TheJapanese rice fish (Oryzias latipes), also known as themedaka,[2] is a member of genusOryzias (ricefish), the only genus in the subfamily Oryziinae. This small (up to about 3.6 cm or 1.4 in) native ofJapan is a denizen ofrice paddies, marshes, ponds, slow-moving streams andtide pools.[3][4] It iseuryhaline, occurring in bothbrackish 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 greentransgenic populations, similar toGloFish, have also been developed, but are banned from sale in theEU.[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 theanal fin for a period before depositing them on plants or similar things.[5]
Medaka live in small ponds, shallow rivers, and rice fields.[7] They can survive in a wide range of water temperatures (0–42 °C or 32–108 °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 ofeast andmainland southeast Asia, but in recent decades most of these populations have been split off as separate species based onmorphological (morphometrics andmeristics) andgenetic evidence.[4][13] This limits the native range of definiteO. latipes to Japan: eastern and southernHonshu,Shikoku,Kyushu, and smaller southern islands in the country.[13] Formerly included in this species but now regarded as separate areO. sakaizumii in northwestern Honshu in Japan (locally, ithybridizes withO. latipes), andO. sinensis (Chinese rice fish) in much of China, westKorea and parts of mainland southeast Asia.[4][13][14] Thetaxonomic 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 ofO. sinensis, but the Laos specimens are relatively large, similar toO. latipes rather than the tinyO. sinensis.[4] The east Korean population is part of aclade withO. sakaizumii andO. latipes. Based on morphology it is closer toO. sakaizumii thanO. latipes, but it may be anundescribed species.[13]
O. latipes has beenintroduced toHokkaido 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 involveO. sinensis (Chinese rice fish).[15][16]
Origin of Southern and Northern Japanese populations
Phylogenetic analysis shows that the southern Japanese population was derived from that of the northern Kyushu area and spread into Honshu. On the other hand, the northern population was derived from a population from theTajima-Tango region and spread alongside theSea of Japan coast.[17]O. latipes is known to have nine sub-populations: East Japanese type, East Setouchi type, West Setouchi type, San'in type, Northern Kyushu type, Osumi Type, Ariake type, Satsuma type and Ryukyu type. These sub-populations have been mixed with each other due to artificial releasing and decreasing local genetic diversity.[citation needed]
An orange aquarium variant (himedaka) achieved throughselective breeding, photographed from above. Not to be confused with the brightly coloredtransgenic aquarium variants.
Oryzias latipes is amodel organism and is extensively used in many areas of biological research, most notably intoxicology. 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 thelife cycle of medaka have been analyzed by researchers includingsexual behavior, genetic inheritance of coloration,spawning habits, feeding, pathology,embryological development,ecology, etc.[18][19] It has a relatively small genome (~800 megabase pairs, half the size of the genome of another popular model fish, thezebrafish) 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 humanhormones, expresspromoter sequences from other fish, and to make antimicrobial proteins and a protein that makes the medaka glowfluorescent 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]
O. latipes carries the distinction of having been the first vertebrate to mate inorbit.[24] The result of the mating was a brood of healthy fry, hatched on the Space ShuttleColumbia in 1994.O. latipes returned to space in 2012, launched aboard a Soyuz spacecraftSoyuz TMA-06M and housed in an aquarium aboard theInternational Space Station.[citation needed]
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 10inbred strains.[26] These 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]
Medaka reproduce on a daily basis, which is an optimal trait for studying their reproductive biology. Researchers have studiedHPG axis activities intensively in this species.[30][31] Moreover, the medaka is the first non-mammalian vertebrate species for which asex-determination gene (DMY) has been identified,[32] their sex is reversible bysex steroid manipulation,[33] and they exhibit morphologicalsexual dimorphism between males and females. Furthermore, some methods such asovariectomy[34] and altered light-dark cycles[35] have been developed to study the mechanism of reproduction in medaka.[citation needed]
The discovery thatT-lymphocytes home to thethymus in medaka has led to an understanding that this is not specific to mammalians but can be found in other vertebrates.[citation needed]
The medaka is listed as a least-concern species in the IUCN red list. The justification of this categorization is that this species is living in widespread habitat (755,000 km2) and is relatively abundant in its various habitats.[1] However, it is considered an endangered species by Japan'sMinistry of the Environment.[37] Many local communities try to preserve wild medaka in Japan.[38][39][40]
There are two major concerns about medaka conservation: habitat degradation and hybridization with domesticated medaka (himedaka). Due to modernization of rice fields and irrigation canals, optimal places for medaka reproduction are massively decreasing.[7] In addition, recent studies confirmed that himedaka have been introduced into many local regions by artificial release.[41] This will eliminate localgenetic adaptations of each sub-population of medaka.[41][42] Furthermore, since himedaka have vivid orange body color, the hybrids will attract more predators and thus decrease the total medaka population.[43] In 2011, researchers discovered that almost 15% of wild-caught medaka inNara had a himedaka-specific gene marker.[44] In addition to these concerns, invasive species likemosquitofish compete with medaka by sharing the same habitat. A study reported that over 70% of medaka had their tail fins injured by attacks from mosquitofish.[45] The damage at the anal fin will decrease medaka offspring by preventing courtship behavior.[46] In 2006, it was found that a transgenic line of medaka was brought to Japan from Taiwan for commercial purposes. This transgenic line had an introduced gene which expresses green fluorescence, making the body glow. Now, this transgenic line has been released into the wild and is causinggenetic pollution.[47] There is no comprehensive study of population size of medaka, but the genomic analysis of one medaka sub-population indicates that theireffective population size is around 25000–70000.[48]
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 forfishkeeping.[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.[citation needed]
^Sakamoto, Tatsuya; Kozaka, Tomohiro; Takahashi, Akiyoshi; Kawauchi, Hiroshi; Ando, Masaaki (2001-02-15). "Medaka (Oryzias latipes) as a model for hypoosmoregulation of euryhaline fishes".Aquaculture.193 (3):347–354.Bibcode:2001Aquac.193..347S.doi:10.1016/S0044-8486(00)00471-3.ISSN0044-8486.
^abcdeAsai, T.; Senou, H.; Hosoya, K. (2011). "Oryzias sakaizumii, a new ricefish from northern Japan (Teleostei: Adrianichthyidae)".Ichthyol. Explor. Freshwaters.22 (4):289–299.ISSN0936-9902.
^Kanda, Shinji (2018-11-27). "Evolution of the regulatory mechanisms for the hypothalamic-pituitary-gonadal axis in vertebrates–hypothesis from a comparative view".General and Comparative Endocrinology.284: 113075.doi:10.1016/j.ygcen.2018.11.014.ISSN0016-6480.PMID30500374.S2CID54468539.
^Matsuda, Masaru; Nagahama, Yoshitaka; Shinomiya, Ai; Sato, Tadashi; Matsuda, Chika; Kobayashi, Tohru; Morrey, Craig E.; Shibata, Naoki; Asakawa, Shuichi; Shimizu, Nobuyoshi; Hori, Hiroshi (May 2002). "DMY is a Y-specific DM-domain gene required for male development in the medaka fish".Nature.417 (6888):559–563.Bibcode:2002Natur.417..559M.doi:10.1038/nature751.ISSN1476-4687.PMID12037570.S2CID4363239.
^Weber, D. N.; Spieler, R. E. (1987-06-01). "Effects of the light-dark cycle and scheduled feeding on behavioral and reproductive rhythms of the cyprinodont fish, Medaka,Oryzias latipes".Experientia.43 (6):621–624.doi:10.1007/BF02126355.ISSN1420-9071.PMID3595795.S2CID11727260.
^Nakao, Ryohei; Kitagawa, Tadao (2015). "Differences in the behavior and ecology of wild type medaka (Oryzias latipes complex) and an orange commercial variety (himedaka)".Journal of Experimental Zoology Part A: Ecological Genetics and Physiology.323 (6):349–358.Bibcode:2015JEZA..323..349N.doi:10.1002/jez.1916.ISSN1932-5231.PMID26054930.
