attached to the hulls of pleasure boats, were transported from Britain to Ireland. These boats were lifted from British waters onto trailers, transported to Ireland by ferry and lifted into Irish waters within a day. Length-frequency distributions of dead and living mussels on one vessel imported 3 months earlier revealed a traumatic occurrence caused by the overland, air-exposed transportation. Results show that a large number of individuals survived after re-immersion in Irish waters and continued to grow. 3. Zebra mussels from populations in Ireland, Great Britain, the Netherlands, France and North America, were analysed using amplified fragment length polymorphisms (AFLP)-fingerprinting to determine the origin of the Irish invasion. Phylogenetic analysis revealed that Irish and British mussels clustered closely together, suggesting an introduction from Britain. 4. Ireland remained un-invaded by the zebra mussel for more than 150 year. The introduction of the zebra mussel to Ireland occurred following the abolition of value added tax in January 1993 on imported second-hand boats from the European Union (UK and continental Europe). This, together with a favourable monetary exchange rate at that time, may have increased the risk of invasion of the zebra mussel.