A new species of calliopiid amphipod, Calliopius ezoensis, from the eastern coasts of Hokkaido, Japan, is described and named. Calliopius ezoensis is similar to the North Atlantic C. sablensisBousfield & Hendrycks, 1997, but can be discriminated from it in possessing two rows of calceoli on the peduncular articles of antennae 1 and 2 of the female, having a mandibular palp of article 3 longer than article 2, and setation of epimeral plate 3 on the ventral submargin. The new species is distinguished from North Pacific congeners by a combination of characteristics: in having dorsally smooth pereonites, the number of marginal and submarginal setal rows on epimeral plate 2, setation of palp article 2 of maxilla 1, indistinct sexually dimorphic gnathopods 1 and 2, and the length ratio of the inner and outer rami of uropod 2. In addition to previously provided DNA sequences of nuclear histone H3 and mitochondrial 16S rRNA genes, nuclear 28S rRNA and mitochondrial cytochrome c oxidase subunit I sequences of this new species are provided for the future comparative purposes.

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