Copepods have been poorly studied in subterranean habitats in Korea. Previous records have indicated mostly the presence of species already described from Japan, with very few endemic elements. This commonality has usually been explained by repeated dispersal across the land bridges that connected the two countries several times during the Pleistocene glacial cycles. However, the Korean Peninsula is known for pockets of Cambrian and Ordovician carbonate rocks, with more than 1,000 caves alreadybeen explored. The relative isolation of these carbonate pockets makes for an enormous speciation potential, and the development of a high level of short-range endemism of subterranean copepods should be expected. Representatives of the genus Diacyclops Kiefer, 1927 are here investigated from a range of subterranean habitats in South Korea, with comparative material sampled from central Honshu in Japan. Morphological analyses of microcharacters, many of which are used in cyclopoid taxonomy for the first time herein, reveal high diversity in both countries. No subterranean species is found in common, although the existence of four sibling species pairs in Korea and Japan may be indicative of relatively recent speciation. We describe seven new stygobiotic species, including three from Korea (D. hanguk sp. n., D. leeae sp. n., and D. parasuoensis sp. n.) and four from Japan (D. hisuta sp. n., D. ishidai sp. n., D. parahanguk sp. n., and D. pseudosuoensis sp. n.). Diacyclops hanguk, D. parasuoensis, D. ishidai, and D. parahanguk are described from newly collected material, while the other three new species are proposed for specimens previously identified as other, widely distributed species. Diacyclops brevifurcus Ishida, 2006 is redescribed from the holotype female, and D. suoensis Ito, 1954 is redescribed from material newly collected near the ancient Lake Biwa in Japan. This research provides evidence for the importance of subterranean habitats as reservoirs of biodiversity, and also demonstrates the inadequacy of current morphological methods of identifying closely related species of copepods. The disproportionately high diversity discovered around Lake Biwa provides further evidence in support of the hypothesis about the role of ancient lakes as biodiversity pumps for subterranean habitats. A key to the East Asian species of the languidoides-group is provided.

Proserpinicaris young sp. nov., Proserpinicaris wangpi sp. nov. and Proserpinicaris imjin sp. nov. are described from subterranean waters of South Korea. They are short-range endemics, allopatric in distribution and closely related to each other, and to two other Far Eastern congeners. Distinguishing features are limited to the general habitus shape, proportions of the caudal rami and degree of sexual dimorphism. The genus Proserpinicaris Jakobi, 1972 Jakobi, H. 1972. Trends (Enp.P4 Mänchen) innerhalb der Parastenocarididen (Copepoda Harpacticoidea). Crustaceana, 22: 127–146. [Crossref] [Google Scholar] , as redefined here, is Palaearctic in distribution, with the centre of diversity in southern Europe, and 20 valid members, all of which share a large hyaline spiniform structure on the male fourth leg basis as a synapomorphy. A key to species is provided. Genera Niponnicaris Jakobi, 1972 Jakobi, H. 1972. Trends (Enp.P4 Mänchen) innerhalb der Parastenocarididen (Copepoda Harpacticoidea). Crustaceana, 22: 127–146. [Crossref] [Google Scholar] and Pannonicaris Jakobi, 1972 Jakobi, H. 1972. Trends (Enp.P4 Mänchen) innerhalb der Parastenocarididen (Copepoda Harpacticoidea). Crustaceana, 22: 127–146. [Crossref] [Google Scholar] are established as its junior subjective synonyms, Lacustricaris Jakobi, 1972 Jakobi, H. 1972. Trends (Enp.P4 Mänchen) innerhalb der Parastenocarididen (Copepoda Harpacticoidea). Crustaceana, 22: 127–146. [Crossref] [Google Scholar] is formally synonymized with Parastenocaris Kessler, 1913, and Parastenocaris lacustris Chappuis, 1958 Chappuis, PA. 1958. Le genre Parastenocaris Kessler. Vie Milieu, 8: 423–432. [Google Scholar] is designated as incertae sedis in Fontinalicaridinae Schminke, 2010 Schminke, HK. 2010. High-level phylogenetic relationships within Parastenocarididae (Copepoda, Harpacticoida). Crustaceana, 83: 343–367. [Crossref], [Web of Science ®] [Google Scholar] .

Definition of monophyletic supraspecific units in the harpacticoid subfamily Stenheliinae Brady, 1880 has been considered problematic and hindered by the lack of molecular or morphology based phylogenies, as well as by incomplete original descriptions of many species. Presence of a modified seta on the fifth leg endopod has been suggested recently as a synapomorphy of eight species comprising the redefined genus Stenhelia Boeck, 1865, although its presence was not known in S. pubescens Chislenko, 1978. We redescribe this species in detail here, based on our freshly collected topotypes from the Russian Far East. The other species redescribed in this paper was collected from the southern coast of South Korea and identified as the Chinese S. taiae Mu & Huys, 2002, which represents its second record ever and the first one in Korea. A fragment of the mtCOI gene was successfully PCR-amplified from two specimens of each species, which represents the first molecular data for this genus, and from additional 19 specimens belonging to six different species of other stenheliins from Korea and Russia. Reconstructed phylogenies confirm previously postulated monophyly of Stenhelia and polyphyly of the closely related genus Delavalia Brady, 1869. Average pairwise maximum likelihood distances between S. pubescens and S. taiae are only slightly above 10%, suggesting a very close relationship despite numerous newly discovered micro-morphological differences and despite macro-morphological similarities being probable plesiomorphies.