Research Activities
Confirming a Non-Native Freshwater Fish With an Extreme Resemblance to an Extinct Species

Rhodeus smithii smithii from Saga Prefecture (above) and
Rhodeus suigensis from Okayama Prefecture (below)

Yasunari Tanaka of the Kwansei Gakuin University Graduate School of Science and Technology (2nd-year Master’s student) and  science (biology) instructor Koji Tominaga at the Kwansei Gakuin Senior High School confirmed the presence of Rhodeus smithii smithii—a non-native freshwater fish that is extremely similar to Rhodeus suigensis, ⁠which previously went extinct in Hyogo Prefecture⁠—in the Chikusa River system, the habitat of the Rhodeus suigensis. The results of this research project were published early on April 28 in the online edition of the Japanese Journal of Ichthyology, a publication by the Ichthyological Society of Japan. 

Rhodeus smithii smithii (pictured above) is a freshwater fish belonging to the Acheilognathinae subfamily in the carp family, and is distributed throughout northwestern Kyushu. In the past, Rhodeus smithii smithii and the extremely similar-looking Rhodeus suigensis (pictured below), which has a subspecies (referring to populations of the same species that are regionally isolated and morphologically distinct from each other) connection with the former, were distributed throughout the Chikusa River system, located in the western part of Hyogo. However, their presence had not been confirmed since 1977, and they were thought to have gone extinct. The Chikusa River system is the eastern limit of the distribution of Rhodeus suigensis, which is currently found only in Okayama and Hiroshima prefectures. The Rhodeus suigensis is listed as an endangered species on the Ministry of the Environment’s red list as having a very high risk of going extinct in the wild in the near future, and is also designated by the Act for the Conservation of Endangered Species of Wild Fauna and Flora.

Although there are slight morphological differences between Rhodeus smithii smithii and Rhodeus suigensis, they look extremely similar and it is difficult to tell them apart based on their appearance. However, since their distributions do not overlap and they have been separated over a long period of time, they can be clearly differentiated through their genes.  In this research project, the authors examined the external morphology and genetics of fish collected in the Chikusa River system with characteristics corresponding to either of the previously-mentioned subspecies. As a result, it was found that both the external morphology and the genes of the fish corresponded to Rhodeus smithii smithii, and it was presumed from the results of genetic analysis that it was highly likely that they were brought in from Saga Prefecture.

The Acheilognathinae have a unique reproductive ecology wherein they lay their eggs in the gills of living freshwater bivalves, and during spawning season, their bright breeding colors appear, which has made them popular as ornamental fish and targets for fishing. Many of the Acheilognathinae native to Japan are listed on the red list, and from that conservation awareness, there have been many reports of confirmed cases where the same species of Acheilognathinae brought in from another area, and even species that were not originally distributed in that area, were released. Such releases can result in the loss of locally endemic genes and competition with native species of Acheilognathinae.

Even in the case of this research project, in the event there were Rhodeus suigensis that had survived in the Chikusa River system, they would have been genetically contaminated through crossbreeding with Rhodeus smithii smithii, and there may have been adverse effects such as competition with other native Acheilognathinae that inhabit the Chikusa River system. It is also possible that by mistaking Rhodeus smithii smithii for Rhodeus suigensis, people may get the wrong impression that an extinct species has been rediscovered. On the other hand, Rhodeus smithii smithii is also an endangered species on the Ministry of the Environment’s red list. This study makes a report about this native freshwater fish, which includes various problems, and will provide key findings for taking measures in the future.