Medaka
The Japanese rice fish (Oryzias latipes), also known as the medaka, is a member of genus Oryzias (ricefish), the only genus in the subfamily Oryziinae. This small (up to about 3.6 cm or 1.4 in) native of Japan is a denizen of rice paddies, marshes, ponds, slow-moving streams and tide pools. It is euryhaline, occurring in both brackish and freshwater. It became popular as an aquarium fish because of its hardiness and pleasant coloration: its coloration varies from creamy-white to yellowish in the wild to white, creamy-yellow, or orange in aquarium-bred individuals. Bright yellow, red or green transgenic populations, similar to GloFish, have also been developed, but are banned from sale in the EU. The medaka has been a popular pet since the 17th century in Japan. After fertilization, the female carries her eggs attached anterior to the anal fin for a period before depositing them on plants or similar things.
As originally defined, O. latipes was native to much of east and mainland southeast Asia, but in recent decades most of these populations have been split off as separate species based on morphological (morphometrics and meristics) and genetic evidence. This limits the native range of definite O. latipes to Japan: eastern and southern Honshu, Shikoku, Kyushu, and smaller southern islands in the country. Formerly included in this species but now regarded as separate are O. sakaizumii in northwestern Honshu in Japan (locally, it hybridizes with O. latipes), and O. sinensis (Chinese rice fish) in much of China, west Korea and parts of mainland southeast Asia. The taxonomic position of certain populations, including some in China, Laos and east Korea, is unclear and require further study. It is possible that all these Chinese populations are part of O. sinensis, but the Laos specimens are relatively large, similar to O. latipes rather than the tiny O. sinensis. The east Korean population is part of a clade with O. sakaizumii and O. latipes. Based on morphology it is closer to O. sakaizumii than O. latipes, but it may be an undescribed species.
O. latipes has been introduced to Hokkaido in northern Japan (where ricefish are not native). There are other reports of introductions around the world, but at least most of those in mainland Asia and Europe involve O. sinensis (Chinese rice fish).
Medaka live in small ponds, shallow rivers, and rice fields. They can survive in a wide range of water temperatures (0–42 °C or 32–108 °F), but they prefer a water temperature of 15–28 °C (59–82 °F). Since they eat juvenile mosquitoes and small plankton, they are known as a beneficial organism for humans. They produce 10–20 eggs per birth, and they can produce eggs every day in laboratory conditions. They are seasonal breeding animals and usually lay eggs between spring and summer. They prefer to lay eggs around water grass and often prefer living in rice fields. The egg usually requires 4–10 days to hatch. They have an advanced renal function, which enables them to live in saltwater and brackish water. The average life span of this species in the wild is estimated to be 2 years, though in laboratory conditions they can survive 3–5 years. They live in schools, and they can recognize the faces of other individual medaka.
Medaka reproduce on a daily basis, which is an optimal trait for studying their reproductive biology. Researchers have studied HPG axis activities intensively in this species. Moreover, the medaka is the first non-mammalian vertebrate species for which a sex-determination gene (DMY) has been identified, their sex is reversible by sex steroid manipulation, and they exhibit morphological sexual dimorphism between males and females. Furthermore, some methods such as ovariectomy and altered light-dark cycles have been developed to study the mechanism of reproduction in medaka.
The medaka is listed as a least-concern species in the IUCN red list. The justification of this categorization is that this species is living in widespread habitat (755,000 km2) and is relatively abundant in its various habitats. However, it is considered an endangered species by Japan's Ministry of the Environment. Many local communities try to preserve wild medaka in Japan.
