Genetically distinct three types (designated as A, B and C) in the common freshwater shrimp Palaemon paucidens in Japan have been determined using allozyme, 18S rDNA and 16S rDNA analyses. Based on the morphological and genetic analyses, Katogi et al. (2019) proposed type B population distributing in the northern Japan to be a new species Palaemon septemtrionalis. In the present study, phylogenetic analysis in amphidromous type B was performed using 16S rDNA sequences of 185 individuals collected throughout the distributional range of this species in Japan. Two highly divergent clades (designated as B-I and B-II) were observed in the phylogenetic tree, in which the mean nucleotide sequence divergence (K2P) between the clades was 3.9 %. B-II clade was further subdivided into subclades B-IIa and B-IIb, in which the mean K2P between the subclades was 1 %. Geographic distributions of these clades were clearly separated. B-I corresponding to P. septemtrionalis was distributed in the northern Japan (north of Miyagi Prefecture to Hokkaido) and rivers on the Sea of Japan side. B-IIa and B-IIb were observed in rivers on the Pacific Ocean side. B-IIa was distributed from the northern Japan (north of Miyagi Prefecture) to central Japan (Kanagawa Prefecture) and B-IIb from central Japan (Shizuoka Prefecture) to southern Japan (Kyushu). Northern area of Miyagi Prefecture was determined to be the boundary zone between B-I and B-IIa, where B-I and B-IIa individuals were observed to coexist in a river of the Oshika Peninsula. Sagami Bay at central Japan was considered to be the boundary between B-IIa and B-IIb. In the appendage morphology, longer propodus than carpus in the second pereiopod, that was reported as one of specific characteristics of P. septemtrionalis, but we found this to be size dependent characteristic and not to be applicable for species identification.
View full abstract