First record of colonial ascidian, Botrylloides diegensis Ritter and Forsyth, 1917 (Ascidiacea, Stolidobranchia, Styelidae), in South Korea.
Botrylloides species are important members of the fouling community colonizing artificial substrates in harbors and marinas. During monitoring in 2017-2020 of non-indigenous species in Korea, one colonial ascidian species was distinctly different from other native colonial ascidians, such as B. violaceus and Botryllus schlosseri, in South Korea. This species was identified as B. diegensis. DNA barcodes with mitochondrial COI were used to identify one-toned and two-toned colonies of B. diegensis. Intraspecific variations between Korean and other regions of B. diegensis from the NCBI ranged from 0.0% to 1.3%. The Korean B. diegensis was clearly distinct from other species of Botrylloides at 15.8-24.2%. In phylogenetic analysis results, Korean B. diegensis was established as a single clade with other regions of B. diegensis and was clearly distinct from Korean B. violaceus. After reviewing previous monitoring data, it was found that two-toned B. diegensis was already found in six harbors by July 2017. It has now spread into 14 harbors along the coastal line of South Korea. This means that B. diegensis might have been introduced to South Korea between 1999 and 2016.