Oryzias latipes, commonly known as the Medaka Ricefish, is a small freshwater fish that are extremely popular in Asia and are pretty rare in America and Europe.
In my opinion, this is one of the most underrated and overlooked fish species in the aquarium hobby. Why?
The Medaka Ricefish are easy to care for and breed quickly. These fish are very hardy and can tolerate extreme water parameters which makes them a good choice even for beginner aquarium enthusiasts. In addition, these fish are peaceful and do not require a lot of space in the tank.
If you are interested in keeping Medaka Ricefish as aquarium pets or want to learn more about these interesting fish, this species profile will tell you everything you need to know about them, including their behavior, feeding preferences, lifespan, ideal tank setups, and how to care for them.
|Although these fish are considered as a least-concern species in the IUCN red list, this species was listed within the rank of ‘Threatened’ in the 1999 Red List of Threatened Animals of Japan (Ministry of the Environment Japan, 1999).
The Japanese wild population of Oryzias latipes has been decreasing rapidly, mainly because of the destruction of their habitat.
Quick Notes about Medaka Ricefish
|Other Names||Japanese Medaka, Japanese killifish, Youkihi Medaka Ricefish|
|Scientific Name||Oryzias latipes|
|Water type||Freshwater / brackish water|
|Tank size (optimal)||10 gallons (~40 liters)|
|Size||1.2 – 1.6 inches (3 – 4 cm)|
|Optimal Temperature||64 – 82 °F (18 – 28 °С)|
|Optimal PH||6.5 – 8.0|
|Optimal GH||6 – 20|
|Nitrate||Less than 80|
|Life span||up to 4 years|
|Color Form||Opaque, white, yellow, golden, red, pink, brown, olive, metallic, greenish, blue|
Interesting fact: Oryzias latipes used to be an integral part of everyday life in Japan from the Edo period of the 17th to the 19th centuries.
Etymology of Medaka Ricefish
The genus name Oryzias is based on the Latin word ‘Oryza’ meaning ‘Rice’.
The specific name ‘Latipes’ is derived from the Latin words ‘Latus’, meaning “Wide”, and ‘Pes’, meaning “foot”.
The term Medaka in Japanese means a tiny fish with big eyes (“Top eye”).
Taxonomy Problems of Medaka Ricefish
These fish were first described scientifically in French under the name of Poecilia latipes based on collections by the German physician and naturalist Philipp Franz von Siebhold.
Following the Japanese designation, it was recorded and published in the book “Fauna Japonica”, in the Netherlands in 1846.
In 1906, scientists Jordan D.S. and Snyder J.O. renamed this species to Oryzias latipes.
Currently, at least 4 populations referred to this species. Oryzias latipes have been recognized based on allozyme and karyological differentiation:
- Northern Japan,
- Southern Japan,
- East Korea,
- China-west Korea.
Northern and southern Japanese medaka populations have long been known to be differentiated genetically as well as isolated geographically.
Distribution of Medaka Ricefish
The Medaka Ricefish (Oryzias latipes) is widely distributed throughout eastern China (including Hainan Island, Laos, and Taiwan), east Korea, and throughout the Japanese archipelago.
Habitat of Medaka Ricefish
These fish are restricted to shallow, thickly-vegetated, swamps, ponds, sloughs, ditches, rice fields, and irrigation canals. They prefer very slow-moving or stagnant waters.
The Medaka Ricefish inhabit fresh and brackish waters.
Note: It is often found in rice fields, which earned it its common English name of Ricefish.
Description of Medaka Ricefish
This is a very small fish. Fully grown adults range between 1.2 – 1.6 inches (3 – 4 cm) in length and 0.5–0.7 g in weight.
Distinguishing characteristics of Medaka Ricefish:
- The body is compressed and elongated.
- Their lower jaw is slightly projecting.
- The eyes are larger than the snout.
- Their body is translucent and the top of the head is dark. The body in the upper half is grayish-black, whilst the portion below the lateral line series are pale grey brawn. Wild-type Medaka Ricefish adults do not exhibit a particularly ordered body pigmentation. However, there are lots of selectively bred color morphs that include: opaque, white, yellow, golden, red, pink, brown, olive, metallic, greenish, blue, and many others.
Note: Nowadays, there are over 800 color variations of Medaka Ricefish. Depending on the strain, some of them are so rare that their price can be around a thousand dollars for a pair of fish!
Lifespan of Medaka Ricefish
According to multiple studies, the lifespan of Oryzias latipes in nature is about 1 year at about 80°F (27°C) indicating that temperature greatly affects its lifespan.
However, in captivity, under optimal conditions and lower temperatures, these fish may live for up to 4 years.
Note: The maximal survival time of Medaka Ricefish maintained outdoors was 1838 days (close to 5 years).
Typical Behavior of Medaka Ricefish
Medaka Ricefish are quite peaceful fish. They will not bother anybody in the community tank. However, they may compete for dominance and food between each other.
According to the study, when the food supply was limited, a social hierarchy developed in which large fish were socially dominant, chased small fish away from food and grew faster than small fish. Agonistic behavior includes nips, chases, frontal and lateral threats.
Although both males and females are able to assume social dominance, eventually, males dominated females regardless of body size.
Medaka Ricefish is a social species. Although they like to be in a group, they do not stay in close proximity all the time. Thus, it is not schooling but rather a shoaling fish.
In the aquarium, you need at least 6 of them to start. According to the same study, agonistic behavior is not triggered when space is limited. Large groups make them happier.
They are moderately active fish. Medaka Ricefish are also very shy and get scared easily.
|Warning: When stressed Medaka Ricefish may jump.|
Placement in Tank:
Generally, they spend most of their time at the top and middle parts of the tank.
- Social: Yes
- Activity: Moderate
- Placement: Top and middle dwellers
- Peaceful: Yes
- Nippers: Yes (rarely)
- Jumpers: Yes (rarely)
Diet of Medaka Ricefish
Oryzias latipes is omnivorous. In nature, these fish feed on microscopic organisms, including algae, phytoplankton, and zooplankton.
Medaka Ricefish are surface-feeders, as the mouth is upwardly directed because of the jaw structure.
Because of their small mouths, they can only eat only tiny particles of food. In the aquarium, we can feed them:
- brine shrimp (artemia salina),
- artemia nauplii,
- fruit flies, etc.
Large worms (grindal worms, bloodworms, blackworms, etc.) should be chopped into small pieces and flake food should be crumbled.
The great thing about Medaka Ricefish is that they are not picky eaters. They will gladly accept frozen and commercial food as well, as far as it is small enough to fit into their mouth for mastication and digestion. Some examples (links to Amazon):
- Fluval bug bites,
- Hikari first bites,
- TetraMin Crisps,
- any kind of really small crushed-up tiny flake food.
How to Feed Medaka Ricefish?
At feeding time they start swimming rapidly back and forth among the food particles until everything is eaten. However, if there are bigger or more active fish, Medaka Ricefish can be outcompeted for food.
- They are diurnal animals, thus these fish should be fed during the day.
- As surface feeders, their mouths are developed to catch prey above them. They rarely pick anything on the substrate. Ideally, you need some food that will stay in the water column for some time.
- Be prepared to cut in pieces or grind up some flakes.
- Make sure it is mostly a protein-based diet.
- Clean up leftover food to avoid fouling the water.
- Diet Type: Omnivore
- Food Preference: Protein-rich
- Feeding Frequency: Daily
How Often to Feed Medaka Ricefish?
Ideally, Medaka Ricefish should be fed at least twice per day.
Are Medaka Ricefish Plants Safe?
Yes, Medaka Ricefish are plant safe. They do not eat live plants.
Even more, planted tanks are highly recommended for them, especially for breeding purposes.
Keeping and Housing Medaka Ricefish
Interesting fact: Because of its hardy nature, Oryzias latipes has been one of the most widely used species in experimental vertebrate biology for over a century.
Nonetheless, as aquarists, we should not test their limits. It is crucial to understand that to keep them healthy and happy, we have to mimic their natural habitat. Stress will significantly reduce their lifespan.
Oryzias latipes are not very active swimmers and do not require a lot of volume. You can easily keep a few dozen in a 10-gallon tank (40 liters). A small group of these fish will be a great choice for nano tanks.
Nonetheless, having a larger tank is always preferable for the stability of water chemistry.
The main problem with small tanks is that it can be difficult to constantly control water parameters. In small tanks, everything can go wrong so fast that you do not have time to fix things.
|Important: Medaka Ricefish are jumpers, however, it does not happen very often. Nonetheless, to prevent it from happening, there are a few things you can do:
Temperature: Medaka Ricefish is a eurythermal fish, and can survive in outdoor containers (ponds) even when the water temperature falls to 32°F (0°C) during the winter and can rise to almost 104°F (40°C) in summer.
In the aquarium, they will thrive in any temperature conditions between 64 – 82°F (18 – 28°С).
pH: Although Medaka Ricefish can tolerate a wider pH range of 6.0 – 9.0, they will be more comfortable in neutral water. The ideal pH range should be between 6.5 – 8.0.
Hardness: Medaka Ricefish can live in any water but they will appreciate optimal GH between 6 – 20 and carbonate hardness around 5 – 10 KH.
Salinity: Oryzias latipes is a euryhaline teleost. This species generally lives in freshwater but is able to acclimate to full-strength seawater. Its embryos and larvae are able to tolerate direct transfer to 25 parts per thousand (ppt) saltwater.
Medaka Ricefish prefers subdued lighting.
However, if you decide to breed them, it is essential to provide them with a long photoperiod. Their breeding pattern is synchronized with the light/dark cycles.
Medaka Ricefish do not like strong water flow. In the natural ecosystem, they inhabit shallow waters where the current is absent or very slow-moving.
Still or very slow water currents are recommended for tanks housing this species.
No special requirements. Medaka Ricefish will be absolutely fine even in a bare-bottom tank.
If you have plants, the substrate should be adapted to the needs of plants in your tank.
There are no special requirements as well. As long as you have got the filter that works great with the size of the tank you have got you will be fine.
If you have fast-growing plants to absorb nitrogen you do not even need filtration.
Medaka Ricefish can be kept even outside.
Plants and Decorations:
The natural habitat of Medaka Ricefish is characterized by the presence of a lot of aquatic plants and leaf litter, and this should be replicated in a tank.
Floating plants, fake plants, driftwood, rocks, etc. provide good hiding and spawning spots for these fish.
Regardless of their hardy nature, before putting Medaka Ricefish into the tank, you need to at least temperature acclimate them.
Give them time to acclimatize before adding to the tank. Do not rush the process! Sudden changes in habitat can harm them.
Breeding Medaka Ricefish
Medaka Ricefish are egg scatterers. They will repeatedly produce eggs without any effort on the part of the aquarist when conditions are right for them.
This is an oviparous fish, with eggs fertilized and embryogenesis occurring externally.
Medaka Ricefish reach maturity when the body length is about 1 inch (2.5 cm) long, which under proper conditions is generally achieved within 2 – 3 months after hatching.
- Males are distinguished from females on account of their dimorphic dorsal fins. In males, they are convex with filamentous fin rays, whereas females have a straight or slightly concave distal margin of the anal fin.
- Males have longer dorsal fins, longer anal fins, and shorter pelvic fin than females.
- In females, pelvic-fin rays are elongated and nearly meeting anal-fin origin. Posterior anal-fin rays medial pectoral-fin rays with bony contact organs in males.
- The genital papilla is a short, slightly conical, tube in males and is a bilobed structure in females.
In the natural environment, the breeding season of Oryzias latipes usually lasts from mid-April to late September in Japan.
Under regular daily photoperiod with more than 13 hours of lighting, Medaka Ricefish mate at the break of dawn. This is because in females ovulation occurs about 1 hour before the onset of the light period.
Mating involves an intricate courtship dance during which males repeatedly approach females and exhibit diverse displays. Males and females begin to sink down gently to the bottom, keeping the crossing posture.
The whole body and fins of both sexes quiver finely but rapidly – lowering their heads or raising their tails for 15-30 seconds.
After that, they contact side by side almost completely. So, the crossing angle between both individuals becomes zero.
Note: It was recorded that sexually mature males of Medaka Ricefish may compete vigorously with each other and would even nip females which do not respond during courtship. However, they do not do a lot of harm anyway.
Temperature and photoperiod play a crucial role in breeding Oryzias latipes
Mating and spawning are tightly correlated to the light cycle.
According to the study, embryo production stops after photoperiod reduction from 16L:8D to 8L:16D (at 77°F or 25°C).
For successful daily breeding in captivity, temperatures ranging from 77°F to 82°F (25 – 28°C) are most suitable.
A severe decline in production is also observed after a temperature decrease of 50°F (10°C).
Under reduced photoperiod, the histologic analysis showed no mature ova and moderate oocyte atresia in all individuals. However, reduced temperature (15°C) produced only mild oocyte atresia and fewer mature ova.
During the breeding season, Medaka Ricefish spawns every day around sunrise.
Mature and healthy females can spawn between 10-30 eggs for the duration of the breeding season. The maximum recorded number was 48 eggs.
The eggs are transparent and are roughly 1 mm in diameter.
Note: Ripe unfertilized eggs appear less transparent than fertilized ones.
The cluster of fertilized eggs stays attached to the mother’s ovarian cavity via attaching filaments for up to a few hours. Eventually, females deposit them on the leaves or roots of floating plants by swimming through them.
Based on the diagnostic features of the developing embryos, the process of embryonic development has 39 stages.
Depending on the temperature, juveniles of Medaka Ricefish hatch after 7–10 days (10 days at 79°F or 26°C) and grow to maturity within about two and a half months.
After hatching, embryos feed on the yolk for the first 7 days of their life.
One of the advantages of Medaka Ricefish is that their fry can tolerate a wide temperature range. It was recorded that they will survive at temperatures as low as 39°F (4°C) up to 95°F (35°C) for a considerably long time.
Some useful tips for breeding Medaka Ricefish
It was noticed that when grown and bred outside, Oryzias latipes become stronger and have more vivid coloration.
Lots of lighting and gradual temperature fluctuations benefit these fish.
Continuous breeding (at high temperatures) leads to physical exhaustion in females because of the high energetic costs associated with breeding in species with direct development. As a result, they almost stop breeding after 1 year.
In terms of color, they reach their full potential only when they are fully grown.
In addition, if you want them to breed true, you will certainly have to keep the strain separate from the others.
Medaka Ricefish and Suitable Tankmates
Species-only tank is preferable. Medaka Ricefish are small and timid fish, they cannot compete with other species and might be targeted for bullying.
These fish can be kept only with fish that is not big enough to eat or harass them such as Pygmy Cory, Boraras, Panda Garra, Clown Killifish, Guppies, Harlequin Rasboras, Otocinclus, etc.
Some tank mates to avoid are Cichlids, Goldfish, Monos & Scats, Jack Dempsey, Oscars, etc. Even Betta, Paradise fish, Siamese Algae Eater, Angelfish should never be placed in a tank with Medaka Ricefish.
Medaka Ricefish can be kept with dwarf shrimp. They have very small mouths to be a threat to adults or even juvenile shrimp.
However, newly-hatched shrimp are tiny enough to fit in their mouth. So, there is always a chance that they might snack on them.
Note: Shrimplets that are 1 – 2 weeks old will be too big for them to eat.
They are compatible with any freshwater snail.
- Large and/or aggressive, and/or boisterous fishes.
- Keep them away from all types of crayfish and most types of freshwater crabs.
Oryzias latipes is a very low-maintenance fish. They are very easy to keep and breed due to their simple dietary and habitat requirements.
These colorful small fish compensate for their size with lots of color morphs and hardy nature.
Medaka Ricefish can be kept even outside.
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