Oryzias latipes

Oryzias latipes, also known as Medaka and Japanese killifish, is a member of genus Oryzias (ricefish), the only genus in the subfamily Oryziinae. This rather small (2-4 cm long) native of Southeast Asia is a common denizen of rice paddies in coastal Asia. It became popular as an aquarium fish because of its hardiness and pleasant coloration: 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 female carries her eggs attached between her anal fins.

Oryzias latipes is a model organism and is extensively used in many areas of biological research. 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. It has relatively small genome (~800 Mb, 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) also hardier growth in a broad temperature range (6-40 degrees C).

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. 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.

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.