Sakhalin ninespine stickleback Pungitius tymensis distributed in the Yubetsu River, eastern Hokkaido, Japan

Two specimens of Sakhalin ninespine stickleback Pungitius tymensis were collected from the Yubetsu River system in eastern Hokkaido, Japan. Morphological analysis showed that the lateral plates of P. tymensis in the Yubetsu River were significantly different from those of sympatric P. sinensis. Molecular phylogenetic analysis of mitochondrial cytochrome b gene sequences revealed that P. tymensis from the Yubetsu River had unique mtDNA haplotypes, differing from a previous study. The haplotype of P. tymensis and that of sympatric P. sinensis differed by nine nucleotide substitutions, suggesting reproductive isolation between the two species. However, the haplotype of P. tymensis constituted the same clade with haplotypes of widely distributed P. sinensis, indicating that the haplotype was introgressed from P. sinensis. The haplotype of P. tymensis in the Yubetsu River was closely related to two haplotypes of P. tymensis in the Teshio River, suggesting that P. tymensis in the Yubetsu River spread from the northern part of Hokkaido and evolved independently since then. As this is the first report with voucher specimens of P. tymensis from the Yubetsu River system, it provides a basis for the species conservation in the area.