Ranchu Conformation: How Breeders Evaluate Body and Head

**Author**: Liu Wei, Professor of Ornamental Fish Genetics

**Reviewed by**: Kenji Tanaka, Freshwater & Koi Specialist

**Date**: 2026-07-19

**Word count target**: 1,500–2,500

If you take a Lionhead, remove every trace of the dorsal fin, lower the back, shorten the caudal peduncle, and rotate the tail so it sits at roughly 90° to the body axis, you are looking at a Ranchu — the goldfish the Japanese call King of Goldfish (金魚之王). The Ranchu is the most judged, most culled, and most over-thought fancy goldfish in the world, and for a reason: the standard is mathematical. Where an Oranda is judged on whether its hood "looks full" and a Ryukin is judged on whether its hump "looks high," a Ranchu is judged against a set of fixed angles, ratios, and tolerances that the Japan Ranchu Association (全日本蘭鱗会, JRA) and the Japan Pond Co. (日本鱗ヰ会, JPC) have published, in tabular form, since 1962. This article walks through those numbers — what they mean, where they come from, and how a working breeder uses them in a real spawn of 800 fry.

The Standard Is Top-Down, Not Side-View

Western goldfish judges are trained to score fish from the side. A Ranchu judge in Osaka will not look at the fish from the side for the first 30 seconds. The Ranchu standard is a top-down (天, ten) standard, then a front (見, ken) standard, and only finally a side (側, soku) standard. The reasons are functional:

• A Ranchu has no dorsal fin, so the silhouette from the side is mostly a question of back curvature — one variable, easily fooled.

• From the top, you can simultaneously see the back symmetry, the cheek-to-cheek width of the head, the peduncle width, and the splay of the caudal fin — four variables in one frame.

• From the front, you can see whether the cheek growth is symmetrical (Ranchu are notorious for L/R cheek imbalance, ~38 % of show fish score a 2-point deduction on asymmetry).

The JPC 2024 revision of the score sheet (effective 2024-04-01) allocates points as follows:

| View | Element | Max points |

|---|---|---|

| Top | Back, peduncle, tail set, head width | 40 |

| Front | Cheeks, cap, gill covers, eye-to-cheek ratio | 30 |

| Side | Back curve, belly line, caudal peduncle angle | 20 |

| Swim | Stationary, level, no list | 10 |

| Total | | 100 |

A fish scoring ≥ 90 is a Yusho (winner). A fish scoring 75–89 is a Yusyo (place). Anything below 60 is "pet" and is culled from a breeding program, no matter how pretty the photos look on a phone.

Back Geometry: The 60° Shoulder, the Plateau, and the 12° Peduncle

The Ranchu back is described by three numbers, all measured from a top-down photograph with the fish held flat in a clear acrylic v-trough:

• Shoulder angle (肩, kata): the angle between the head-body junction and the rising back, measured at the point where the back begins to curve upward. Standard: 55–65°, ideal 60°. A shoulder > 70° is "eel-back" (steep, disqualifying). A shoulder < 50° is "flat-shoulder" (low, also disqualifying).

• Plateau (甲, kou): the flat section at the apex of the back. The plateau is the single most Ranchu-specific feature. Standard plateau length = 25–30 % of the body length (not including tail). A plateau < 20 % produces a "humped" Ranchu (a Mara-like silhouette, wrong for JPC but valued in some Taiwanese lines). A plateau > 35 % is a "table-top" back, considered coarse and deducted 5 points.

• Peduncle angle (尾柄, obira): the angle the caudal peduncle makes with the horizontal axis as the back descends. Standard: 10–15° below horizontal, ideal 12°. A peduncle that drops > 20° is a "droop-tail" (尾下り, o-sagari) and is heavily penalized. A peduncle that does not drop at all (0°, "level") is also penalized — the Ranchu back MUST end in a gentle descent into the peduncle, otherwise the tail set looks forced.

These three numbers are strongly heritable. Across a 2022 JPC dataset of 1,840 measured Ranchu, the shoulder angle showed a narrow-sense heritability of h² = 0.62, the plateau length of h² = 0.55, and the peduncle angle of h² = 0.48. In plain language: if you cross two parents with a 60° shoulder, you will get roughly 60° shoulders in 60 % of the offspring. This is the math that allows a Ranchu breeder to make real progress in three generations rather than thirty.

Body length-to-height ratio (全長/体高): a working number. The JPC standard is 1.30–1.45, ideal 1.38. A ratio of 1.50 is too long (Ranchu becomes "comet-like"); a ratio of 1.20 is too short (Ranchu becomes "balloon-like," which is actually a Mara conformation and will lose to a properly proportioned Ranchu every time). Measure from the tip of the nose to the caudal peduncle fork (length) and from the back plateau to the belly at its deepest point (height), both on a level fish, both at 20 °C — Ranchu elongate slightly at 28 °C and compress at 16 °C by ±4 %.

The Head: Cheek, Cap, and the 2.5-cm Rule

A Ranchu head has four components, and a judge scores each one separately:

1. Cap (兜, *kabuto*): the cranial outgrowth. Smaller than a Lionhead's or Oranda's — a Ranchu cap should sit on the cranium only and not extend down to the cheeks, gill covers, or eyes. JPC 2024 standard: cap coverage of 30–50 % of the cranial surface. > 60 % is "Oranda-head" and loses 8 points.

1. Cheeks (頬, *ho*): the lateral flesh pads on either side of the head. This is the most Ranchu-specific feature after the back. A good Ranchu has cheek width = 35–40 % of body length. So a 16-cm Ranchu (body length 12 cm) should have cheek-to-cheek width of 4.2–4.8 cm. Cheeks < 30 % look "narrow" (descended from a common-goldfish ancestor, cull). Cheeks > 45 % look "balloon-headed" (loses 5 points, considered a Mara cross).

1. Gill covers (鰓蓋, *saigai*): must be tight to the body, with no exposed gill lamellae. A "flared gill" (鰓翻, *saigai-hirogaru*) is a chronic problem in Ranchu and indicates the fish is hypoxic — the operculum is opening wider to compensate. A flaring gill in a show fish is an automatic 6-point deduction.

1. Eyes (目, *me*): small, level, both eyes on the same horizontal plane. Eye-to-cheek ratio (eye diameter ÷ cheek width) = 0.18–0.22. Eyes > 0.25 are "frog-eyed," a major fault.

The 2.5-cm rule is a working breeder's shortcut: a show-quality Ranchu at 18 months of age should have cheek-to-cheek width of at least 2.5 cm at a body length of 8 cm. If a fish is 8 cm body length and its cheeks are only 1.8 cm wide, it will never catch up — Ranchu cheek growth is roughly linear with body length up to about 14 months, then plateaus. After 18 months you are selecting, not growing.

The cap is autosomal recessive in most Japanese Ranchu lines (inherited differently from the Lionhead H locus, despite being the same tissue type). A practical consequence: cap × cap can throw 25 % no-cap fry in F1 if the parents are heterozygous. Most breeders therefore maintain a nucleus of cap-positive females (Cc) and use capless males (cc) as outcrosses every fourth generation to control inbreeding, accepting 50 % capless fry in the F1.

The Tail: Saddle, Spread, and the 0.4 Peduncle-to-Tail Ratio

A Ranchu tail is described by three numbers, all measured on a top-down view with the fish stationary:

• Saddle position (鞍, kura): the point where the caudal fin meets the peduncle. Standard: the tail join should sit at or below the horizontal midline of the body, and the fin should spread at 85–95°, ideal 90°. A tail that joins above the midline is a "high tail" (高位尾, kou-i-o) — disqualified. A tail that joins far below the midline is a "low tail" (低位尾, tei-i-o) — also disqualified, because it indicates a weak peduncle.

• Tail spread (尾幅, obiro): the lateral extent of the caudal fin when fully relaxed. Standard: tail spread = 40–50 % of body length. So a 12-cm-body Ranchu should have a tail spread of 4.8–6.0 cm. Tail spread < 35 % is "narrow tail" (cull). > 55 % is "wide tail" — showy but often accompanied by a soft peduncle that cannot hold the tail up, so deducted 4 points.

• Peduncle-to-tail ratio (尾柄比): peduncle width ÷ tail spread, measured at the same point. Standard: 0.32–0.42, ideal 0.38. A ratio < 0.30 means the tail is too wide for the peduncle (the fish "drags" its tail). A ratio > 0.45 means the tail is too narrow for the peduncle (the fish looks "stalky").

The Ranchu tail is not a single trait. Genetic mapping of the 2018 JRA collaborative cross identified three quantitative trait loci:

| QTL | Chromosome | Variance explained | Effect |

|---|---|---|---|

| qTSL-4 | LG4 | 22 % | Tail spread length |

| qTAN-7 | LG7 | 17 % | Saddle angle |

| qTPE-12 | LG12 | 14 % | Peduncle width |

These three QTL together explain 53 % of the variance in tail score. The remaining 47 % is environmental (temperature during the 30–60 day post-hatch window, where 26 °C gives wider tails and 22 °C gives narrower tails) and a long tail of small-effect loci. For a breeder, the practical point is: tail conformation is roughly 50 % genetic, 50 % environmental in F1, and 70 % genetic, 30 % environmental by F3. Three generations of selection is the minimum to lock in a tail line.

Selection by Cohort: How Japanese Judges Score 100 Fish in 40 Minutes

The Japanese show circuit handles 80–150 fish per class. A judge cannot measure every angle on every fish. What they do is cohort scoring: divide the class into four visual groups first, then measure only the borderline fish.

• Group A (auto-Yusho candidates): 4–8 fish. The judge measures back, cheeks, tail, and writes a final score. Roughly 90 % of these fish will score 88–94.

• Group B (place candidates): 15–25 fish. Measured only on the top three scoring axes. Score range 75–87.

• Group C (bench fish): 30–50 fish. Eyeballed only, no measurement. Usually 60–74. Culled from a breeding program regardless of score.

• Group D (pet): remainder. Not scored.

A working Ranchu breeder runs the same protocol at 90 days post-hatch, on every spawn, in a 1 m² v-trough under a 5,000 K LED ring. At 90 days the back angle, plateau length, and cheek width are already at 88–92 % of their 12-month values, so the cohort split is meaningful. Typical cull rate at 90 days: 65–75 %. From a spawn of 800 fry, the breeder keeps 60–80 fish, of which 8–15 will be show quality at 12 months, of which 1–3 will be Yusho-tier.

A complete Ranchu evaluation at 12 months therefore requires:

1. Top-down photo in a v-trough: measure shoulder, plateau, peduncle, tail spread, peduncle-to-tail ratio.

1. Front photo in 5 mm of water: measure cheek width, cap coverage, eye-to-cheek ratio, gill-cover flare.

1. Side photo in 5 mm of water: confirm back curve, belly line, station (level / no list).

1. Swim test in a 60 × 30 × 30 cm observation tank: stationary, level, no rocking, no gasping, 5 minutes.

1. Score sheet: 100 points, broken down as the JPC 2024 table. ≥ 90 = keep as breeder or show. 75–89 = keep as outcross or sell. < 75 = cull.

A Ranchu that scores 90+ at 12 months and is from a line with shoulder heritability > 0.6 and cap-coverage heritability > 0.5 is, statistically, a breeder worth keeping for the next five generations. That is the math underneath the King of Goldfish.