Taxonomic Reassessment of the Izumo Lineage of Hynobius utsunomiyaorum: Description of a New Species from Chugoku, Japan

SIMPLE SUMMARY: In the study, we investigated the true taxonomic composition of Hynobius utsunomiyaorum from Chugoku District, Japan. Our detailed morphological and molecular comparisons showed that H. utsunomiyaorum is clearly divided into two species: the true H. utsunomiyaorum and the Izumo Lineage of H. utsunomiyaorum, i.e., Hynobius kunibiki sp. nov. This new species does not satisfy the diagnosis of H. utsunomiyaorum, while these two species do not form a monophyletic group based on molecular data. Hynobius setoi is morphologically more similar to the new species than is H. utsunomiyaorum. Previous studies have suggested that H. setoi is distributed across the San’in region, i.e., the northern part of Chugoku District, Japan. However, our research revealed that H. setoi is not distributed in the northern part of Shimane Prefecture, located in the western part of the San’in region in Chugoku District. Rather than H. setoi, we found that the new species was distributed in the northern part of Shimane Prefecture. Additionally, results of phylogenetic analyses using all valid Japanese Hynobius species are provided in our study. Based on these results, we show that Japanese Hynobius species included in the subgenus Hynobius may be divided into four genetic clades. The information in our study will be vital for developing conservation management strategies and policies for these species. ABSTRACT: Here, we describe a new species of the genus Hynobius from Chugoku, Japan. In populations from central to eastern Shimane Prefecture, the Izumo Lineage of Hynobius utsunomiyaorum was clearly distinguished from the true Hynobius utsunomiyaorum based on morphological and molecular evidence. Thus, we describe the former lineage as a new species, Hynobius kunibiki sp. nov. Morphological comparisons revealed that H. utsunomiyaorum lacks a distinct yellow line on the ventral side of its tail, whereas the new species possesses this yellow line; most H. utsunomiyaorum individuals have distinct white spots on the lateral sides of their body and lack a fifth toe, whereas the new species largely lacks these spots, and all examined individuals had a fifth toe. The two species also differed significantly by several other morphological characteristics. The lentic species Hynobius setoi is morphologically similar to H. kunibiki sp. nov., but they differ significantly by various morphological characteristics. Despite their partial morphological similarity, these two species differed substantially in terms of their genetics. Finally, we show, in a phylogenetic tree including all Japanese Hynobius species, that the subgenus Hynobius can be divided into four genetic clades. Overall, this information will help develop conservation management strategies and policies for these species.