The Kyushu bitterling (Rhodeus atremius) is a temperate freshwater fish belonging to the Acheilognathinae subfamily of the Cyprinidae family. It originates on Kyushu Island in Japan. It was originally described as Acanthorhodeus atremius by Jordan & Thompson in 1914. The fish reaches a size of up to 6 cm (2.5 in), and is native to freshwater habitats with a pH of 6.8 to 7.8, a hardness of 20 DH, and a temperature of 10 to 25 °C (50 to 77 °F). When spawning, the females deposit their eggs inside bivalves, where they hatch and the young remain until they can swim.