The Japanese rice fish (''Oryzias latipes''), also known as the medaka,
is a member of genus ''Oryzias'' (
ricefish), the only genus in the subfamily Oryziinae. This small (up to about ) native of
East Asia
East Asia is the eastern region of Asia, which is defined in both Geography, geographical and culture, ethno-cultural terms. The modern State (polity), states of East Asia include China, Japan, Mongolia, North Korea, South Korea, and Taiwan. ...
is a denizen of
rice paddies
A paddy field is a flooded field of arable land used for growing semiaquatic crops, most notably rice and taro. It originates from the Neolithic rice-farming cultures of the Yangtze River basin in southern China, associated with pre-Au ...
, marshes, ponds, slow-moving streams and
tide pool
A tide pool or rock pool is a shallow pool of seawater that forms on the rocky intertidal shore. Many of these pools exist as separate bodies of water only at low tide.
Many tide pool habitats are home to especially adaptable animals that ...
s.
It is
euryhaline, occurring in both
brackish
Brackish water, sometimes termed brack water, is water occurring in a natural environment that has more salinity than freshwater, but not as much as seawater. It may result from mixing seawater (salt water) and fresh water together, as in estu ...
and freshwater.
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
A transgene is a gene that has been transferred naturally, or by any of a number of genetic engineering techniques, from one organism to another. The introduction of a transgene, in a process known as transgenesis, has the potential to change the ...
populations, similar to
GloFish
The GloFish is a patented and trademarked brand of genetically engineered fluorescent fish. They have been created from several different species of fish: zebrafish (''Danio rerio'') were the first GloFish available in pet stores, and recently te ...
, have also been developed, but are banned from sale in the
EU.
The medaka has been a popular pet since the 17th century in Japan. After fertilization, the female carries her eggs attached anterior to the
anal fin for a period before depositing them on plants or similar things.
Ecology
Medaka live in small ponds, shallow rivers, and rice fields.
They can survive in a wide range of water temperatures (), but they prefer a water temperature of . 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. They prefer to lay eggs around water grass and often prefer living in rice fields.
The egg usually requires 4–10 days to hatch. They have an advanced renal function, which enables them to live in saltwater and brackish water. 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.
Taxonomy and range
![School of Japanese killifish,Katori-city,Japan](https://upload.wikimedia.org/wikipedia/commons/5/5a/School_of_Japanese_killifish%2CKatori-city%2CJapan.JPG)
As originally defined, ''O. latipes'' was native to much of
east
East or Orient is one of the four cardinal directions or points of the compass. It is the opposite direction from west and is the direction from which the Sun rises on the Earth.
Etymology
As in other languages, the word is formed from the fac ...
and
mainland southeast Asia, but in recent decades most of these populations have been split off as separate species based on
morphological (
morphometrics
Morphometrics (from Greek μορϕή ''morphe'', "shape, form", and -μετρία ''metria'', "measurement") or morphometry refers to the quantitative analysis of ''form'', a concept that encompasses size and shape. Morphometric analyses are co ...
and
meristics Meristics is an area of ichthyology and herpetology which relates to counting quantitative features of fish and reptiles, such as the number of fins or scales. A meristic (countable trait) can be used to describe a particular species of fish, or us ...
) and
genetic evidence.
This limits the native range of definite ''O. latipes'' to Japan: eastern and southern Honshu
, historically called , is the largest and most populous island of Japan. It is located south of Hokkaidō across the Tsugaru Strait, north of Shikoku across the Inland Sea, and northeast of Kyūshū across the Kanmon Straits. The island se ...
, Shikoku
is the smallest of the four main islands of Japan. It is long and between wide. It has a population of 3.8 million (, 3.1%). It is south of Honshu and northeast of Kyushu. Shikoku's ancient names include ''Iyo-no-futana-shima'' (), '' ...
, Kyushu, and smaller southern islands in the country.[ 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
Korea ( ko, 한국, or , ) is a peninsular region in East Asia. Since 1945, it has been divided at or near the 38th parallel, with North Korea (Democratic People's Republic of Korea) comprising its northern half and South Korea (Republic o ...
and parts of mainland southeast Asia.[ The taxonomic position of certain populations, including some in China, Laos and east Korea, is unclear and require further study.][ 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''.][ 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
In taxonomy, an undescribed taxon is a taxon (for example, a species) that has been discovered, but not yet formally described and named. The various Nomenclature Codes specify the requirements for a new taxon to be validly described and named. U ...
.[
''O. latipes'' has been introduced to ]Hokkaido
is Japan's second largest island and comprises the largest and northernmost prefecture, making up its own region. The Tsugaru Strait separates Hokkaidō from Honshu; the two islands are connected by the undersea railway Seikan Tunnel.
The lar ...
in northern Japan (where ricefish are not native). 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).[
]
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 the Tajima-Tango
Tango is a partner dance and social dance that originated in the 1880s along the Río de la Plata, the natural border between Argentina and Uruguay. The tango was born in the impoverished port areas of these countries as the result of a combina ...
region and spread alongside the Sea of Japan
The Sea of Japan is the marginal sea between the Japanese archipelago, Sakhalin, the Korean Peninsula, and the mainland of the Russian Far East. The Japanese archipelago separates the sea from the Pacific Ocean. Like the Mediterranean Sea, i ...
coast. ''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.
Use in science
''Oryzias latipes'' is a model organism and is extensively used in many areas of biological research, most notably in toxicology
Toxicology is a scientific discipline, overlapping with biology, chemistry, pharmacology, and medicine, that involves the study of the adverse effects of chemical substances on living organisms and the practice of diagnosing and treating e ...
. 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
Life cycle, life-cycle, or lifecycle may refer to:
Science and academia
*Biological life cycle, the sequence of life stages that an organism undergoes from birth to reproduction ending with the production of the offspring
* Life-cycle hypothesis ...
of medaka have been analyzed by researchers including sexual behavior
Human sexual activity, human sexual practice or human sexual behaviour is the manner in which humans experience and express their sexuality. People engage in a variety of sexual acts, ranging from activities done alone (e.g., masturbation) t ...
, genetic inheritance of coloration, spawning habits, feeding, pathology, embryological development, ecology
Ecology () is the study of the relationships between living organisms, including humans, and their physical environment. Ecology considers organisms at the individual, population, community, ecosystem, and biosphere level. Ecology overl ...
, etc. It has a relatively small genome (~800 mega base pairs, half the size of the genome of another popular model fish, the zebrafish
The zebrafish (''Danio rerio'') is a freshwater fish belonging to the minnow family (Cyprinidae) of the order Cypriniformes. Native to South Asia, it is a popular aquarium fish, frequently sold under the trade name zebra danio (and thus often ca ...
) as well as a generation time of 7 weeks (rather than 9 weeks for zebrafish) and hardier growth in a broad temperature range ().
Transgenic
A transgene is a gene that has been transferred naturally, or by any of a number of genetic engineering techniques, from one organism to another. The introduction of a transgene, in a process known as transgenesis, has the potential to change the ...
medaka are relatively easy to produce. They have been genetically modified to secrete various human hormone
A hormone (from the Greek participle , "setting in motion") is a class of signaling molecules in multicellular organisms that are sent to distant organs by complex biological processes to regulate physiology and behavior. Hormones are require ...
s, express promoter sequences from other fish, and to make antimicrobial proteins and a protein that makes the medaka glow fluorescent
Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, ...
green, yellow or red.[ 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.
]
In space
''O. latipes'' carries the distinction of having been the first vertebrate to mate in orbit
In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as ...
. 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
Soyuz TMA-06M was a spaceflight to the International Space Station launched on 23 October 2012, transporting three members of the Expedition 33 crew. TMA-06M was the 115th flight of a Soyuz spacecraft, the first flight launching in 1967. Soyuz TM ...
and housed in an aquarium aboard the International Space Station
The International Space Station (ISS) is the largest modular space station currently in low Earth orbit. It is a multinational collaborative project involving five participating space agencies: NASA (United States), Roscosmos (Russia), JAXA ( ...
.
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. By 1979, researchers had generated 10 inbred strain
Inbred strains (also called inbred lines, or rarely for animals linear animals) are individuals of a particular species which are nearly identical to each other in genotype due to long inbreeding. A strain is inbred when it has undergone at least ...
s. These inbred lines made medaka a model species for scientific research in genetics. In 2014, work began on generating 111 different inbred lines derived from a single population collected in the wild.
Sex and reproduction
Medaka reproduce on a daily basis, which is an optimal trait for studying their reproductive biology. Researchers have studied HPG axis activities intensively in this species. Moreover, the medaka is the first non-mammalian vertebrate species for which a sex-determination gene (DMY) has been identified, their sex is reversible by sex steroid manipulation, and they exhibit morphological sexual dimorphism
Sexual dimorphism is the condition where the sexes of the same animal and/or plant species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction. The condition occurs in most an ...
between males and females. Furthermore, some methods such as ovariectomy and altered light-dark cycles have been developed to study the mechanism of reproduction in medaka.
Immunology
The discovery that T-lymphocytes
A T cell is a type of lymphocyte. T cells are one of the important white blood cells of the immune system and play a central role in the adaptive immune response. T cells can be distinguished from other lymphocytes by the presence of a T-cell r ...
home to the thymus
The thymus is a specialized primary lymphoid organ of the immune system. Within the thymus, thymus cell lymphocytes or ''T cells'' mature. T cells are critical to the adaptive immune system, where the body adapts to specific foreign invaders. ...
in medaka has led to an understanding that this is not specific to mammalians but can be found in other vertebrates.
Conservation
Status
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. However, it is considered an endangered species by Japan's Ministry of the Environment. Many local communities try to preserve wild medaka in Japan.
Concerns
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. In addition, recent studies confirmed that himedaka have been introduced into many local regions by artificial release. This will eliminate local genetic adaptations of each sub-population of medaka. Furthermore, since himedaka have vivid orange body color, the hybrids will attract more predators and thus decrease the total medaka population. In 2011, researchers discovered that almost 15% of wild-caught medaka in Nara
The National Archives and Records Administration (NARA) is an " independent federal agency of the United States government within the executive branch", charged with the preservation and documentation of government and historical records. It ...
had a himedaka-specific gene marker. In addition to these concerns, invasive species like mosquitofish 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. The damage at the anal fin will decrease medaka offspring by preventing courtship behavior. 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 causing genetic pollution
Genetic pollution is a controversial term for uncontrolled gene flow into wild populations. It is defined as "the dispersal of contaminated altered genes from genetically engineered organisms to natural organisms, esp. by cross-pollination", but ...
. There is no comprehensive study of population size of medaka, but the genomic analysis of one medaka sub-population indicates that their effective population size
The effective population size (''N'e'') is a number that, in some simplified scenarios, corresponds to the number of breeding individuals in the population. More generally, ''N'e'' is the number of individuals that an idealised population w ...
is around 25000–70000.
Social importance in Japan
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. Currently, 456 commercial strains are documented and available for fishkeeping
Fishkeeping is a popular hobby, practiced by aquarists, concerned with keeping fish in a home aquarium or garden pond. There is also a piscicultural fishkeeping industry, serving as a branch of agriculture.
Origins of fishkeeping
Fish hav ...
. 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.
See also
* Mummichog
The mummichog (''Fundulus heteroclitus'') is a small killifish found along the Atlantic coast of the United States and Canada. Also known as Atlantic killifish, mummies, gudgeons, and mud minnows, these fish inhabit brackish and coastal waters in ...
(''Fundulus heteroclitus''), first fish sent to space in 1973
References
External links
*
Medaka genome
in Ensembl
Ensembl genome database project is a scientific project at the European Bioinformatics Institute, which provides a centralized resource for geneticists, molecular biologists and other researchers studying the genomes of our own species and other v ...
.
{{Authority control
Japanese rice fish
Fish of Japan
Fish of East Asia
Animal models
Taxa named by Coenraad Jacob Temminck
Taxa named by Hermann Schlegel
Japanese rice fish