Japanese rice fish

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

The Japanese rice fish (Oryzias latipes), also known as the medaka,

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]

Taxonomy and range

As originally defined, O. latipes was native to much of

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 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 region and spread alongside the Sea 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]}

Use in science

Oryzias latipes is a

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.^{[citation needed]}

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] 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]

Sex and reproduction

Medaka reproduce on a daily basis, which is an optimal trait for studying their reproductive biology. Researchers have studied

The discovery that T-lymphocytes home to the thymus in medaka has led to an understanding that this is not specific to mammalians but can be found in other vertebrates.^{[citation needed]}

Skeletal Research

This species is increasingly used as a model in research relating to skeletal diseases in humans,^[36] such as osteoporosis.

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.^[37] However, it is considered an endangered species by Japan's Ministry of the Environment.^[38] Many local communities try to preserve wild medaka in Japan.^[39][40][41]

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.^[7] In addition, recent studies confirmed that himedaka have been introduced into many local regions by artificial release.^[42] This will eliminate local genetic adaptations of each sub-population of medaka.^[42][43] Furthermore, since himedaka have vivid orange body color, the hybrids will attract more predators and thus decrease the total medaka population.^[44] In 2011, researchers discovered that almost 15% of wild-caught medaka in Nara had a himedaka-specific gene marker.^[45] 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.^[46] The damage at the anal fin will decrease medaka offspring by preventing courtship behavior.^[47] 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.^[48] 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 is around 25000–70000.^[49]   

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.^[50] Currently, 456 commercial strains are documented and available for fishkeeping.^[51] 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]}

See also

• Mummichog (Fundulus heteroclitus), first fish sent to space in 1973

References

External links

Look up Medaka in Wiktionary, the free dictionary.

Wikimedia Commons has media related to:
Oryzias latipes (category)

Wikispecies has information related to Oryzias latipes.

• View the oryLat2 genome assembly in the UCSC Genome Browser.
• Medaka genome in
  Ensembl
  .
• Medaka Fish Farm