Japanese water Crabs species Heikea japonica, being called as Heikagani crabs. These crabs’ hard shell have the pattern resembles a furious face of samurai.
The legend of Heikagani crabs was back in the year 1185. In the battle of Danno-Ura, an epic war between two Japanese clans, Heikea and Genji. Heikea was defeated in the sea battle.
Heikagani crabs are beliefs to be the souls of the drowned Heike warriors who are still lurking around, searching for its empire.
Source:
The Greatest Show on Earth: The Evidence for Evolution, by Richard Dawkins
Crabs Folklore 