Ascidiella aspersa, originally native to the northeastern Atlantic, has emerged as a prolific invasive species in coastal waters worldwide. In 2010, it was identified as an alien species in Republic of Korea, rapidly colonizing artificial harbor structures and outcompeting native species. This study employs morphological analyses and genetic sequencing, focusing on mitochondrial DNA (cytochrome oxidase subunit I; mt-COI) and nuclear markers (18S rRNA), to unravel the genetic structure and haplotype diversity (Hd) of A. aspersa populations in Republic of Korea and globally. The analysis of 154 mt-COI and 127 18S rDNA global population sequences, as well as 80 mt-COI and 79 18S-rDNA Korean population sequences, revealed distinct genetic patterns. Among global populations, the mt-COI gene displayed significant genetic diversity, with 21 distinct haplotypes distributed across 41 polymorphic sites, which is indicative of extensive genetic variability. In contrast, the 18S rDNA marker exhibited limited diversity, with only four haplotypes identified at three polymorphic sites. In Korean populations, the mt-COI gene also exhibited substantial genetic diversity, with 14 distinct haplotypes displaying genetic variations at 29 polymorphic sites. Conversely, the 18S rDNA marker in Korean populations revealed a unique genetic pattern, with only one shared haplotype. These findings emphasize the complex genetic diversity within A. aspersa populations, both globally and in Republic of Korea. This genetic analysis provides valuable insights into the species’ colonization history and adaptation mechanisms, shedding light on the factors shaping its genetic structure. Further research is warranted to elucidate the ecological implications of these genetic patterns in the context of invasion biology.

The solitary ascidian Ascidiella aspersa is an emerging invasive species in coastal ecosystems worldwide, including Korean waters. We developed and validated a species‐specific quantitative PCR (qPCR) assay targeting the mitochondrial COI gene, showing high specificity against 128 non‐target taxa and robust performance (Efficiency = 110.1%, R ² = 0.9962, LOD = 12 copies/reaction, LOQ = 598 copies/reaction). The assay was applied to over 300 environmental DNA (eDNA) samples collected across 18 harbors between 2019 and 2021, and to an additional survey in 2022 that targeted four representative harbors (Bieung, Tongyeong, Yangpo, and Jeju) selected from the East, West, South, and Jeju Seas. Seasonal detection patterns showed peak DNA concentrations during summer, coinciding with the reproductive season of A. aspersa . Geographic analysis revealed a broader distribution than previously recorded, including new detections in Jeju Island and eastern ports. These findings highlight the assay's utility for early detection, risk assessment, and surveillance of A. aspersa , supporting its integration into national biosecurity frameworks. This study demonstrates the power of eDNA‐based diagnostics for managing marine invasive species through scalable and non‐invasive monitoring strategies.

The first record of Herdmania pallida (Ascidiacea: Stolidobranchia: Pyuridae) in South Korea is presented based on specimens collected from Jejudo Island. The specimens displayed morphological characteristics consistent with H. pallida, such as body form, branchial sac structure, and gonad arrangement, thus clearly differentiating them from other Herdmania species found in Korea (H. japonica, H. mirabilis, and H. momus). Seven partial sequences (856 bp) of the mitochondrial cytochrome c oxidase subunit I gene were obtained from the Korean specimens. Pairwise distance analysis indicated an average divergence of 9.8 % from three H. pallida reference sequences obtained from GenBank. The phylogenetic analysis further supported that the Korean specimens constitute a distinct clade and appear to be paraphyletic relative to the GenBank H. pallida sequences. These findings suggest potential cryptic diversity within the species and highlight the necessity for more extensive research. The study emphasizes the importance of broader geographic and population-level sampling, which will contribute significantly to better understanding species boundaries and clarifying the taxonomic status of H. pallida.

The complete mitochondrial genome of Bionychiurus tamilensis Thunnisa & Sumithra, 2021 was sequenced, assembled, and annotated. The mitochondrial genome of B. tamilensis is 14,937 bp in length and contains 13 protein-coding genes (PCGs), 22 transfer RNA (tRNA) genes, and two ribosomal RNA (rRNA) genes. The overall nucleotide composition was 35.3% A, 15.8% C, 9.8% G, and 39.6% T, indicating an obvious A + T bias (74.9%). B. tamilensis was closely clustered with the following species of Onychiuridae: Allonychiurus kimi, Tetrodontophora bielanensis, Onychiurus orientalis, and Orthonychiurus folsomi.

The mitochondrial genome of Decametra tigrina (A.H. Clark, 1907) was fully sequenced and characterized. This circular, double-stranded genome spans 15,794 bp and encompasses 13 protein-coding genes (PCGs), 22 tRNA genes, and two rRNA genes. The arrangement of genes remains conserved, matching those of Cenometra bella and the MW405444 (submitted to GenBank as Oligometra serripinna). The base composition consists of 24.6% A, 47.6% T, 16.2% G, and 11.6% C. Most PCGs use ‘ATG’ as the initiation codon, with NADH4L and NADH5 initiating with ‘GTG’. Each PCG terminates with a complete stop codon (‘TAA’ or ‘TAG’). Twelve tRNA genes, three NADH genes, and two rRNA genes are located on the L strand, while the remaining genes reside on the H strand. Phylogenetic analysis utilizing all 13 PCGs demonstrated that D. tigrina forms a clade with MW405444 (submitted to GenBank as O. serripinna, reassessed here as D. tigrina) and Florometra sp. (MN883538). Decametra tigrina forms a sister group with C. bella; together, they cluster with Stephanometra indica (Mariametridae) and Zygometra comata (Zygometridae) in the ML tree.