Japanese rice fish

Medaka live in small ponds, shallow rivers, and rice fields.[7] They can survive in a wide range of water temperatures (3–42 °C or 37–108 °F), but they prefer a water temperature of 15–28 °C (59–82 °F).[8] 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.[9] They prefer to lay eggs around water grass and often prefer living in rice fields.[7] The egg usually requires 4–10 days to hatch.[10] They have an advanced renal function, which enables them to live in saltwater and brackish water.[11] 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.[12]  

A group in a shallow ditch, a typical habitat of the species (Katori, Japan)

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.[4][13] This limits the native range of definite O. latipes to Japan: eastern and southern Honshu, Shikoku, Kyushu, and smaller southern islands in the country.[13] 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.[4][13][14] The taxonomic position of certain populations, including some in China, Laos and east Korea, is unclear and require further study.[4][13] 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.[4] 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.[13]

O. latipes has been introduced to Hokkaido in northern Japan (where ricefish are not native).[15] 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).[15][16]

An orange aquarium variant (himedaka) achieved through selective breeding, photographed from above. Not to be confused with the brightly-colored transgenic aquarium variants.

Oryzias latipes is a model organism and is extensively used in many areas of biological research, most notably in toxicology. Medaka have a short gestation period, and are reproductively prolific—characteristics that make them easy to rear in the laboratory. They can withstand cold and can be shipped easily. Nearly all aspects of the life cycle of medaka have been analyzed by researchers including sexual behavior, genetic inheritance of coloration, spawning habits, feeding, pathology, embryological development, ecology, etc.[18][19] It has a relatively small genome (~800 mega base pairs, half the size of the genome of another popular model fish, the zebrafish) as well as a generation time of 7 weeks (rather than 9 weeks for zebrafish) and hardier growth in a broad temperature range (6–40 °C or 43–104 °F).[20][21]

Transgenic medaka are relatively easy to produce. They have been genetically modified to secrete various human hormones, express promoter sequences from other fish, and to make antimicrobial proteins and a protein that makes the medaka glow fluorescent green, yellow or red.[5][22] There are also many mutations that show up in medaka at random, for example, a mutant strain that lacks scales, and one with extra-long fins. Haploid embryonic stem cell lines have been established.[23]

The medaka has been kept as a domesticated pet in Japan for centuries. In recent years the fish has gained further popularity, with some rarer breeds valued at over 1 million yen (approximately US$10,000)—though the most common varieties (like himedaka) can be purchased for around 50 yen per fish.[49] Currently, 456 commercial strains are documented and available for fishkeeping.[50] Medaka are not only kept as pets but also widely utilized in education; Japanese elementary school classes often raise medaka in order to give the students firsthand experience with caring for live organisms, as well as to foster more broad appreciation for animals' life cycles.

• Mummichog (Fundulus heteroclitus), first fish sent to space in 1973

Look up Medaka in Wiktionary, the free dictionary.

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Oryzias latipes (category)

Wikispecies has information related to Oryzias latipes.

• View the oryLat2 genome assembly in the UCSC Genome Browser.
• Medaka genome in Ensembl.