Syllabic (left) and traditional (right) writing of medaka in japanese language by Ishida Kayoko.
Medaka, Oryzias latipes, is a small, egg-laying freshwater teleostean fish primary found in Japan , but also in many countries of eastern Asia . Like in other Asian countries Japanese people has a special interest in fish and particularly in medaka. Since the fish was worthless for commerce because of its small size, its name never need to be unified. There are actually nearly 5000 denominations for medaka throughout Japan, a record in this language. The most wide-spread name is nevertheless “me-da-ka” which is written in syllabic alphabet “me-da-ka” or in traditional writing. Ideograms designing medaka are “eye” and “big” which allude to the big size of eyes compared to body in the fish. (Opposite figure )
Easy to breed medaka has been used since the beginning of the last century in experimentation in Japan . As soon as 1913, gene transmission, sex determination and pigmentation were studied in the medaka. It is actually the first fish in which Mendelian inheritance ( Toyama 1916) and partial inheritance linked to Y chromosome (Aida 1921) was shown.
For about twenty years medaka has become a lab animal in USA and Europe as well. The expansion of the use of this model is linked to advances in embryology, genetics and ecotoxicology.
A few words on medaka biology...
Medaka is a very hardy and robust animal which tolerates a wide range of temperatures (10-40°C) and salinity though it is a fresh water fish.
Living near water surface it is a microphagous fish which feeds on phyto- and zooplankton.
Medaka are egg-laying fishes. Sexual dimorphism is rather discreet: males and females can be distinguished by the shape of the dorsal fin.
Mating induces spawning and liberation of sperm ; fecundation is external. After spawning eggs are connected to the ventral side of the female body by attachment filaments. Mating seasons correlate with light cycles. Between 30 and 50 eggs are spawned every day; up to 3000 eggs are produced during a mating season.
Spawning occurs in females only in presence of males with a mating behavior. However to obtain unfertilizized oocytes, sterile males obtained by interbreeding two different Oryzias species, O. celebensis and O. latipes for example can be used to induce spawning. Males obtained are sterile but by their behavior induce spawning by the females.
Medaka development is fast. First cleavages take about 30 minutes at 28°C, gastrulation begins 8,5h post-fecundation and neural tube is prominent after15h. Eggs hatch as feeding young adult after 7 days. In the laboratory, medaka eggs may be raised in Petri dishes at a maximum density of 200 eggs per 9 cm diameter dish. Up to three months, early medaka embryos can be maintained at temperatures as low as 4°C to slow down their development.
-WITTBRODT J, SHIMA A, SCHARTL M. Médaka-A model organism from the far east. Nature reviews, 2002, 3 , 53-64.
-SHIMA A, MITANI H. Medaka as a research organism: past, present and future, Mechanisms of Development, 2004, 121 , 599-604.
Digital atlas of topographic histology