High haplotype diversity and strong phylogeographic structure in the invasive jellyfish Blackfordia virginica population highlight its complex demographic history in Korea

The hydrozoan Blackfordia virginica Mayer (1910), recognized as a successful invader, has been identified in brackish waters globally, posing a growing threat to ocean biodiversity. The species was discovered at Sihwa Lake in the western region of Korea in 2013, where the jellyfish blooms almost every year. However, there was a sudden bloom in 2021 at the Seomjin River estuary in the southern region of Korea. This study investigated the population genetic structure of B. virginica using mitochondrial DNA cytochrome c oxidase I (COI) from the samples collected from Sihwa Lake (SWL) and Seomjin River Estuary (SRE). We found high COI haplotype diversity within the Korean population, comprising 16 unique haplotypes that were highly differentiated based on geographical locations. Analysis of molecular variance (AMOVA) indicates significant genetic variation (F_ST = 0.6), which suggests multiple introduction events. Furthermore, a comparison of B. virginica COI data from Korea with GenBank records from the USA, China, India, and the Baltic Sea uncovers a global genetic structure within the species. Notably, the Korean population exhibited significant genetic differences from other populations, further making it challenging to determine the haplotype source origin. Nevertheless, the emergence of high genetic diversity and considerable genetic distance between the Korean population and other populations poses an intriguing puzzle, warranting further investigations.