I’m going to share with you two stories about the Samurai Crab, Heikea japonica; one is a ancient Japanese legend, and the other is a modern piece of scientific folklore. First, the historical background of the Japanese myth…
April of 1185 was the end of the line for the Heike Empire in Japan. Their rival clan, the Genji, swept across the southern Inland Sea of Japan and annihilated the final, desperate armada of the Heike at the battle of Dan-no-ura. The defeated Heike child-emperor and his samurai, refusing to surrender, committed suicide by throwing themselves from their boats to drown.
The battle of Dan-no-ura represented a massive cultural and political shift in Japanese history. It was the end of the imperial Age of Courtiers, and the beginning of the Feudal Era of Japan, lasting from 1185 to 1868. During this time, the majority of Japan was ruled by a powerful military dictator, the shogun, and his samurai warriors. Naturally, such geopolitical upheaval has strongly ingrained itself in the culture of Japan, and folklore stemming from the battle of Dan-no-ura survives today.
Popular legend alleges that, following the battle Dan-no-ura, the souls of drowning Heike samurai warriors were transformed into crabs. These crabs are distinguished by having the faces of the fallen samurai on their backs. To this day the Heike crabs roam the depths of the oceans around Japan, searching for the lost heirlooms of their empire.
This story has also generated a bit of modern scientific folklore; put forth by the likes of Julian Huxley (biologist and grandson of “Darwin’s bulldog”), Carl Sagan, and Richard Dawkins, who approaches the idea with appropriate skepticism. The story goes that, the resemblance of the crab’s carapace to the stylized face of an enraged samurai is the product of artificial selection; an evolutionary selective pressure applied by humans. I personally have always thought that it is unnecessary to artificially remove ourselves from under the tent of natural selection, but that is a different discussion. The primary examples of artificial selection include the domestication of plants and animals.
Those who believe that H. japonica gained its carapace patterns from artificial selection claim that generations of superstitious Japanese fishermen have selectively released any crabs with even the most remote resemblance to a human face. Thus, the samurai-face-like crabs were more likely to pass on their genes to the next generation. Eventually, this resulted in facial characteristics becoming evolutionarily fixed in the population of H. japonica. Here is Carl Sagan’s infinitely more eloquent description of the idea:
Though the samurai face on the back of H. japonica is a interesting story with which to demonstrate the power of selection, it is, unfortunately, most likely a scientific urban legend. Problems with this hypothesis include the fact that these crabs only reach a maximum carapace length of 1.2 inches. Anyone who has eaten steamed crabs can immediately recognize that such a minuscule crab as H. japonica is not worth the effort it would take to extract its meat. Therefore, it is unlikely that these animals were ever caught for food in the first place.
In addition, H. japonica is not the only crab with a human-looking face on its back. A variety of crabs from the family Dorippidae all appear to have human faces on their backs. Some of these crabs do not live within human fisheries. It turns out that the groves and ridges on the backs of crabs have nothing to do with artificial selection; they represent muscle attachment sites and organ chambers, reflected in the internal viscera of the animals. Instead, any resemblance the carapace of H. japonica bears to a samurai warrior is likely the result of simple pareidolia. Pareidolia is an illusory manifestation wrought by the human brain’s innate ability to recognize faces and human forms. Put two dots side by side above a horizontal line and your brain says, “Face!” Pareidolia is responsible for a multitude of pop-culture phenomena including the face on mars, the man on the moon, the Virgin Mary on toast, and the faces of Heike samurai on the backs of crabs.
This one small crustacean, H. japonica, has inspired mystical reverence to ancient legends, as well as scientific investigation into the power of selection. The story of the Japanese Samurai Crabs has transcended down through generations and across cultures. They aptly illustrate the power of an idea; how it can spread, and evolve, and cast its roots, searching for purchase upon the soil of thought.