Phyllodoce koreana was first described in 1985 in Gwangyang Bay, a semi-enclosed bay in Korea affected by significant organic input from the Seomjin River and dredging activities near the Gwangyang Port. Since then, this Korean endemic species has received limited attention in taxonomic and ecological studies. Phyllodoce koreana is known for its resilience to mild disturbances but is vulnerable to severe environmental changes. In this study, P. koreana specimens were collected from organically polluted Asian stalked tunicate aquaculture farms at eight sites in Jindong Bay, a location with environmental conditions similar to those of Gwangyang Bay, over the course of five sampling events from March to November. Both bays experience benthic hypoxia in summer due to elevated water temperatures and organic matter accumulation. Phyllodocid specimens were primarily collected in March and November 2023, non-hypoxic periods, suggesting potential seasonal adaptations to environmental fluctuations. The morphological features of the collected specimens were consistent with the original description of P. koreana, confirming their identification. Additionally, we reported previously overlooked morphological details, contributing to a more comprehensive taxonomic understanding of the species. We also present, for the first time, the complete mitochondrial genome of this species, comprising 15,559 bp, which provides essential genetic data for future taxonomic and phylogenetic studies. The phylogenetic analysis of protein-coding genes shows that, among 17 related polychaete species, P. koreana (family Phyllodocidae) is closely related to the family Goniadidae. Future research should expand our knowledge of polychaete taxonomy by integrating additional mitochondrial genomes and investigating the role of conserved gene synteny within Polychaeta.

In the western subarctic Pacific, zooplankton biomass is dominated by large copepods such as Neocalanus spp. and Eucalanus bungii, whereas small copepods dominate numerically. To evaluate how mesh size affects long-term monitoring, zooplankton were collected from 0 to 150 m with 100 µm and 335 µm mesh nets in mid-May of 2003–2018 and analyzed by ZooScan imaging. Abundance in the 335 µm net was only 11.7% of that in the 100 µm net, and the numerically dominant taxon shifted from Oithona spp. to Neocalanus spp. Total biovolume differed little between the nets and was dominated by Neocalanus spp. in both. A regime shift, a decline in 2008, was detected in E. bungii with both nets, whereas no shift was detected in Neocalanus spp., consistent with their reproduction in the epipelagic layer and at depth, respectively. Mean body size of both species was larger in the 335 µm net, which does not retain the smallest individuals. Each mesh size therefore gives a more reliable estimate for the taxa it retains, and the two nets together separate biomass-related from abundance-related patterns.