A new species of Diosaccus Boeck, 1873 (Arthropoda, Hexanauplia, Harpacticoida) was recently discovered in Korean waters. The species was previously recognized as D. ezoensis Ito, 1974 in Korea but, here, is described as a new species, D. koreanus sp. nov., based on the following features: 1) second inner seta on exopod of fifth thoracopod apparently longest in female, 2) outer margin of distal endopodal segment of second thoracopod ornamented with long setules in male, 3) caudal seta VII located halfway from base of rami (vs. on anterior extremity in D. ezoensis), and 4) sixth thoracopod with three setae in female (vs. 2 setae in D. ezoensis). In addition, there is also a mitochondrial COI sequence difference of more than 19.93% with D. ezoensis registered in NCBI. A key to Diosaccus species of the world is also provided, and new morphological features and DNA sequences are presented for two other harpacticoid species, Parathalestris verrucosa Ito, 1970 and Peltidium quinquesetosum Song & Yun, 1999. In order to clearly identify harpacticoids at the species level, both morphological and DNA sequence characteristics should be considered.

This study reports the discovery of four new benthic harpacticoids from Korean waters in the Pacific Ocean. The harpacticoids were collected using SCUBA or a grab sampler and sorted using a stereomicroscope for both molecular and morphological analysis. The study employed standard DNA sequencing methods (COI and 18S rRNA) and detailed morphological characterization through microscopy and scanning electron microscopy. As a result, three new species belonging to the family Orthopsyllidae and classified as a new genus were discovered. Although the new genus lacks brush setae on leg 1, a diagnostic feature of Orthopsyllidae, it shares similarities with other orthopsyllids in characters such as the large thorn-like process on the antennule, leg 5, and sexual dimorphism. This finding necessitates an expanded diagnosis of Orthopsyllidae to reflect the characters of the new genus based on the three new species. In addition, a new species belonging to the genus Orthopsyllus was discovered. This study is the first to describe a new species of Orthopsyllus from Korea, where the genus has not been previously reported at the species level. This study provides a key for classifying genera and species within the Orthopsyllidae family.

Benthic harpacticoids were collected from Korean waters. Two species were identified as members of the genus Longipedia Claus, 1863, because they have an extremely elongated distal segment of the P2 endopod. Longipedia koreana sp. nov. is morphologically most closely related to L. nichollsi Wells, 1980 and L. scotti Sars, 1903, but it can clearly be distinguished from both species based on the following morphological characteristics: P1 coxa with strong spinules near the outer margin and the distal element being much bigger than the proximal elements, P2 coxa with a small inner seta on the anterior surface, P4 exopod first segment without an inner element, and the P5 with a rectangular exopod (more than 3.5 times as long as wide). L. ulleungensis sp. nov. is similar to L. brevispinosa Gurney, 1927, L. spinulosa Itô, 1981, and L. weberi Scott A., 1909. However, L. ulleungensis sp. nov. is characterized by the P2 coxa with a reduced inner seta, the P4 exopod second segment without an inner seta, and the anal operculum with a long median projection, a single spine, and a group of outer spines on each side. In a molecular analysis using the mitochondrial cytochrome c oxidase subunit I (COI) and 18S ribosomal RNA (18S rRNA) genes, the inter-specific variation was 22.525–23.102% and 1.325–1.382% of COI and 18S rRNA between the two new species, respectively. A key to the family Longipediidae is provided herein.

Two new species of the genus Scottolana Huys, 2009, were collected from Korean waters. S. daecheonensis sp. nov. is morphologically similar to S. jasani Song, Kangtia, Khim, and Chullasorn, 2018, from the Yellow Sea, Korea, and S. gomezi Nazari, Mirshamsi, Sari, Aliabadian, and Martinez Arbizu, 2018, from Bandar Abbas, Iran. However, it can clearly be distinguished from its congeners by rostrum with two tiny lateral sensilla, 4-segemented antennule in female, and 5-segmented antennule in male, 3-segmented mandibular exopod, P3 basis with an inner spinous process on the posterior surface, P3 endopod-3 without a modified tube-pore along the inner margin in male, P4 endopod-2 crumpled inner margin with strong spinules in male, P5 second outermost seta naked, caudal rami with an unguiform extension on the inner margins, and caudal seta V with a normal base in female. S. wonchoeli sp. nov. is similar to S. geei Mu and Huys, 2004 from the Bohai Sea, and S. longipes (Thompson I.C. and Scott A., 1903) sensu Wells and Rao (1987) from the Andaman Islands, but it can clearly be distinguished from its congeners by a combination of the following features: rostrum with a broadly rounded apex, 3-segemented antennule in female and 5-segmented in male, antennary exopod 8-segmented, female genital field relatively small, extending about one-third of genital double-somite, genital somite with paired lateral spinous processes in male, P6 with a long proximal element in male, and maxillular endopod with 11 setae in total. In a molecular analysis using the mitochondrial cytochrome c oxidase subunit I (COI) and 18S ribosomal RNA (18S rRNA) genes, the interspecific distance was 20.031% for COI and 3.214% for 18S rRNA between the two new species. A key for the identification of the species of Scottolana is included.

Three new species of the copepod family Canuellidae Lang, 1944 (Copepoda, Canuelloida) are described from the Western Pacific Ocean, based on specimens collected from coastal waters of Korea (Jeju-do Island) and the Republic of Palau. This study provides detailed morphological descriptions and illustrations for Elanella jejuensis sp. nov ., Brianola coreana sp. nov ., and Scottolana picrca sp. nov . Key diagnostic characters differentiating them from congeners include the unique bulbous modification of caudal ramal seta IV in E. jejuensis sp. nov ., the presence of an inner seta on the P1 exopod-2 in B. coreana sp. nov ., and features including reduced urosome segmentation and a modified female caudal seta II that place S. picrca sp. nov . within the longipes -group. Molecular phylogenetic analysis using mitochondrial cytochrome c oxidase subunit I (COI) and nuclear 18S ribosomal RNA (18S rRNA) sequences recovers each new species as a distinct, well-supported lineage and resolves two major clades within Canuellidae: ( Elanella + Brianola ) and ( Scottolana + Canuella ). The phylogeny further indicates that Elanella and Brianola are sister taxa, and places S. picrca sp. nov . within the longipes -group; however, the placement of S. picrca sp. nov . as sister to S. wonchoeli is weakly supported (57% bootstrap). These findings significantly advance our understanding of Canuellidae diversity and evolution in the understudied Western Pacific region.