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 complete mitochondrial genome of Tubifex tubifex was analyzed using the MGISEQ-2000 platform. The size of the complete mitochondrial genome was 15,972 bp. Data pertaining to the genome, such as the presence of 13 protein-coding genes (PCGs), two rRNA genes, 22 tRNA genes, and a putative control region were submitted to NCBI (MW690579). A phylogenetic tree was constructed with the sequences of the 13 PCGs using the maximum-likelihood method. Despite only a few references available on the complete mitochondrial genome of other aquatic oligochaetes, our phylogenetic analysis revealed that the freshwater oligochaetes T. tubifex and Limnodrilus hoffmeisteri are in a cluster different from that of the earthworm group.

This taxonomic study investigates marine tubificids, with a focus on the genera Smithsonidrilus, Heterodrilus, Limnodriloides, and Tubificoides and presenting a new species, Heterodrilus koreanus n. sp., discovered in the around Ulleungdo and Dokdo Islands in the East Sea of the Republic of Korea. The new species, Heterodrilus koreanus n. sp., is characterized by a very long and slender atrium with prostate glands attached to the ventral side, with indistinct ducts and large round ampullae in the spermathecae. The four recorded species are: H. pentcheffi Erséus, 1981, Limnodriloides anxius Erséus, 1990, Smithsonidrilus exspectatus Erséus, 1993, and Tubificoides heterochaetus Michaelsen, 1926. This study not only enhances our comprehension of the intricate morphology within these genera but also contributes to the broader understanding of marine oligochaetes, particularly in the Korean marine ecosystem.

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.

Freshwater oligochaetes, belonging to the class Oligochaeta, are vital components of aquatic ecosystems globally, contributing significantly to nutrient cycling, sediment dynamics, and overall ecosystem function. Despite their ecological importance, freshwater oligochaetes in Korea remain relatively understudied compared to other aquatic organisms. This study aimed to enhance our understanding of the diversity and ecological roles of freshwater oligochaetes in Korean aquatic ecosystems. We identified nine species across six genera and two subfamilies from samples collected from various freshwater environments in Korea. This research contributes valuable taxonomic knowledge and highlights the ecological significance of freshwater oligochaetes in Korean aquatic ecosystems.

The complete mitochondrial genome of Nais communis was analyzed using the Illumina Hiseq 2000 platform. The length of the complete mitochondrial genome was 15,685 bp, and the data were submitted to NCBI (MW770354). The genome contained 13 protein-coding genes (PCGs), 2 rRNA genes, 22 tRNA genes, and a putative control region. A phylogenetic tree was constructed based on the sequences of 13 PCGs identified by the maximum-likelihood method. Regardless of the lack of studies on the complete mitochondrial genome of other aquatic oligochaetes, the phylogenetic tree showed N. communis to cluster with Tubifex tubifex and Limnodrilus hoffmeisteri with high support value, and the freshwater oligochaete and earthworm groups to be sister groups.