A new species of Jesogammarus from the Iki Island, Japan (Crustacea, Amphipoda, Anisogammaridae).

A new species of anisogammarid amphipod, Jesogammarus (Jesogammarus) ikiensis sp. n., is described from freshwaters in the Iki Island, Nagasaki Prefecture, Japan, based on results of morphological and molecular analyses. The new species is distinguished from all members of the genus by the combination of small number of setae on dorsal margins of pleonites 1-3, short and small number of setae on posterior margins of peduncular articles of antennae, mandibular article 1 without setae, well developed posterior lobes of accessory lobes of coxal gills on gnathopod 2 and pereopods 3-5, and pectinate setae on palmar margin of female gnathopod 2. A key to all the species of Jesogammarus is provided.