The palaemonid shrimp genus Palaemon Weber 1795 is currently represented by 87 species worldwide, of which 36 species inhabit freshwater environments. In this study, we describe a new species of the genus, P. septemtrionalis, primarily based on material collected from rivers in Miyagi Prefecture, Tohoku District, northeastern Japan. The present new species is morphologically and genetically close to Palaemon paucidens De Haan, 1841, but it is morphologically distinguishable from the latter by the chela of the pereopod 2 being longer than the carpus (versus shorter than the carpus in P. paucidens) and the possession of a low, laminar convexity on the flexor margin of the pereopod 3 dactylus just proximal to the base of the unguis (such a laminar structure is absent in P. paucidens). Comparison of partial sequences of the mitochondrial 16S rRNA gene supports the recognition of the new species. Examination of museum collections and a BLAST search on GenBank revealed that the geographical range of the new species includes the Sea of Japan side ranging from Hokkaido to Hyogo Prefecture and the Pacific side ranging from Aomori to Miyagi Prefecture. An identification key to the 13 Japanese species of the genus is presented.

The life history of Palaemon paucidens, a common freshwater shrimp inhabits many freshwater areas in Japan, remains unclear. In the present study, we surveyed a single population in a stream in Kesennuma, Miyagi Prefecture, Japan to elucidate the life history of this species. The spatio-temporal distribution pattern of the surveyed population was investigated in samples from seven stations established along the upstream and downstream sections of the stream. Size composition, state of sexual maturation, and appearance of hatched larvae were observed in samples collected from the mouth of the stream. Molecular species identification was used to confirm that the samples were P. paucidens. The results indicated that the total population consisted of three year-classes, and that many of the matured individuals(one or two years old)of both sexes seasonally migrated to the mouth of the stream. However, because some individuals remained in the freshwater area upstream, the migration of this species at our study site was partial. Larvae appeared at the same time as adults migrated downstream, and larvae hatched only at night. Based on age estimations in the field and the spawning intervals of females in a laboratory, it was peculated that two-year-old individuals would die after reproduction while one-year-olds would migrate upstream again. As the seasonal migration of both males and females corresponded to reproduction, we conclude that P. paucidens at our study site show diadromous migration and that their main migration pattern is catadromy.

We developed short tandem repeat markers for Palaemon septemtrionalis and tested their utility for analysing population genetics. Six markers were selected for their genotyping ability and suitability for population genetic analyses, and the genetic differences between two populations from different rivers were evaluated using these markers. The pairwise fixation index value between the two populations was 0.01756 (P＝0.036), weak but statistically significant, suggesting a low level of genetic differentiation between the two populations. Although the results were not fully conclusive, we are confident these markers can be utilised for investigating the genetic population structure of P. septemtrionalis.