A new monospecific genus of the family Nannopodidae Brady, 1880 is proposed, based on specimens of both sexes of Concilicoxa hispidagen. et sp. nov. collected from subtidal sandy sediments in the Yellow Sea of South Korea. The presence of a coxal outer projection on the first to fourth legs and reduction of both rami of the second to fourth legs in this new genus show a clear relationship with a clade, which is characterised by the modified thoracopods for burrowing ability, comprising Huntemannia Poppe, 1884, Rosacletodes Wells, 1985, Laophontisochra George, 2002, Acuticoxa Huys & Kihara, 2010 and Talpacoxa Corgosinho, 2012 in Nannopodidae. Within this clade, C. hispidagen. et sp. nov. is most closely related to L. maryamae George, 2002 in having the prehensile endopod in the first leg, broad intercoxal sclerite on the second to fourth legs and the female fifth leg being composed of separate exopod and baseoendopod, but is distinguished by the absence of mandibular exopod, two-segmented mandibular endopod, presence of four setae on the distal exopodal segment of the first leg, and fusion of the intercoxal sclerite to the coxae in the third and fourth legs. These four features are considered as autapomorphies of the new genus. The possible relationship amongst members of the nannopodid clade is further discussed. Additionally, some comments on the taxonomic position of L. terueae Björnberg, 2014 are given, resulting in the transfer of the species to Acuticoxa as A. terueaecomb. nov.

Two species belonging to the family Laophontidae Scott T., 1905 were identified among harpacticoid copepods collected from marine plastic debris (MPD) stranded along the Korean coastline. These species were assigned to the genera Pseudonychocamptus Lang, 1944 and Heterolaophonte Lang, 1948. Pseudonychocamptus setadefectussp. nov. conforms to the generic diagnosis, displaying sexual dimorphism in the armature of the swimming legs. It is distinguished from its six congeners by possessing only two endites on the maxillary syncoxa, notably lacking the proximal endite bearing the seta that is present in all other known species within the genus. Although most morphologically similar to P. colomboi Ceccherelli, 1988, the new species differs in several key characteristics, including the number of setae on the maxillary endopod, the relative length ratios of setae on the female P5 exopod, the presence of an inner seta on the male P5 exopod, and the asymmetry of the male P6. The second species, Heterolaophonte discophora (Willey, 1929), was previously reported from the Atlantic coast of Canada and the Pacific coasts of the USA and Japan. The Korean specimens closely correspond to these previous records, yet exhibit intraspecific variation, particularly in the setal count on the female P3 enp-2. Additional morphological differences were also observed, including variations in the segmentation of the male P3 endopod and in the number of setae on the maxillary endopod. Based on the sexually dimorphic traits observed in males, we propose the subdivision of the genus Heterolaophonte into four distinct species groups. The discovery of P. setadefectussp. nov. and H. discophora on anthropogenic MPD underscores the potential role of such substrates as microhabitats for benthic harpacticoid copepods.

Two new species, Cerviniella bisegmenta sp. nov. and C. permixta sp. nov., are described in detail with illustrations based on females from the Korean Yellow Sea. These species lacking the fourth leg endopod belong to the mirabilipes group, one of two species groups within the genus Cerviniella Smirnov, 1946. Both species can be distinguished from each other by the surface ornamentation of the cephalothorax, shape of the rostrum tip, antennule segments, armature formula of thoracic legs 1–4, and length ratio of the caudal rami. Cerviniella bisegmenta sp. nov. is characterized by a short caudal ramus and a two-segmented antennary exopod, which are unique within the genus. Cerviniella permixta sp. nov. differs from other congeners of the mirabilipes group by the seven-segmented antennule, the armature formulae of the exopod of the antenna and thoracic legs 1–4, and the modified apical inner element of the second endopodal segment of the second leg. The present study is the first to identify the genus Cerviniella in Korean waters, resulting in extension of its distribution area to East Asia.

Zosime destituta sp. nov. is described from shallow subtidal habitats in the southern sea of Korea. The new species is similar to Z. valida Sars, 1919, but differs from it primarily by the lack of an inner seta on the third endopodal segment of the fourth swimming leg. Based on morphological comparison of female character states, the genus Zosime can be subdivided into three provisional groups, anneae-, incrassata-, and typica-group. These groups are distinguished from each other by differences in the antennule, antennary exopod, mandibular rami, armature of swimming legs, fifth leg, and caudal rami. In addition, Zosime incrassata bathybia Bodin, 1968 is removed from its synonymy with Z. incrassata Sars, 1910 and upgraded to species level. An amended key to all known species of Zosime Boeck, 1873 is given, along with a table of all major characters. The present study is the first report of the family Zosimeidae Seifried, 2003 from Korean waters.

The genus Smacigastes Ivanenko & Defaye, 2004 (Harpacticoida, Copepoda) is the most primitive genus in the family Tegastidae Sars, 1904, occurring in deep-sea chemosynthetic environments, such as hydrothermal vents, cold seeps, whale falls and wood falls. Our exploration of the Onnuri Vent Field, the sixth active hydrothermal vent system in the Central Indian Ridge, resulted in the discovery of a new species in the genus Smacigastes. A detailed morphological analysis of S. pumilasp. nov. reveals that it most resembles S. barti Gollner, Ivanenko & Martínez Arbizu, 2008, described from a hydrothermal vent in the East Pacific Ridge; the new species can be distinguished from the existing species by the 8-segmented female antennule, the absence of an abexopodal seta on the antennary basis, the mandibular exopod represented by a single seta and the exopod of the first leg with five setae. This is the first record of Smacigastes in the Indian Ocean. A dichotomous key to species of the genus Smacigastes worldwide is provided.

A new eunicid species, Leodice duplexa sp. n., from intertidal and subtidal habitats in the eastern coast of South Korea is described. The new species is assigned to the C-2 group, and is similar to Leodice antennata, the type species of the genus, in having the following combination of characteristics: moniliform antennae and palps, bidentate compound falcigers, articulated peristomial and notopodial cirri, pectinate branchiae showing bimodal distribution of branchial filaments, and yellow aciculae. However, L. duplexa sp. n. is readily distinguished from L. antennata by the following features: the aciculae are 2–4 in number, with blunt or pointed tips and hammer-headed or bifid tips, and the subacicular hooks are paired in some chaetigers. A detailed description and illustrations are provided for the new species. The validity of the new species is also supported by a genetic comparison using sequences of the mitochondrial cytochrome c oxidase subunit I (COI). A revised key to known Leodice species is provided with a comparison of their morphological characteristics.