The morphological comparison between extant and fossil shells of Meretrix lusoria and M. petechialis from Japan and South Korea

초록

Shell morphology of Meretrix lusoria and M. petechialis was compared between extant and fossil shells from Japan and South Korea. Torii et al. (2010) established a method to identify M. lusoria and M.petechialis from shell morphology. The modified discriminant score used 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 living individuals of M. lusoria and M.petechialis in 98.89% correct percentage. Then, objectives of the present study are 1) to distinguish fossil specimens of Meretrix using this discriminant score and elliptic Fourier analysis, and 2) to examine the temporal change of distribution of M. lusoria and M. petechialis in Japan and South Korea. Fossil shells of Meretrix were collected from 9 localities in Japan and South Korea; Satohama shell mound and San-no shell bed (Miyagi), Takahama shell bed (Fukui), Matsuzaki shell mound (Aichi), Kumamoto shell bed in Japan, and Gimhae shell mound, Daesan shell mound, Oido shell mound and Ganghwado tidal flat in South Korea. The outside of the right valve was photographed with a digital camera and delineated by an elliptic Fourier descriptor software (SHAPE ver. 1.3), and 10 parts of shell morphology, such as shell height and shell length, were measured using image analysis software (Scion Image ver. 1.63) and a slide caliper. Multivariate analysis of shell morphology revealed that fossil shells from all localities of Japan and southern coast of South Korea are identified as M. lusoria. These results are same as the living populations. However, fossil shells from western coast of South Korea, where the present living population is M. petechialis, were also identified as M. lusoria. The shells from the tidal channel of Ganghwa tidal