Once upon a time in the Straits of Shimonoseki there was an epic battle between the Minomotas and the Tairas. The Taira tribe took a total pasting and were more or less wiped out or drowned in the inland sea.
The HEIKE crabs which abound in this area have shells indented with what look remarkably like scowling faces. These are said to be the spirits of the departed Tairas. Not surprising as they have a lot to scowl about. They lost absolutely everything including the sacred imperial sword. Now they are doomed to wander around under the waters for eternity.
Those of you into evolutionary biology may like to ponder the significance of artificial selection in the case of the Heike Crab. The theory goes that no ghost-fearing person is likely to kill and eat a crab bearing such an image, so over the years, normal crabs are more likely to get eaten while the strangely-marked ones survive to pass on their characteristics. Result: Human superstition conspires to breed a race of crabs with startlingly realistic Samurai faces.
However, there are several other crab species outside of Japan with similar markings, so perhaps the Heike warriors were inspired by the crabs instead. If only mythology could be explained so easily, we could have a day off now and then.