The Medaka Classification Field Guide

JMA 2025, pp. 5–14, 68–73 · JMA primary source

1. What a classification can—and cannot—tell you

Ornamental medaka have accumulated an extraordinary range of visible features. Sellers often name a newly bred line to honor the breeder, describe its look, or distinguish a commercial selection. A name on a sales listing may therefore communicate a family history, a memorable brand, a phenotype, or some mixture of all three. Classification becomes useful when those different meanings are made explicit.

The central practical distinction is between a trait and a line. A trait is something observers can describe in the fish: a pattern on the scales, an eye condition, a particular form of reflective light, or a fin and body shape. A line describes ancestry or deliberate selection over generations. Two unrelated lines can independently produce similar-looking fish; offspring from the same line can vary dramatically in appearance. Neither relationship should be inferred solely from a nickname.

The Japan Medaka Association has developed standardized vocabulary so that a fish may be described through combinations of visible characteristics rather than by house name alone. The association has revised its guidance as new forms appear. Its nomenclature is voluntary and is not a genetic test, legal registration, or proof that a seller's pedigree is genuine.

Aquapedia records these as separate fields: observed phenotype; breeder or commercial name; claimed parental history with provenance; the evidence supporting a specific gene association; and confidence or unresolved questions. A label that is useful in the marketplace is not automatically a scientific genotype.

JMA 2025, pp. 11–15 and trait chapters · JMA primary source

2. A repeatable identification workflow

Begin with a healthy mature fish in more than one viewing position. From above, evaluate the overall silhouette, dorsal reflections, scattered scale sparkle, color patches, and the caudal fin. From the side, inspect the cheek and gill cover, the eye, the belly and peritoneum, the dorsal and anal fin shapes, and pigment present in fin rays or membranes. Photograph each view using neutral lighting.

Use consistent settings whenever you compare siblings. The perceived darkness of a fish can change with the container color, illumination, camera white balance, and its acclimation history. A black fish photographed in a dark tub should not be treated as equivalent evidence to the same fish after a controlled observation in a pale tub. Record the setup; do not subject the animal to abrupt temperature or water-quality changes simply to improve photographs.

Identify broad categories separately instead of forcing a famous strain name onto a fish: body color, scale transparency, eyes, reflective-pigment effects, pattern, fin structure, and overall body form. Then record any additional descriptors. If the trait is subtle or appears in only one individual, note it as provisional and seek repeat observations or a knowledgeable second reviewer.

Do not confuse phenotype with genetic certainty. Family photographs and repeated generations can support heritability, but a visually obvious effect does not by itself establish a named allele, its mode of inheritance, or genetic identity between independent breeder lines.

JMA 2025, pp. 19–28 · JMA primary source

3. Body color: what the background really shows

Color terms in the Japanese classification system are practical visual descriptions, not a single-color digital standard. Viewing light, maturity, sex, nutrition, and the combination of pigment cells influence what the eye sees. A sample judged yellow under one light source may look warmer under another, so keep lighting and background consistent and document the exact conditions of comparison.

The core 2025 vocabulary includes brown, yellow, white, blue, black, golden, amber, vermilion red, orange, and pink. The informal marketing term attached to a fish may name a color more poetically than the formal trait term does. It helps to preserve the original Japanese term where its conventional English translation could be confused with a specific commercial strain.

Separate a base body color from black mottling, reflective light, and colored fins. A fish that appears white with black patches is not automatically an independently inherited two-color genotype; it may be a body-color designation plus an observed mottled pattern. Likewise, intense coloration in fins may warrant a fin-related qualifier without changing the body-color term.

Some blue-class fish are described as green-tinted. The 2025 guidance treats certain green expressions within blue-related descriptions even though their pigment biology is more complicated than a literal reading of 'blue without yellow' might suggest. Aquapedia marks such placements as classification conventions rather than settled genetic proofs.

JMA 2025, pp. 29–31, 68–70; 2025 trait revisions · JMA primary source

4. Transparent, semi-transparent, and mirror-scale effects

Transparency is best evaluated at defined anatomical landmarks. The cheek and gill cover are especially useful: a classic transparent-scale appearance can expose red coloration from underlying tissue because reflective cells are reduced there. Not all lightly colored or translucent-bodied fish belong to that category, and an individual may look different on its two sides.

Semi-transparent scale—frequently encountered under the historical hobby term 'Aurora'—describes a more subtle appearance associated with altered reflective coverage. 'Aurora ancestry' and 'Aurora-looking fish' are not interchangeable statements. The first refers to a lineage claim; the second is a visual description that can be assessed directly.

Strongly see-through appearances, panda eyes, and albino conditions can overlap in ways that change what is visible through the head and abdomen. Classify eye pigment independently from scale transparency rather than assuming that one automatically proves the other.

Kagami (カガミ) is recorded in the 2025 material as a mirror-scale-related common descriptor. It should be treated as a scale characteristic, not as a standalone body color or proof of a single lineage. Record whether mirror-like plate reflections are uniform, how many scales are involved, and whether their expression persists as the fish matures; confirm the original Japanese wording before assigning an official composite name.

JMA 2025, pp. 32–38 · JMA primary source

5. Eye changes: an identification guide

Several eye traits that share a dramatic appearance have different diagnostics. For example, an albino-like red eye, a ruby-colored eye with some residual pigment, and a dark panda-style eye cannot be safely grouped under one genetic label. Compare the eye from above, the side, and a slight diagonal angle, without shining a harsh light directly into the animal's eyes.

Shape also matters. A protruding eye is not the same thing as a large eye: one describes the eye's position or projection, the other its size relative to the head. An eye angled toward the front creates yet another appearance. Reduced-eye traits can occur asymmetrically. Make records for left and right eyes separately when needed.

The more unusual eye forms, including bubble-like structures or highly reflective iris appearances, should be documented with clear photos and followed by an animal-welfare assessment. Never breed or grade a fish solely for an extreme eye condition without considering visibility, feeding success, and normal swimming.

JMA 2025, pp. 39–46 · JMA primary source

6. Understanding lamé, external light, and internal light

The word 'shimmer' hides several different structures. Lamé describes conspicuous sparkling points associated with individual scales. External body light is a broader reflective effect appearing on or along the surface of the back and sometimes other positions; it is central to the historic Miyuki phenotype. Internal body light, whole-body internal effects, and peritoneal reflections are not equivalent just because all can look bluish or metallic.

Under a diffuse overhead light, move the camera rather than the fish to test how a reflection behaves. Lamé remains associated with separate scales; external light often forms an elongated field or localized dorsal patch; internal-looking highlights appear beneath the skin in a different optical position. Multiple effects can coexist in one individual, so record each independently.

The Japanese vocabulary distinguishes colors, locations, and shapes of external illumination. A silver-white streak along the back, a pale head-only reflection, and an outline that follows individual scales should not all be labeled with one trade name. Scale-outlined light (Rinkō) and scattered Lamé may produce superficially similar photographs but differ on close inspection.

A Hikari body form can show dorsal silver reflection. That does not automatically mean the fish has the external-light trait: Hikari is fundamentally a body/fin configuration, while external light can appear in several body forms. Check the caudal and dorsal fins to separate those descriptions.

The genetics behind many ornamental reflective phenomena are not established at the level of a single causal variant in every commercial strain. Distinguish JMA's usable visual vocabulary from genomic research claims.

JMA 2025, pp. 47–48 · JMA primary source

7. Patterned fish: patches versus scale edging

Two dark-pattern categories are easily confused in small images. Buchi is irregular, patch-like black marking that can occupy distinct parts of the body. Black Rim emphasizes scales, giving the appearance of dark edging or organized pigment across multiple adjacent scales. One close-up image of a flank is rarely enough for a defensible assessment.

Dark markings may change in strength depending on the rearing background. Some selected lines retain deep black coloration even against a pale surface. The response to background belongs in observation notes, not in an invented claim about a universal gene. When recording red-and-white or three-color fish, write down the actual colors and black-pattern expression visible in each individual.

A breeder nickname may describe a visually balanced pattern even though pattern placement varies from offspring to offspring. Quality grading can be a separate judgment from trait identification: 'this fish has distinct black patches' is observational; 'this is the best balanced tricolor' is a preference that needs a disclosed judging standard.

JMA 2025, pp. 49–57 · JMA primary source

8. The many different ways fins change

Fin terminology is unusually prone to name collisions. A divided dorsal fin, a diamond-shaped caudal fin, a broad fin, and dramatically elongated fin tissue may all appear on fish marketed with fanciful names. Accurate identification starts by asking which fin is affected, which portion changed, and whether the modification is symmetric.

Samurai concerns splitting of the dorsal-fin structure. Hishio, a diamond-tail form, relates to the tail shape in fish that do not otherwise express the full Hikari body configuration. Maruko denotes absence of the dorsal fin. Meller and Swallow describe other distinctive patterns of altered fin development; these are not catch-all synonyms for ordinary long-finned fish.

Several different long-fin categories appear in modern Japanese terminology. A fish should not be labeled 'real longfin' just because it appears larger or older than an average fish. Measure and photograph comparable fish of similar body length and stage of maturity.

In show records, include the dorsal, caudal, anal, and pelvic fin appearances and whether asymmetry is intentional, stable, or plausibly due to damage. A torn fin or poor regrowth is not a breeding trait. Pay special attention to mobility and feeding when selecting fish with extreme fin length.

JMA 2025, pp. 58–60 · JMA primary source

9. Standard, Hikari, Daruma, and combined body forms

Hikari body form is recognized chiefly by the caudal and dorsal/anal fin configuration, including a conspicuous diamond-like tail. Strong dorsal brightness can accompany it, but lack of obvious dorsal shine does not necessarily rule it out. It is important not to diagnose the form on light alone.

Daruma fish are shortened-bodied. Breeders also use semi-Daruma for intermediate forms. Different degrees of shortening can exist among related offspring, so an unusually deep body on its own should not be used as a final classification. Short-bodied individuals may require extra attention to water quality, stable temperatures, feeding, and swimming ability.

Hikari-Daruma combines the visible shortened-body and Hikari fin/body characteristics. When recording such fish, separate the two components rather than assuming their combined expression has a uniquely verified genotype. Check for spinal curvature or other health issues before assigning breeding quality.

JMA 2025, pp. 61–67; September 2025 supplementary table · JMA primary source

10. Supplementary terms and the 2025 additions

JMA's supplementary vocabulary captures special appearances that may not be a top-level body color, pattern, or body-form category. Examples include unusually striking fin coloration (Hirebi), blue peritoneal appearance, changes in internal black pigmentation, certain illumination locations, background-response behavior, and newer common descriptors. These should be recorded as supplements to the main observed phenotype, not automatically promoted into wholly separate varieties.

The September 2025 list includes terms such as Kagami, Hige, Apex/Top Eye, Mosaic Panda, See-through, Side Light, and Powder among the common supplementary descriptors. Their casual English renderings are only orientation aids; the Japanese official terminology and its specific diagnostic criteria remain authoritative. Some terms describe localized structures or combinations that would be misleading if simplified to a general color.

A practical record should pair the Japanese term with a photograph of the feature, the viewing angle, and a status such as 'visible,' 'possible,' or 'not established.' Where the diagnostic detail in the original material is unclear, use the Japanese name and mark the English gloss provisional rather than creating a definitive new definition.

JMA 2025, pp. 68–73 and 74–84 · JMA primary source

11. Traits, inheritance, and why dominance cannot be guessed

A heritable visible characteristic is not automatically a simple dominant allele. Incomplete dominance, multiple contributing loci, modifier genes, developmental variation, age, sex, and environmental influences can produce overlapping appearance. A single attractive F1 fish does not establish how the feature will behave through later generations.

Use a pedigree with dated crosses, photographed parents, offspring counts, and known rearing conditions. For each generation, count phenotype classes and compare them with the predictions of several inheritance models. Where a marker or causal gene has actually been identified in primary research, cite the specific study; otherwise label 'dominant,' 'recessive,' and 'polygenic' as hypotheses rather than facts.

Background adaptation, nutrition, lighting, and maturation can strengthen or weaken visible colors. These can change an individual's appearance without creating a new heritable form. Evaluate stability across siblings, settings, and generations before claiming a distinct strain.

A fish can possess genetic variants that do not visibly express a named feature, and appearance cannot reliably reveal every hidden allele. JMA's visual classification is designed for practical communication; it is not a system for sequencing or assigning complete genotypes.

JMA 2025, pp. 70–84 · JMA primary source

12. From an attractive fish to a documented breeding line

Start with the selection goal stated in observable terms: for example, consistent side-scale reflection, a particular combination of body color and distinctive fin shape, or a reliable lack of background fading. Keep the original breeder name if it is known, but record the physical traits separately and do not add unverified parentage to a public genealogy diagram.

Photograph the founders, record mating dates, hatch and survival counts, and tag each generation. When offspring vary, report the full distribution rather than showing only the best specimen. A useful project log tracks the proportion expressing the target trait, sex ratios where measurable, deformities, fertility, and whether the color expression changes after maturation.

A new trait discovery differs from a new arrangement of already recognized traits. Both may be meaningful breeding achievements. In either case, documenting multigenerational transmission is stronger evidence than a one-time cross. If a show or association recognition is claimed, link to the dated judging or recognition record rather than repeating promotional copy as an established fact.

Preserve animal welfare in the selection criteria. Severe spinal changes, impaired eyesight, or extreme fins may reduce swimming, feeding, and survival. The breeding log should record those effects transparently instead of treating every visually unusual outcome as a desirable new variety.

Cross-cutting Aquapedia editorial synthesis · JMA primary source

13. Reading breeder listings and entering data into Aquapedia

An online listing can truthfully describe what the fish looks like yet overstate its documented ancestry. Ask for original images of the actual stock, parent and sibling photographs where available, the breeder's stated strain name in Japanese and romanization, and whether a line is a locally bred descendant or a recent import. Record provenance rather than inferring it from an attractive photograph.

Use separate fields for 'trade name,' 'JMA-observed traits,' 'breeder-reported ancestry,' 'published evidence,' and 'DNA-confirmed genotype.' If only the color/fin combination is visible, that still makes a useful descriptive record. It does not license an invented genealogy linking unrelated breeders.

In Aquapedia, label uncertainty visibly: VERIFIED when an authoritative source establishes a fact, BREEDER-REPORTED when a named breeder asserts it, COMMUNITY USAGE for common trade terminology, and UNKNOWN when parentage or the causal genetic mechanism is not documented. Link the source next to the claim and date the observation.