Abstract
The Pacific cupped oyster, Crassostrea gigas, is one of the major aquacultural shellfish species that has been introduced to Europe and America from its native source in the West Pacific. In Taiwan, the cultivated cupped oysters along the west coast have been identified as C. gigas for over centuries; however, several molecular phylogenetic studies have cast doubt upon the existence of this species in Taiwan and adjacent waters. Indeed, our analyses of mitochondrial cytochrome oxidase I (COI) sequences from 313 Crassostrea collected from 12 locations along Taiwanese and southern Chinese coastlines confirm that all samples were the Portuguese oyster, C. angulata, rather than C. gigas. Multiple lines of evidence, including haplotypic and nucleotide diversity of the COI gene, demographic history, and population genetics, suggest that Taiwanese C. angulata is unique, probably experienced a sudden population expansion after the Last Glacial Maxima around 20,000 years ago, and has a significantly limited genetic connectivity across the Taiwan Strait. Our study applies an extended sampling and DNA barcoding to confirm the absence of C. gigas in natural and cultivated populations in Taiwan and southern China, where we only found C. angulata. We highlight the importance of conserving the gene pool of the C. angulata population in Taiwan, particularly considering the current threats by large-scale environmental disturbances such as marine pollution, habitat destruction, and climate change.
Highlights
Southern China, with the Yangtze River separating it from C. gigas in northern China[8,9]
Instead of grouping with C. gigas, all 43 cytochrome oxidase I (COI) haplotypes formed a monophyletic group with the Portuguese oyster, C. angulata (DQ659374), with high bootstrap support (99) and Bayesian posterior probability (100)
Our results confirmed that C. gigas does not occur along the coasts of Taiwan and southern China, regardless of whether the oysters were from marine farms or natural populations
Summary
Southern China, with the Yangtze River separating it from C. gigas in northern China[8,9]. Geological events such as the last glacial maximum (LGM~20 ka BP) are speculated to have affected the phylogeography of marine biota in the western Pacific[19,20,21] and might play an important role in shaping the demographic history of Crassostrea species in Taiwan. We applied DNA barcoding of a COI fragment to examine the status of cultivated Crassostrea species from eight populations in Taiwan and four along the southern China coast (Supplementary Fig. 1S). The published COI DNA sequences of C. gigas from the northwestern Pacific (China, Japan, and Korea) were retrieved from GenBank[20] for phylogenetic and phylogeographic comparisons
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