Pseudogagrella amamiana (Opiliones: Sclerosomatidae) from Kanto District, Japan, as the first probable cases of domestically introduced species in Japanese harvestmen

A population of Pseudogagrella amamiana (Nakatsudi 1942) (Arachnida: Opiliones: Sclerosomatidae) was found from a town park in the suburbs of Yokohama City, Kanagawa Prefecture, Honshu, Japan, in 2017. Furthermore, another population of the species was found from Unomisaki Point, Ishihama Beach, Hitachi City, Ibaraki Prefecture, in 2019. This species has been known from the Ryukyu Islands (in and north of Kumejima Island), and sporadically from Kyushu and westernmost part of Honshu (Yamaguchi Prefecture). It is highly probable that these populations in Yokohama and Unomisaki Point have originated from a few males and females or a gravid female or soil litter containing eggs somehow introduced from the native ranges of the species, because the present localities are more than 700 km apart from the nearest known locality of the species in Yamaguchi Prefecture. The chromosome number of these populations showed 2n = 18, hence it seems that the population came from the native ranges of the species other than Okinawa and Kumejima Islands where it is 2n = 20.