Morphological and genetic traits of Meretrix lusoria and M. petechialis were compared among individuals from Japan and Korea. Multivariate analysis of shell morphology revealed that M. lusoria from all localities of Japan (Aomori to Kyushu) and from the southern and southwestern coasts of Korea (Sacheon Bay and Gangjin Bay) have some common characters, namely more linear shape in posterior-dorsal margin, smaller width of socket and larger shell breadth rather than M. petechialis from the western coasts of Korea (Baeksu and Saemangeum). Among M. lusoria, individuals from the Japan/East Sea coasts (Yuya Bay, Aso Sea and Mutsu Bay) have more linear shape in posterior-dorsal margin than those from other localities. The distribution border between M. lusoria and M. petechialis is located around the southwestern coasts of Korea (from Gangjin Bay to Baeksu). Analyses of mitochondrial COI and nucleus ITS also revealed that individuals from Japan and the southern coasts of Korea (Sacheon Bay) were classified as M. lusoria, and those from the western coasts of Korea (Baeksu and Saemangeum) as M. petechialis. However, all individuals from Gangjin Bay were classified as M. petechialis based on the analysis of mitochondrial COI, although most individuals were classified as M. lusoria by the analysis of nucleus ITS. These results suggest that hybridization between M. lusoria and M. petechialis occurs around Gangjin Bay. Further, we established a method to identify M. lusoria and M. petechialis from shell morphology. The modified discriminant score using the 5 selected characters, i.e. shell length (L), shell breadth (B), width of socket (SW), length of posterior-dorsal margin (LPM) and height of posterior-dorsal margin (HPM), is D=110.26 61.61(log B/log L)+10.90(log SW/log L)-81.72(log LPM/log L)+27.27(log HPM/log L). Using this discriminant score, we can identify M. petechialis and M. lusoria with 98.89% correct percentage.
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