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Kidadl Team

AUGUST 06, 2021

15 Fin-tastic Facts About The Japanese Rice Fish For Kids

Learn about an aquarium favorite with Japanese rice fish facts.

If you think you know about all the various fishes that could possibly be raised in a freshwater aquarium, what about the Japanese rice fish?

Also known by other common names such as the Youkihi medaka rice fish and Japanese killifish, the Japanese rice fish or Japanese ricefish (Oryzias latipes or O. latipes) is a small and peaceful fish species of the genus Oryzias. The genus Oryzias comprises not more than three dozen different species of small fish with a geographical distribution spreading from India and Southeast Asia to Japan and Korea.

Mostly inhabiting the freshwaters, the rice fish is also found in marine and brackish environments, often traveling between the freshwater and saltwater niches. They are called rice fish because they are a common occurrence in the shallow and flooded rice paddies found throughout their range. However, they are in no way related to the killifish and only called so because they tend to share the same habitat as the killies do.

What makes the ricefish stand out is the fact that even though the wild ones appear drab and dull with occasional flecks of metallic blue-green, the bodies of the genetically modified individuals glow in the dark. The most famous flashy variants are the moonlight medaka and the gold medaka. While the gold medaka is pretty ancient, the moonlight medaka is a recently bred color morph.

Whether you want to pep up your aquarium hobby or are simply reading this out of curiosity, these facts about the Japanese ricefish will surely grasp your attention!

For more relatable content, check out these piranha fish facts and x-ray tetra facts for kids.

Japanese Rice Fish Interesting Facts

What type of animal is a Japanese rice fish?

The Japanese rice fish (Oryzias latipes or O. latipes) is a popular freshwater fish species.

What class of animal does a Japanese rice fish belong to?

The Japanese ricefish or the Japanese medaka belongs to the class Actinopterygii of the family Adrianichthyidae and order Beloniformes.

How many Japanese rice fish are there in the world?

The exact population size of the Japanese ricefish is not known. However, the International Union for Conservation of Nature (IUCN) reports that the genetically pure population of Oryzias latipes has declined in several parts of its native range due to deliberate genetic modifications for ornamental use and hybridization with other species of the genus Oryzias.

Where does a Japanese rice fish live?

The Japanese medaka lives in shallow rivers, slow streams, small ponds, canals, rice fields, and aquariums. Since they are adapted to both fully marine environments and freshwater areas, the medakas can be found in freshwater rivers as well as in the ocean.

What is a Japanese rice fish's habitat?

The natural habitat of the Japanese ricefish includes the slow-moving waters in Japan, China, Korea, and Vietnam. In Japan, their natural range includes Kyushu, Shikoku, central and northern Ryukyus, and Honshu (except for the Sea of Japan side of Hokuriku district, Tohoku district, and around the Wakasa Bay).

Even though these fishes can tolerate water temperatures ranging from as low as 37 °F (2.8 °C) to as high as 108 °F (42.2 °C), they usually prefer the water temperature to be between 59-82 °F (15-27.8 °C). The dense rice paddies seem to be the fish's favorite dwelling, with egg-laying taking place in areas around water grass. The small size of the fish does not call for a large aquarium, but replicating their natural environment with the right kind of water temperature and vegetation is desirable. A 10-gallon tank or aquarium is best if you want to keep a single fish or a pair. However, a school will require a larger aquarium.

Who do Japanese rice fish live with?

The Japanese rice fish live in groups known as schools. The schools are not tight-knit groups, and the medakas can usually recognize the faces of their own kind. The Japanese medaka may also sometimes be seen traveling alone or in pairs. The medakas are a peaceful lot of fish that can co-exist in a community aquarium or tank with other small species of fish of a peaceful temperament.

How long does a Japanese rice fish live?

The Japanese rice fish has an average life span of four years. Although, the medaka lives for around two years in the wild and for three to five years under laboratory conditions.

How do they reproduce?

The Japanese medakas are seasonal breeders that lay eggs between the spring and the summer seasons. The medaka prefers to lay its eggs around water grass or other aquatic vegetation that helps in catching the fertilized eggs and providing them protection until the baby medaka fish hatch. When kept in a breeding aquarium or tank, spawning mops or fine-leaved plants can do the job of protecting the eggs.

The medakas usually spawn at dawn. Many of the eggs of the female that are fertilized by the male remain attached to the female's belly. The males likely fertilize the eggs internally. After spawning, when the female swims around in the water, eggs detach from its belly and attach to the plants. The females lay about 10-20 eggs at a time, and they hatch in around 10 days.

What is their conservation status?

According to the International Union for Conservation of Nature (IUCN) Red List of Threatened Species, the Japanese rice fish is a species of Least Concern.

Japanese Rice Fish Fun Facts

What do Japanese rice fish look like?

Wild populations of the Japanese medaka do not glow in the dark.

While the wild form of the Japanese medaka is not particularly dazzling and eye-catching, the genetically modified or transgenic varieties are known for their semi-transparent gold or orange hues. In the wild, their color can be dull yellowish or creamy-white with streaks of blue-green iridescence, and those of the aquarium can be orange, creamy-yellow, or white. The eyes are large and appear to shine. Transgenic populations may be a bright fluorescent shade of red, yellow, or green. The moonlight and gold medakas are fluorescent ornamental varieties that have been bred in captivity. The body of this fish is slender and elongated, with the back slightly arched. Like all other fishes, the medakas have an anal fin, dorsal fins, pectoral fins, pelvic fins, and caudal fin.

How cute are they?

The large eyes of the Japanese medaka on a relatively small body sort of make it look cute. The transgenic variety with the fluorescent protein looks particularly brilliant when it glows in the dark.

How do they communicate?

Much information is not available regarding the communication system of the Japanese medakas. However, the fact that these fishes can recognize the faces of other medaka individuals indicates that they may have a well-developed mechanism to identify their own kind.

How big is a Japanese rice fish?

The Japanese medaka is a small-sized species of fish that can grow up to 1.6 in (4 cm). They are about half an inch longer than the Chinese rice fish (O. sinensis) of the family Adrianichthyidae and order Beloniformes. The teira batfish is 15 times bigger than the Japanese ricefish.

How fast can a Japanese rice fish swim?

No data is available on exactly how fast the Japanese medakas can swim.

How much does a Japanese rice fish weigh?

An adult medaka fish weighs approximately around 0.01-0.02 oz (0.3-0.6 g). They are in the weight range of guppies and neon tetras.

What are the male and female names of the species?

Japanese rice fish males and females do not have any distinct names.

What would you call a baby Japanese rice fish?

A baby Japanese rice fish would be called a fry.

What do they eat?

The Japanese medaka is an omnivorous fish that primarily eats plankton and juvenile mosquitoes. Medakas kept in an aquarium or tank will eat all types of food, including freeze-dried food and flake food. A densely planted aquarium might help in protecting dwarf shrimp fry from the fish.

Are they rare?

Considering that the Japanese medaka is exclusively found in Japan and the Southeast Asian regions, they may be rare. However, they are a hardy and adaptable aquarium species of fish that makes them a common pet in many parts of the world, besides Japan. Color morphs may be relatively rarer.

Would they make a good pet?

Yes, the Japanese medaka is a quiet and peaceful fish and an excellent choice for a pet, especially for beginners. The best part about them is that they can be easily kept along with other small species of fish in a community tank. The medakas are not messy, and hence, maintaining them is relatively easy. Further, given their hardy nature, they are not very demanding and can adapt to almost any tank condition. They are also easy to feed and will eat almost any standard fish food.

Did you know...

It is not easy to differentiate between the males and females of the Japanese medaka species. However, the males tend to be more slender than the females with larger anal and dorsal fins.

Since the Japanese rice fish development is well-researched and understood, it is relatively easy to produce transgenic (genetically modified) rice fish for various scientific purposes such as the production of human hormones, making antimicrobial proteins, and producing a fluorescent protein that makes the fish glow red, yellow, or green.

The gene for fluorescent protein was extracted from jellyfish and introduced into the Japanese medaka's embryo to produce the fluorescent varieties.

In some species of rice fish, the males have tiny contact organs on the back and middle rays of the anal fin that reportedly help in mating.

Besides the zebra danio, the Japanese rice fish is the most widely studied species of vertebrates.

The Japanese medaka is the first vertebrate to have mated in orbit aboard the space shuttle Columbia (1994), and it resulted in a brood of healthy offsprings. The next time the fish went into space was in 2012 aboard the Soyuz TMA-06M.

Japan's Ministry of Environment considers the medaka to be an endangered species.

Are Japanese rice fish livebearers?

No, Japanese rice fish are not livebearers since they do not produce live young. The females lay eggs that hatch externally, outside of the body of the fish.

Having your own Japanese rice fish

The Japanese rice fish is a great choice for beginners who are just starting out on the aquarium hobby and can be bought from aquarium shops. If kept alone or in a pair, the fish requires a minimum tank size of 10 gal (45.5 L). A larger tank of at least 20 gal (90.9 L) may be suitable if you intend to keep a school of medakas. Water changes and good filtration are mandatory, and they require weekly water changes of 30 percent. Since the medakas like an unheated water environment, the use of aquarium heaters is not advisable. In addition, the tank or the aquarium should have a tight-fitting lid because the medakas tend to be quite jumpy.

The substrate of the tank may be of small gravel or sand and should be provided with dense floating vegetation that replicates their natural environment. The water temperature must be around 64-75 °F (17.8-23.9 °C), and the water pH level should be 7-8. The Japanese medaka is pretty inexpensive and commonly available, except for the genetically modified versions that are banned from sale in the EU and some states of the U.S.

Here at Kidadl, we have carefully created lots of interesting family-friendly animal facts for everyone to discover! Learn more about some other fish from our oilfish facts and French angelfish facts pages.

You can even occupy yourself at home by coloring in one of our free printableJapanese rice fish coloring pages.

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