The Mystery of the Samurai Crab

The story starts in 8th Century Japan, with Emperor Saga-tennō fathering 49 children by at least 30 different women.

This had some unintended consequences for the succession of the empire, and who would take over.

After his death, his children roughly formed two clans, the Taira and the Minamoto and over the next two hundred years, battled it out to claim the throne.

This came to a head in 1185 CE at the battle of Dan-no-ura where the child Emperor Antoku and his army were defeated. Rather than falling into the enemy’s hands, the 6 year old boy and his grandmother, along with a large portion of his army, committed suicide by jumping into the sea.

This is where the crabs come in.

There is a species of crab in the area of Dan-no-ura that has a striking resemblance to the grimacing face of a Samurai warrior. In the centuries following the battle, these crabs were thought to be the reincarnated spirits of these warriors.

Traditionally these crabs were thrown back in the sea to honour the spirits. Which has led to the belief that by letting the crabs that resemble samurai faces back into the sea, humans have artificially selected crabs with these characteristics.

Artificial selection is nothing out of the ordinary, just look at pugs. But in this instance, these crabs are the result of natural selection rather than human selection.

These crabs are of the genus Dorippidae, and all the species have these ‘facial’ markings whether they’re found in Japan or not.

The markings are actually the external view of ridges on the inside called apodemes, which support muscles. These ridges allow more space on the inside of the crab for all its squishy bits.

The fact that we see faces in these crabs might have more to do with face pareidolia – the phenomenon of seeing faces in everyday objects. Which in humans is evolutionarily linked to being better at recognising other humans.

We still like the story.