Distribution and morphological variation of the freshwater halacarid mite Lobohalacarus weberi (Romijn & Viets, 1924) (Acari, Halacaridae) in Japan

The morphological variations of the subterranean freshwater halacarid mite Lobohalacarus weberi (Romijn & Viets) were examined for five regional populations in the Japanese Archipelago. When examining a total of 12 females and four deutonymphs, one of the four females in a single population had almost no frontal spine. Variations in the number of genital acetabula, perigenital setae, and leg chaetotaxy accorded well with the results of morphological variations in previous studies. The present distributional results and previous records of L. weberi in Japan showed the possibility of the species being distributed throughout the Japanese Archipelago. Considering the entire distributional records on the continent neighbouring Japan and the Japanese Archipelago lying relatively close to the continent, relictualism seems to effectively explain the current distribution of L. weberi in the Japanese Archipelago and on the continental landmass.