Eight species of copepods belonging to the family Dirivultidae (Siphonostomatoida) are described from deep-water hydrothermal vent fields on the Central Indian Ridge in the Indian Ocean. Aphotopontius limatulus Humes, 1987, previously known only from the East Pacific, is included. The other seven species are new, as follows: Aphotopontius kiost n. sp., A. muricalus n. sp., Benthoxynus constrictus n sp., Stygiopontius spinifer n. sp., S. horridus n. sp., S. geminus n. sp., and S. quadripaxillifer n. sp. The copepodid I stage of S. horridus n. sp. is also described. This is the first record on copepods living on hydrothermal vent fields in the Indian Ocean.

Surveys of caves in South Korea yielded a new species of the ostracod genus Cavernocypris Hartmann, 1964, herein described as Cavernocypris danielopoli Smith & Kamiya n. sp. This is the fifth described species in this widely distributed genus, and can be discriminated from its congeners by the highly triangular shape of the carapace in lateral view, caused by a large dorsal protrusion on the left valve. A second triangular-shaped morphotype was also collected, differing from the first by a lower carapace and less pronounced dorsal protrusion. As it is not clear at present if this represents a separate species or intraspecific variation, it is left in open nomenclature. The lack of a pigmented eye, a highly triangular carapace and no carapace pigmentation suggests that Cavernocypris danielopoli Smith & Kamiya n. sp. is the most adapted species in the genus to a stygobiotic lifestyle. Another Korean species, Cavernocypris coreana (McKenzie, 1972), found in caves and spring discharges, has a reduced number of antennule segments, common for stygobiotic species, but this feature is lacking in the new species. This suggests that within the genus, species have followed different evolutionary pathways to adapt to the subterranean realm.

Monstrilloid copepods of the genus Monstrillopsis Sars, 1921 are recorded from Korea, as well as from the northwestern Pacific, for the first time. Specimens were obtained using a light trap set overnight at piers or seawalls in small harbors. Males of two new species were found and are described herein with remarks on their affinities and detailed drawings and SEM photographs of their morphological features. Monstrillopsis longilobata sp. nov. is allied to M. chathamensis Suárez-Morales & Morales-Ramírez, 2009 and M. sarsi Isaac, 1974 in having the character combination of a long, claw-like apical spinous process on the male antennule and elongate genital lappets. It differs from them in having conspicuous transverse striations on the dorsal surface of the cephalothorax and extremely long male genital lappets, which extend far beyond the anterior margins of the caudal rami. Monstrillopsis coreensis sp. nov. also has a claw-like apical spinous process of the male antennule and conspicuous transverse striations on the dorsal surface of the cephalothorax, but it differs from M. longilobata in having a much longer and stronger apical spine of the antennule and a relatively shorter and undulating genital apparatus.

A new species of monstrilloid copepod belonging to the genus Monstrilla Dana, 1849 is recorded from South Korea. Specimens were obtained between May of 2012 and January of 2014 from inshore waters along the coasts of the East Sea, South Sea, and Jeju Island, using a light trap installed at quays and wharves overnight. Monstrilla ilhoii sp. nov. is characterized by a remarkable combination of a large body size (females about 3.8 mm long, males about 2.8 mm long), a polygonal ridge pattern (cuticular reticulation) over the entire body surface, and an antennule armature featuring a supernumerary spiniform seta (4d3) in females and a dagger-shaped distal spine (62) in both sexes. In other respects M. ilhoii sp. nov. most closely resembles M. lata Desai & Bal, 1963; both species have one and three setae on the endopodal and exopodal lobes, respectively, of female leg 5, a horseshoe-shaped male genital apparatus with marked undulations along the whole distal margin, and six setae on each caudal ramus in both sexes. This paper provides a description of the new species, taxonomic remarks on its morphological features, and differential diagnoses with respect to its allies. Scanning electron microscopy was used to document significant morphological microcharacters. The species of Monstrilla known from five sub-regions of the north-west Pacific are enumerated with the source references, and keys to the females and males of these species are provided.

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.

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.