Taxonomic description of a new freshwater Gammarus (Crustacea, Amphipoda, Gammaridae) restricted to the upper Hantan River, central Korean Peninsula

A new species of the genus <i>Gammarus</i> Fabricius, 1775 is described, originating from the Hantan River region, South Korea. <i>Gammarus kyungsooki</i> <b>sp. nov.</b> is morphologically characterised by the absence of calceoli on antenna 2, the presence of 6-7 clusters of long setae on the posterior margin of peduncular article 4 in antenna 2, long straight setae along the posterior margins of merus to carpus in pereopod 3, a 0-2-0 dorsal spine formula on urosomite 1, plumose setae on the outer margin of the outer ramus of uropod 3 and three or more clusters of setae on the outer margin of each telson lobe. Genetic distance analyses, based on mitochondrial cytochrome <i>c</i> oxidase subunit I (COI) and nuclear large-subunit ribosomal RNA (28S) gene sequences, support the distinctiveness of the new species, with interspecific divergences of 18.8%-25.2% (COI) and 1.7%-5.1% (28S).