Oryzias latipes

The Medaka (目高) or Japanese killifish, Oryzias latipes, is a very small ricefish, popular as an aquarium fish native to Southeast Asia. It is approximately 4 cm long. Its coloration varies, from brown or yellow-gold in the wild to white, creamy yellow, or orange in aquarium-bred individuals. The medaka has been a popular pet since the 17th century in Japan. It is amphidromous, meaning it moves between salt and freshwater at some point in its life. It is found in both ocean and river habitats. The medaka is a common denizen of rice paddies in coastal Asia. The female carries her eggs attached between her anal fins.

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Oryzias latipes is a common model organism used in biological research. It is a simple, short-lived, hardy species that is reproductively prolific and easy to rear in the laboratory. It can withstand cold and can be shipped easily. Nearly all aspects of the life cycle of the medaka have been repeatedly analyzed by laboratory biologists. Sexual behavior, genetic inheritance of coloration, spawning habits, feeding, pathology, embryological development, and ecology of the fish are all published and replicated. Its neuroanatomy and genome are charted.

Transgenic medaka are relatively easy to produce. The fish has been genetically modified to secrete various human hormones, express promoter sequences from other fish, and make antimicrobial proteins and a protein that makes the medaka glow fluorescent green. 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.

O. latipes has gone into space aboard the space shuttle. It carries the distinction of having been the first vertebrate to mate in orbit. The result of the mating was a brood of healthy fry, hatched on the space shuttle Columbia in 1994.

References

• FishBase summary
• Medaka aboard Columbia
• Leroi, Armand Marie. (2003). Mutants: On Genetic Variety and the Human Body. New York: Viking. ISBN 0-670-03110-0
• Tanaka M, Kinoshita M, Kobayashi D, Nagahama Y. (2001). Establishment of medaka (Oryzias latipes) transgenic lines with the expression of green fluorescent protein fluorescence exclusively in germ cells: a useful model to monitor germ cells in a live vertebrate. Proc Natl Acad Sci USA 98(5):2544-9.