Molecular classification and morphological traits of two species of Meretrix (Mollusca,Bivalvia)

Although many species of the genus Meretrix are commercially-important cultured bivalves,the taxonomy of Meretrix is still in argument over several discrepancies among scientists in the world.Compared with the groups of Meretrix meretrix (RD & BH),two groups of Meretrix clam(CN & BW),collected from Cangnan and Beihai,were phylogenetically analyzed and molecularly classified based on partial sequences of the mitochondrial COⅠ gene.The results indicated that the base sequence of CN group and M.lamarckii with similar composition was the highest in homology of 100.0% and 99.2%,respectively.Phylogenetic NJ and UPGMA tree of Meretrix based on genetic distance also showed that CN1 and CN2 were clustered together with M.lamarckii firstly.Therefore we classified CN group as M.lamarckii,which extended the geographic distribution of M.lamarckii from Taiwan to the southern coast of Zhejiang and corrected the existing document claims.But,the BW group shared low sequence homology(79.6%-85.3%)with other six species of Meretrix and formed much independently cluster beyond among groups.Number of polymorphic sites,indexes of haplotype diversity and nucleotide diversity revealed that the level of genetic diversity from high to low was BH＞M.lamarckii＞RD＞BW.The COⅠ gene sequences of BW group showed very low genetic diversity and no nucleotide variation among 10 individuals.The morphological characters of M.lamarckii, BW and M.meretrix were analyzed using statistical analysis of AMOVA and Tukey’s multiple comparisons for seven morphological traits.Three morphological parameters,including SH/SL,SMW/TW and IW/TW,of M.lamarckii differed significantly with M.meretrix and BW(P<0.05).BW with relatively expanded shell shape differed significantly with M.meretrix and M.lamarckii in parameters of SW/SL and SW/SH(P<0.05).These results can be expected to provide basic data for the studies of species identification,germplasm conservation and phylogeny.