First Japanese record of brood-caring halacarid mites Limnohalacarus (Acari: Halacaridae), with notes on the biogeographical implications

Faunal investigation of freshwater halacarid mites was carried out on Ishigaki Island in the southern part of the Ryukyus in Japan. The research was executed in five small streams on the island in 2022 and 2023. During the survey, two halacarid species, Limnohalacarus cultellatus and Lobohalacarus weberi were obtained only from a mountain stream rising from Mt. Omoto. The genus Limnohalacarus was reported for the first time from Japan in this study. Based on the present report of the two halacarid species on Ishigaki Island and the former halacarid record from a spring on Miyako Island, the formation history of the distribution of freshwater halacarids in the Southern Ryukyus was presumed. The freshwater halacarids would disperse from the Eurasian continent to these islands when these islands were a part of the western margin of the continent. Afterward, most of the Ryukyus was inundated by the sea. However, some highland areas have probably escaped from sinking into the sea until the present time. Consequently, it is presumed that the current local distribution of freshwater halacarids on Ishigaki and Miyako Islands has been maintained by the relatively highland area that became a refugium.