most likely due to inadvertent human intervention. Invasion in Europe Since being found in Ireland as early as 1837, Potamopyrgus antipodarum has now spread to nearly the whole of Europe. It is considered as about the 42nd worst alien species in Europe and the second worst alien gastropod in Europe. It does not occur in Iceland, Albania, Bulgaria or the former Yugoslavia. Countries where it is found include: Ireland around 1837 Stelfox, A. W. (1926) - probably the first introduction in Europe England since 1859 Germany Poland Western Baltic Sea since 1887 Russia Azov Black Sea region, since 1951, Ukraine since 1951 in brackish waters, and since 2005 in freshwater Catalonia in Spain, since 1952 Mediterranean region of France, since the end of 1950s Italy, since 1961 Turkey Czech Republic, since September 3, 1981 Slovakia, since 1986 Greece, since November 2007 Distribution within the United States First detected in the United States in Idaho's Snake River in 1987, the mudsnail has since spread to the Madison River, Firehole River, and other watercourses around Yellowstone National Park; samples have been discovered throughout the western United States. Although the exact means of transmission is unknown, it is likely that it was introduced in water transferred with live game fish and has been spread by ship ballast or contaminated recreational equipment such as wading gear. The New Zealand mudsnail has no natural predators or parasites in the United States, and consequently has become an invasive species. Densities have reached greater than 300,000 individuals per m² in the Madison River. It can reach concentrations greater than 500,000 per m², endangering the food chain by outcompeting native snails and water insects for food, leading to sharp declines in native populations. Fish populations then suffer because the native snails and insects are their main food source. Mudsnails are impressively resilient. A snail can live for 24 hours without water. They can however survive for up to 50 days on a damp surface, giving them ample time to be transferred from one body of water to another on fishing gear. The snails may even survive passing through the digestive systems of fish and birds. Mudsnails have now spread from Idaho to most western states of the U.S., including Wyoming, California, Nevada, Oregon, Montana, and Colorado. Environmental officials for these states have attempted to slow the spread of the snail by advising the public to keep an eye