Limnoperna fortunei (the golden mussel) is an invasive freshwater bivalve species that is native to China and southeastern Asia. Since L. fortunei has both economic and environmental effects, Japan's Ministry of the Environment has officially designated it as an “Invasive Alien Species”. The occurrence of the mussel was initially reported in 2005 in central Japan, but its spatial distribution in this area was unknown. We examined the spatial distribution of the mussel in the Kanto region from 2006 to 2009.In the lower Tone River, we found the mussel up to about 120 km from the river mouth. The mussel was also found in several other rivers, including the Kokai, Hitachitone, and Edo Rivers; in ponds and lakes (Lake Kasumigaura, Lake Teganuma, Lake Ushikunuma, etc.); and in canals. Size distribution data suggest that the mussel invaded these areas no later than 2004. Previous studies showed L. fortunei to be already present around the Oshio reservoir (Gunma Pref), which is a part of the Tone River system. Spatial distribution and genetic analysis revealed that the samples from the lowlands of the Kanto district (the lower Tone River, Lake Kasumigaura, etc.) show different genetic structure from those of the Oshio reservoir. From these results, we considered that the mussel invasion in this region occurred in at least two separate occasions. The spatial distribution and genetic analysis of the mussel in the lowlands of the Kanto district suggest that expansion of the mussel distribution was brought about by the water current via river, water-way and pipeline. Early detection and monitoring of the mussel invasion will be important, especially in the downstream of the invaded areas.
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