Maruko (Finless-Back) Medaka
The Maruko Medaka is a distinctive improved medaka variety defined by its finless back, a trait first developed by Takeshi Kogure in 2011. Because it lacks a dorsal fin, the metallic body-light stripe along its flanks runs uninterrupted in one continuous line instead of being broken where a dorsal fin would normally sit. This makes it a popular partner for crossing with Mikuyuki-line varieties, and the finless-back blue body-light strain known as "Ryusei" is considered the variety's flagship strain. Adults reach roughly 4 cm in standard length, and Maruko medaka are generally recommended for intermediate keepers. Watching its unbroken silhouette move through the water is much of the appeal.
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Maruko (Finless-Back) Medaka key facts
- Care level
- Intermediate
- Lifespan
- 2-3 years
- Size
- Up to about 4cm SL
- Temperature
- 18〜28℃
- Diet
- Prepared medaka food, green water
- Lighting
- With no dorsal fin the external luminescence runs unbroken along the back, so the fish is best viewed from above. The Medaka Web Encyclopedia recommends outdoor keeping and suggests shading half the container in summer.
- Diet & Nutrition
- Commercial prepared medaka food is the staple, given in an amount finished within a few minutes. Leftovers decay and foul the water, so adjust the quantity. Green water rich in phytoplankton suits fry rearing.
- Water Temperature & Quality
- Slightly acidic to slightly alkaline water is considered ideal, and the water drifts toward acidic as keeping continues. Around one water change per week is the guideline, and because sudden shifts in temperature or pH cause stress, acclimatise new fish by floating the bag. FishBase records the wild water chemistry of this species as pH 7.0-8.0 and hardness 9-19 dH.
- Tank Mates & Compatibility
- A peaceful schooling fish, normally kept as a group of the same variety. One litre of water per fish is the guideline against overcrowding; overstocking causes poor growth, reduced breeding, oxygen shortage and water fouling.
- Breeding
- Outdoors the spawning season runs from about April to October, once water reaches 18C or above with 12-13 hours of daylight. For year-round indoor breeding, 25-28C water and over 13 hours of lighting are the guideline. Eggs hatch at a cumulative 250 degree-days, about 10 days at 25C.
- Health & Notes
- The missing dorsal fin can be judged by eye, and fish retaining a partial dorsal are culled from breeding stock. In lines combined with external luminescence, fish whose light runs unbroken to the head are the preferred specimens.
About Maruko (Finless-Back) Medaka
Maruko Medaka is a finless-back improved medaka whose body-light stripe runs unbroken along its flanks. Often crossed with Mikuyuki lines, it grows to about 4 cm and suits intermediate keepers.
7 things to know before welcoming a Maruko (Finless-Back) Medaka
1No Dorsal Fin
The Maruko Medaka's defining trait is the complete absence of a dorsal fin, a form first fixed in 2011. This gives it a silhouette quite different from ordinary medaka, and its swimming profile is part of the variety's appeal.
2An Unbroken Body-Light Line
Without a dorsal fin in the way, the metallic body-light stripe along the flanks runs as one continuous line instead of being interrupted. This is why Maruko medaka are often crossed with Mikuyuki-line varieties, which have well-developed body light.
3"Ryusei" Is the Flagship Strain
The finless-back blue body-light strain known as "Ryusei" is the best-known Maruko medaka variety. Coloring and how the light develops can vary between individuals, so watching them change as they mature is part of the fun.
4Best Suited to Intermediate Keepers
Maruko medaka are generally considered suitable for intermediate keepers rather than complete beginners. Some prior experience managing water quality and temperature swings is helpful.
5Adult Size Guideline
Adults reach roughly 4 cm in standard length at maturity. Keep this in mind when choosing the size of a tank or outdoor container.
6Fin Shape Can Vary Slightly
While the finless-back trait is a fixed, bred characteristic, how the back silhouette looks can differ slightly between individual fish. It's worth checking an individual's appearance before bringing one home.
7Prepare the Environment First
Like other medaka, Maruko medaka are sensitive to sudden changes in water quality and temperature. Let a new tank or outdoor container stabilize before introducing fish.
Frequently asked questions about Maruko (Finless-Back) Medaka
- Q.Is the Maruko Medaka suitable for beginners?
- A.It's generally considered an intermediate-level variety. Some prior experience with basic medaka care makes water quality and temperature management easier.
- Q.Does having no dorsal fin affect swimming?
- A.The finless back is a fixed, bred trait, and Maruko medaka swim just as actively as ordinary medaka. No special accommodation is needed.
- Q.How large does it grow?
- A.Adults reach roughly 4 cm in standard length. Use this as a guide when choosing a tank or container.
- Q.Why is it crossed with Mikuyuki-line varieties?
- A.Mikuyuki lines have well-developed body light, and combining them with the finless-back Maruko trait produces an unbroken stripe of light along the body. The "Ryusei" strain is the best-known result.
- Q.Are there any legal restrictions on this variety?
- A.The Maruko Medaka is not subject to CITES, so this is not a concern when choosing the variety.
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