Decline factors of Gyrinus japonicus (Coleoptera, Gyrinidae) in Saitama Prefecture, Honshu, Japan, with their distribution in neighboring prefecture, Yamanashi

Abstract

Gyrinus japonicus is one of the endangered species and has decreased rapidly recent years. This species was studied in the low mountain to mountain area of Saitama and Yamanashi prefecture. From this survey, it was found the habitat of G. japonicus has the following characteristics: 1) main habitat is dammed pools (type: debris dam) and scour pools (type: eddy) in slow water of the river, 2) such pools are often in the dark site surrounded by forests, 3) the water surface is almost uncovered by branches and aquatic plants, 4) water quality is high transparency and oligotrophic state, 5) several water beetles and bugs living in/on running water (ex. Orectochilus regimbarti, Platambus pictipennis, Hydrocassis lacustris and Metrocoris histrio) or that live in the dark site surrounded by forests (ex. Gerris gracilicornis, Gerris insularis and Aquarius elongatus) were collected with G. japonicus. The result of this survey suggested that local populations of this species have already isolated at the river (ex. pools) with such characteristics in low mountain area range in Saitama around 2000 to 2010. Afterward, this water beetle was considered to be extinct due to changes in the river environment such as a decrease in water volume.