Blooms of the giant jellyfish Nemopilema nomurai have occurred in waters adjacent to Japan and Korea every year from 2002 to 2007 and in 2009, and have terribly disturbed the marine ecosystem and damaged fisheries. To prevent damage to fisheries by N. nomurai, it is very important to elucidate the origin and dispersal process of the species. Accordingly, we conducted population genetic analyses of N. nomurai. We analyzed specimens collected from 11 localities spanning the species distribution

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... 2007 based on mitochondrial gene sequences. We detected at least two genetically differentiated populations, the Chinese coast and Korean west coast populations, and individuals in eastern Korea and Japan were derived from the two populations. Seasonal changes in the genetic constitutions of the sample population from the East China Sea also revealed mixing of the populations from the Chinese coast and Korean west coast due to low-salinity waters from the Yangtze River in the summer. We presumed that individuals from the mixed group drifted with the Tsushima Current and winds, passed through the Tsushima Strait, and reached the eastern Korean and Japanese coasts.