Two new species of Jesogammarus from Japan (Crustacea, Amphipoda, Anisogammaridae), with comments on the validity of the subgenera Jesogammarus and Annanogammarus

Two new species of anisogammaris amphipod, Jesogammarus (Jesogammarus) bousfieldi and J. (J.) uchiyamaryui, are described from mountain streams in Yamagata Prefecture and from brackish waters on Iki and Fukue Islands, Nagasaki Prefecture, Japan. Jesogammarus bousfieldi is morphologically almost similar to J. paucisetulosus Morino, 1984. However, J. bousfieldi is distinguished from J. paucisetulosus by the more number of marginal setae on the pleonites. Jesogammarus uchiyamaryui resembles J. ikiensis Tomikawa, 2015, but the former differs from the latter by two setae on the posterior margin of peduncular article 2 of antenna 1, short and straight accessory lobes of coxal gills on gnathopod 2 and pereopods 3-5, densely setose ventral margins of coxae of female gnathopods 1 and 2 and pereopod 3, and a shorter inner ramus of uropod 3. Phylogenetic analyses using nuclear 28S rRNA, mitochondrial cytochrome c oxidase subunit I, and the 16S rRNA markers showed the sister relationship between J. bousfieldi and J. paucisetulosus. However, the phylogenetic position of J. uchiyamaryui remains uncertain. Both new species were genetically highly diverged comparable to intraspecific divergence among other Jesogammarus species. The species diversity related to habitat and the subgeneric classification of Jesogammarus are briefly discussed.