According to japanese folklore, heikegani are crabs noted for the peculiar pattern on their shells. When viewed from above, they appear to have the scowling face of a samurai mask imprinted on their carapace. According to legend, this the result of the enmity of drowned samurai from the Taira clan — also known as the Heike — imprinting itself on these crabs.

During the twelfth century, a bloody civil war was fought between Japan’s two most important political clans — the Minamoto, or Genji, and the Taira, or Heike. The war was decisively ended in 1185 at the Battle of Dan no ura. The Genji and Heike fleets met at the narrow strait separating Honshū and Kyūshū. While the battle initially favored the Heike, as the tides changed the advantaged went to the Genji. On top of that, a Taira general defected and revealed the location of the Heike ship carrying the six year old Emperor Antoku and the imperial regalia. Genji archers rained arrows down upon the Heike fleet. When it became clear that defeat was inevitable, the surviving Heike, including the young emperor, flung themselves into the sea to drown rather than be captured. The battle resulted in a decisive victory for the Genji and the complete destruction of the Heike.

Although the Heike were physically exterminated, their souls lived on, carrying the grudge from the war and the betrayal of their general. The souls of the dead Heike warriors lost at sea became lodged in the crabs crawling on the bottom of the sea. The crabs’ shells mutated into the shape of a scowling face, a permanent reminder of the never ending grudge of the Heike.

In Kitakyūshū City, it is said that the souls of the Heike men turned into heikegani, while the souls of the Heike women turned into kappa.