A new species of Mundochthonius Chamberlin, 1929 (Pseudoscorpiones, Chthoniidae) from Tsushima Island, Japan with notes on cryptic diversity and biogeography of the Japanese Archipelago

Species of the pseudoscorpion genus Mundochthonius (Chthoniidae) are difficult to distinguish using traditional morphology, as diagnostic traits exhibit limited variation despite substantial genetic divergence. Here, we used publicly available sequence data to reconstruct the phylogeny of Japanese Mundochthonius and applied multiple species-delimitation methods to assess diversity and biogeographic patterns. Phylogenetic analyses recovered seven major genetic clades consistent with previous studies, while five delimitation approaches indicated substantially higher species diversity than currently recognized.

These clades exhibit strong geographic structuring: clades 1, 3, 5, 6, and 7 are restricted to eastern Japan, whereas clade 4 is broadly distributed in western Japan, a pattern corresponding to the Itoigawa-Shizuoka Tectonic Line, which demarcates the western boundary of the Fossa Magna region. We describe Mundochthonius tsushimaensis sp. nov., a genetically distinct lineage endemic to Tsushima Island, which is situated between the Korean peninsula and Kyushu island of the Japanese Archipelago.

Although the new species lacks clear morphological differentiation from other members of the clade, it displays a COI p-distance of 14 % from all congeners, indicating substantial genetic divergence that is further corroborated by the geographical isolation of the island. Additionally, we elevate Mundochthonius japonicus scolytidis to species rank (Mundochthonius scolytidis stat. nov.) based on genetic distinctiveness.

We propose that the phylogeographic structure of Japanese Mundochthonius reflects a refugial model, where deeply divergent, range-restricted eastern clades originated on the complex geological history of the Japanese Archipelago. These findings underscore the importance of integrating molecular and biogeographic data for resolving taxonomy in morphologically conserved groups.