ANOTHER EXOTIC MOLLUSK IN THE LAURENTIAN GREAT-LAKES - THE NEW-ZEALAND NATIVE POTAMOPYRGUS-ANTIPODARUM (GRAY 1843) (GASTROPODA, HYDROBIIDAE)

In 1991, the hydrobiid snail Potamopyrgus antipodarum (Gray 1843), nat ive to New Zealand, was found 1 km offshore Wilson, New York, in Lake Ontario. This is the first known occurrence of the snail in eastern No rth America. Densities of P. antipodarum have increased since its disc overy, and the snail is expected to spread quickly throughout the Grea t Lakes area. The biofouling potential of P. antipodarum is probably l ow; however, its most serious threat may be resource competition with native molluscs. This species was probably introduced in ballast water from transoceanic vessels, thus increasing the list of nonindigenous aquatic organisms introduced into the Great Lakes since the 1800s to 1 40 different organisms. The risk of other species invasions into the G reat Lakes is still possible, despite the now mandatory ballast water regulations.