Identification Of Saccostrea Mordax And A New Species Saccostrea Mordoides Sp. Nov. (Bivalvia: Ostreidae) From China

The taxonomy of Saccostrea oysters is problematic because of their highly variable shells. Molecular studies have revealed diverse sequence lineages that do not correspond to valid species names including some Saccostrea mordax-like oysters. The S. mordax-like oysters (n = 202) were collected from southern China and classified into three lineages, A, B, and C, based on both shell morphology and mitochondrial 16S ribosomal RNA (16S) and cytochrome oxidase I (COI) sequences. Molecular and morphological analyses indicate lineages A and B are S. mordax, and lineage C is a new species Saccostrea mordoides sp. nov. The shell morphology of the new species is variable, as in other Saccostrea oysters, but distinctive in that shell height is similar to shell length with the right valve much smaller than the left valve and having undulate laminae. The average Kimura's two-parameter distance between S. mordoides sp. nov. and other Saccostrea oysters is 0.046-0.120 for 16S and 0.106-0.240 for COI, which are significantly higher than typical distances among closely related oyster species. This study shows that S. mordoides is a new species different from S. mordax, highlighting rich species diversity of Saccostrea oysters and the need for molecular taxonomy.