In the year 1185, the Emperor of Japan was a seven-year-old boy named Antoku. He was the nominal leader of a clan of samurai called the Heike, who were engaged in a long and bloody war with another samurai clan, the Genji. Each asserted a superior ancestral claim to the imperial throne. Their decisive naval encounter, with the Emperor on board ship, occurred at Danno-ura in the Japanese Inland Sea on April 24, 1185. The Heike were outnumbered, and outmaneuvered. Many were killed. The survivors, in massive numbers, threw themselves into the sea and drowned. The Lady Nii, grandmother of the Emperor, resolved that she and Antoku would not be captured by the enemy. What happened next is told in The Tale of the Heike:

The Emperor was seven years old that year but looked much older. He was so lovely that he seemed to shed a brilliant radiance and his long, black hair hung loose far down his back. With a look of surprise and anxiety on his face he asked the Lady Nii, ‘Where are you to take me?’

She turned to the youthful sovereign, with tears streaming down her cheeks, and … comforted him, binding up his long hair in his dove-colored robe. Blinded with tears, the child sovereign put his beautiful, small hands together. He turned first to the East to say farewell to the god of Ise and then to the West to repeat the Nembutsu [a prayer to the Amida Buddha]. The Lady Nii took him tightly in her arms and with the words ‘In the depths of the ocean is our capitol,’ sank with him at last beneath the waves.

The entire Heike battle fleet was destroyed. Only forty-three women survived. These ladies-in-waiting of the imperial court were forced to sell flowers and other favors to the fishermen near the scene of the battle. The Heike almost vanished from history. But a ragtag group of the former ladies-in-waiting and their offspring by the fisher-folk established a festival to commemorate the battle. It takes place on the twenty-fourth of April every year to this day. Fishermen who are the descendants of the Heike dress in hemp and black headgear and proceed to the Akama shrine which contains the mausoleum of the drowned Emperor. There they watch a play portraying the events that followed the Battle of Danno-ura. For centuries after, people imagined that they could discern ghostly samurai armies vainly striving to bail the sea, to cleanse it of blood and defeat and humiliation.

The fishermen say the Heike samurai wander the bottoms of the Inland Sea still - in the form of crabs. There are crabs to be found here with curious markings on their backs, patterns and indentations that disturbingly resemble the face of a samurai. When caught, these crabs are not eaten, but are returned to the sea in commemoration of the doleful events at Danno-ura.

This legend raises a lovely problem. How does it come about that the face of a warrior is incised on the carapace of a crab? The answer seems to be that humans made the face. The patterns on the crab’s shell are inherited. But among crabs, as among people, there are many different hereditary lines. Suppose that, by chance, among the distant ancestors of this crab, one arose with a pattern that resembled, even slightly, a human face. Even before the battle of Danno-ura, fishermen may have been reluctant to eat such a crab. In throwing it back, they set in motion an evolutionary process: If you are a crab and your carapace is ordinary, the humans will eat you. Your line will leave fewer descendants. If your carapace looks a little like a face, they will throw you back. You will leave more descendants. Crabs had a substantial investment in the patterns on their carapaces. As the generations passed, of crabs and fishermen alike, the crabs with patterns that most resembled a samurai face survived preferentially until eventually there was produced not just a human face, not just a Japanese face, but the visage of a fierce and scowling samurai. All this has nothing to do with what the crabs want. Selection is imposed from the outside. The more you look like a samurai, the better are your chances of survival. Eventually, there come to be a great many samurai crabs.