The Tokyo bitterling (''Pseudorhodeus tanago'') is a

temperate In geography, the temperate climates of Earth occur in the middle latitudes (approximately 23.5° to 66.5° N/S of the Equator), which span between the tropics and the polar regions of Earth. These zones generally have wider temperature ran ...

freshwater Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. The term excludes seawater and brackish water, but it does include non-salty mi ...

ray-finned fish Actinopterygii (; ), members of which are known as ray-finned fish or actinopterygians, is a class of bony fish that comprise over 50% of living vertebrate species. They are so called because of their lightly built fins made of webbings of sk ...

belonging to the

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

Acheilognathidae The bitterling-like cyprinids form the cyprinoid family Acheilognathidae. This subfamily contains seven genera, although the Khanka spiny bitterling is often placed in ''Acheilognathus'', and at least 75 described species to date. Over half of t ...

. This fish is

endemic Endemism is the state of a species being found only in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also foun ...

to the island of

Honshu , historically known as , is the largest of the four main islands of Japan. It lies between the Pacific Ocean (east) and the Sea of Japan (west). It is the list of islands by area, seventh-largest island in the world, and the list of islands by ...

in Japan. The species was first described as ''Rhodeus tanago'' by

Shigeho Tanaka was a Japanese ichthyologist and professor of zoology at the Imperial University of Tokyo. He published numerous works on fishes and sharks and co-authored a book on Japanese fish with famous American scientist David Starr Jordan David Starr Jo ...

in 1909. It is widely known as ''Tanakia tanago'', although a 2014 study suggests it is genetically distinct from other '' Tanakia'' species, and warrants placement of it in the monotypic genus ''Pseudorhodeus''.

Distribution

In the wild, this fish is found only on the

Kantō Plain The , in the Kantō region of central Honshu, is the largest plain in Japan. Its 17,000 km2 covers more than half of the region extending over Tokyo, Saitama Prefecture, Kanagawa Prefecture, Chiba Prefecture, Gunma Prefecture, Tochigi Prefe ...

Japan Japan is an island country in East Asia. Located in the Pacific Ocean off the northeast coast of the Asia, Asian mainland, it is bordered on the west by the Sea of Japan and extends from the Sea of Okhotsk in the north to the East China Sea ...

, 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.

Threats

There is a real risk that it could become extinct in the wild. It also suffers from competition from the related but more aggressive

rosy bitterling The rosy bitterling or Tairiku baratanago (''Rhodeus ocellatus'') is a small freshwater ray-finned fish belonging to the family Acheilognathidae, the bitterlings. This species occurs in East Asia from the Amur River basin to the Pearl River basi ...

. Bitterlings lay their eggs in

freshwater mussel Freshwater bivalves are molluscs of the order Bivalvia that inhabit freshwater ecosystems. They are one of the two main groups of freshwater molluscs, along with freshwater snails. The majority of bivalve molluscs are saltwater species that l ...

shells. The Tokyo bitterling lays its eggs in only one type of mussel shell, limiting its chances of successful breeding. To help protect the fish, it has been declared a "national monument" by the

Japanese government The Government of Japan is the central government of Japan. It consists of legislative, executive and judiciary branches and functions under the framework established by the Constitution of Japan. Japan is a unitary state, containing forty- ...

, and this gives it special protection.

References

Acheilognathidae Natural monuments of Japan Freshwater fish of Japan Endemic fish of Japan Fish described in 1909 Taxa named by Shigeho Tanaka {{Acheilognathinae-stub