The Tokyo bitterling (Pseudorhodeus tanago) is a temperate freshwater fish of the carp family (Cyprinidae).

[3] In the wild, this fish is found only on the Kantō Plain of Japan, an area near the capital city, Tokyo.

The fish was formerly abundant in small streams, but its habitat has been overrun by people and pollution.

Bitterlings lay their eggs in freshwater mussel shells.

The Tokyo bitterling lays its eggs in only one type of mussel shell, limiting its chances of successful breeding.