Japanese rice fish

The Japanese rice fish (Oryzias latipes) also known as the medaka,[2] 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 East Asia is a denizen of rice paddies, marshes, ponds, slow-moving streams and tide pools.[3][4] It is euryhaline, occurring in both brackish and freshwater.[3] 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.[5] The medaka has been a popular pet since the 17th century in Japan.[6] After fertilization, the female carries her eggs attached anterior to the anal fin for a period before depositing them on plants or alike.[5]

For the manga and anime series, see Medaka Box.

Japanese rice fish
Oryzias latipes

Least Concern  (IUCN 3.1)[1]
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Actinopterygii
Order: Beloniformes
Family: Adrianichthyidae
Genus: Oryzias
Species:
O. latipes
Binomial name
Oryzias latipes
Synonyms[2]
  • Poecilia latipes Temminck & Schlegel, 1846
  • Aplocheilus latipes (Temminck & Schlegel, 1846)

Ecology

Medaka live in small ponds, shallow rivers, and rice fields.[7] They can survive in a wide range of water temperatures (3℃-42℃), but they prefer a water temperature of 15℃-28℃.[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 a rice field.[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, yet in laboratory conditions they will survive 3–5 years. They live in schools, and they can recognize other fish's faces.[12]  

Taxonomy and range
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]

Origin of Southern and Northern Population

Phylogenetic analysis shows that the southern population was derived from Northern Kyushu area and spread into Honshu island. On the other hand, Northern population was derived from Tango and spread alongside with Japan sea coast.[17] Orizias Latipes is known to have nine sub populations, East Japanese type, East Setouchi type, West Setouchi type, Saninn type, Northern Kyusyu type, Osumi Type, Ariake type, Satuma type and Ryukyu type. These sub-populations are mixed each other due to artificial releasing and decreasing local genetic diversity.

Use in science
An orange aquarium variant 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 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]

In space

O. latipes carries the distinction of having been the first vertebrate to mate in orbit.[24] The result of the mating was a brood of healthy fry, hatched on the Space Shuttle Columbia in 1994. O. latipes returned to space in 2012, launched aboard a Soyuz spacecraft Soyuz TMA-06M and housed in an aquarium aboard the International Space Station.

Inbreeding lines

The possibility of serial inbreeding facilitates genetic research due to reduction of heterozygous sites in the genome. In medaka it is relatively easy to establish inbred lines, unlike other model species like zebrafish and mice.[25] By 1979, researchers had generated 10 inbred strains.[26] This inbred lines made medaka a model species for scientific research in genetics.[27][28] In 2014, work began on generating 111 different inbred lines derived from a single population collected in the wild.[29]

Understanding Sex and Reproduction by Medaka

Medaka reproduce on daily basis, which is optimal trait to study reproduction biology. The researchers study HPG axis activities intensively in this species.[30][31] Moreover, Medaka is first non-mammal vertebrate species that sex-determination gene(dmy) was identified[32] and their sex is reversible by sex steroid manipulation[33] and they exhibit morphological sexual dimorphism exist between male and female. Furthermore, some methods such as ovariectomy [34] and changing light-dark cycle[35] are developed to study the mechanism of reproduction in Medaka.  

Immunology

Understanding that the homing of lymphocytes T to the thymus is not specific to the mammalians only but we can find it on other vertebrates.

Conservation of Medaka

Status

Medaka is listed as least concern species by IUCN red list. The justification of this categorization is that this species is living in widespread habitat(755,000 km2) and relatively abundant in the various habitat.[36] Whereas, Medaka is considered endangered species by Ministry of Environment in Japan.[37] Many local communities try to preserve wild Medaka in Japan[38][39][40]

Concerns

There are two major conservational concerns about Medaka, habitat degradation and hybridization with domesticated Medaka(Himedaka). Due to modernization of rice field and irrigation canal, the optimal place for Medaka's reproduction is massively decreasing.[7] In addition, recent studies confirmed that Himedaka was introduced into many local regions by artificial release.[41] This will eliminate local genetic adaptation of each sub-population of Medaka.[41][42] Furthermore, Since Himedaka has vivid orange body color, the hybrid will attract more predators and decrease the total population of Medaka.[43] In 2011, the researchers discovered that almost 15% of wild caught Medaka in Nara had Himedaka specific gene marker.[44] In addition to these concerns, invasive species like mosquitofish competes with Medaka by sharing same habitat. The study reports over 70% Medaka was injured their tail fins by attack from Mosquitofish.[45] The damage at anal fin will decrease offspring by preventing courtship behavior.[46] Recently, the transgenic line was brought from Taiwan for commercial purpose. This transgenic line was introduced a gene which can express the green fluorescence and make body glowing. Now, this transgenic line was released into the wild and causing genetic pollution.[47] There is no comprehensive study of population size of Medaka. But the genomic analysis of one of Medaka sub-population indicates that their effective population size is around 25000-70000.[48]   

Social importance in Japan

Medaka has a 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 $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 utilised 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.

See also
  • Mummichog (Fundulus heteroclitus), sent to space in 1973 as the first fish

References
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