Mitochondrial and nuclear DNA sequence variation of presumed Crassostrea gigas and Crassostrea angulata specimens:: a new oyster species in Hong Kong?

In several cases, oyster taxa. have been misidentified owing to their high morphological plasticity, uncertain geographical range and undocumented introductions. Recently though, molecular techniques have been efficiently applied to discriminate between oysters and to quantify genetic divergence within and among species. In the present paper, we report mitochondrial (16S and COI) and nuclear (28S) DNA sequences of presumed Crassostrea gigas and Crassostrea angulata, two taxa of aquacultural importance. Mitochondrial DNA sequences are compared with previously published sequences and PCR-RFLP haplotypes. Within C. gigas, divergence was less than 0.5% for COI, and less than 0.2% for 16S. Within C. angulata, divergence was less than 1.1% for COI and 0.2% for 16S. Our results also confirm the close genetic relationship between C gigas and C angulata and further document their level of divergence: 2-3% for COI and 0.5-1% for 16S. However, the initially presumed C. gigas oysters farmed in Hong Kong (Pearl River delta), presented DNA sequences strongly divergent from both C gigas and C angulata: 13-14% for COI, 3-4% for 16S and 1.2-1.6% for 28S. The closest related species are C. gigas and Crassostrea nippona with a divergence of 12-13% for COI, 3-4% for 16S and 1.2-1.6% for 28S. Comparisons with existing DNA sequence data available in the nucleotide sequence databases shows that this is either a new species or that it corresponds to a species for which no DNA sequence is yet available. Further studies are required to document morphological characteristics and geographical range of this putative new species. (C) 2003 Elsevier B.V. All rights reserved.