The Ranchū (蘭鋳, 蘭虫, 卵虫, "Dutch worm") (also Maruko)[1] is a hooded variety of goldfish native to Japan, which lacks a dorsal fin.

The ranchū tail meets the caudal peduncle at a 45-degree angle, giving the fish a unique swimming motion.

[2] Fry possessing broad foreheads and square noses generally produce better wens.

[5] Japanese, Thai, American, and British ranchu breeders, as well as many goldfish societies, adhere to two strict viewing classifications, namely the top-view ranchū comportment and the side-view selection criteria.

However, traditional Japanese culture, aesthetics, and concepts regard these attributes of both ranchū and sumo wrestlers as an imposing and solid figure composed of massive circles and squares, which are abstract representations of balance, proportion, dignity, graceful movement and power.

[5] In ancient China, goldfish were kept in large jars made of pottery or porcelain, so the only way one could see the fish was from the top.

Judges view the fish from the side and the top, taking note of its comportment and conformation to physical standards, motion, and movements.

[10] Perhaps 700 years ago, egg goldfish were first bred for their lack of a dorsal fin.

Prior to the 17th century, lionhead goldfish with this trait were kept in China and Japan, and the ranchū breed was derived from them.

They must also deal with a tendency to roll to the side while moving or at rest, as well as a lack of directional stability.

[11] Ranchu goldfish are also susceptible to dropsy, bladder issues and too large wen growth that can cover gills and eyes due to selective breeding and inbreeding.