Taxonomic status of four Crassostrea oysters from China as inferred from mitochondrial DNA sequences

It has been presumed that there are four common Crassostrea oyster species along the coast of China: the Pacific oyster (Crassostrea gigas), Zhe oyster (C. plicatula), Suminoe oyster (C. ariakensis), and Dalianwan oyster (C. talienwhanensis). Classification and species identification of these Crassostrea oysters have been difficult because of morphologic plasticity. In this article, phylogenetic analysis was performed to clarify taxonomic status of these species using mitochondrial DNA sequence data. Nucleotide sequences of a 443-bp fragment of ribosomal RNA gene and a 579-bp segment of cytochrome c oxidase I gene were obtained through sequencing and used for analysis. Genetic distances among the four species, using C. virginica as outgroup, were computed based on the sequence data, and phylogenetic trees for the five species were generated. The divergence between C. gigas and C. talienwhanensis was very low, as was that between C plicatula and C. ariakensis. Phylogenetic analysis showed that haplotypes of C. gigas and C. talienwhanensis clustered in one clade and those of C. plicatula and C. ariakensis in another one. Our data suggest that C. gigas and C. talienwhanensis may be the same species. However, the lack of divergence between C. plicatula and C. ariakensis samples may indicate that the C. plicatula specimen we sampled could actually be a morph of C. ariakensis living in high salinity habitats. More work is needed for confirmation.