Samurai Crabs: Transmogrified Japanese warriors, the product of artificial selection, or pareidolia?

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.

The ghost of the Heike general Taira no Tomomori (bottom left) at the bottom of the ocean with the anchor he used to drown himself following defeat at Dan-no-ura. He is joined by Heike crabs bearing the faces and souls of his comrades. By artist Utagawa Kuniyoshi, 17th century. Click to embiggen.

Samurai crab, H. japonica and stylized Kabuki samurai face (inset). Click to embiggen.

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.


  • Martin, J.W. 1993. The Samurai Crab. Terra, 31:4, 30-34.
  • The Samurai Archives: Japanese History Page
  • Dawkins, R. 2009. The Greatest Show on Earth: The Evidence for Evolution. Free Press.
  • Sagan, C. 1981. Cosmos. London, Macdonald.


  • The Ghost of Taira no Tomomori, Utagawa Kuniyoshi, 17th century, public domain.
  • H. japonica photo,, accessed Jan. 26, 2010.
  • Samurai Kubuki print, Utagawa Toyokuni Ill, 19th century, public domain.


58 Responses to “Samurai Crabs: Transmogrified Japanese warriors, the product of artificial selection, or pareidolia?”

  1. 1 Genghis Prawn January 28, 2010 at 11:37 am

    Interesting point about the size of these animals, but consider the fact that even miniscule river crabs (look up “Sawagani”), are harvested and consumed in Japan. I wouldn’t imagine the collection of these swift and diminutive Geothelphusa spp. to be any less effort-intensive than sorting through a net’s worth of their anthropomorphic marine cousins.

    That said, my feeling is that the story does sound a bit *too* neat to be true.

    • 2 _ April 19, 2010 at 2:50 pm

      Dear Genghis Prawn,

      As you may be aware, fresh-water crabs (as your cited Geothelphusa) are geographically restricted to their settings (let’s say rivers and creeks at first instance), and by itself restricted to their shallow waters. Heikeopsis (formerly Heikea), lives in a wide range of bathymetries, and as you may know, they do not have a geographic barrier to avoid their distribution. Having said that, a continental (fresh-water in this case) whatever organism will be ALWAYS easier to capture and manipulate (and artificially select), than vast and ‘theoretically less finite’ entire populations of marine organisms.


  2. 3 Mike Bok January 28, 2010 at 11:56 am

    Martin, J.W. 1993 claims that crabs from the Dorippidae are universally discarded by fishermen around Japan, regardless of whether the particular species has facial markings or not.

    It could be that Geothelphusa spp. have a much softer carapace, allowing them to be cooked and eaten whole (which is apparently how Sawagani is prepared).

    Perhaps someone with first had experience of Dorippidae and Potamidae crabs can shed some light?

  3. 4 Rambling Woods February 1, 2010 at 11:39 am

    How much of the natural world inspires myths and stories, hopefully enough that we learn to take care of it also… Mine is a less scientific look at my yard.. Michelle

    My COSAutumn Meadowhawks Mating.

  4. 5 Rich Daniel (@rhdaniel) December 18, 2011 at 9:54 pm

    Thank you so much for citing the Martin article! I’m writing a paper on a film called Kuroneko that utilizes these crabs as symbols, and his article was immensely helpful for my argument.

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    If the fishermen didn’t eat these crabs, is it possible that they took care to throw back the ones with a resemblance to a samurai but left others to die in their boats? Maybe I’m grasping at straws, but I’d heard the story of these crabs as an example of evolution without knowing that they are so tiny and therefore probably not eaten, and now I’m a bit disappointed that the explanation from Huxley and others seems unlikely. Thank you for your excellent explanation though. Perhaps this is an example of just how powerful pareidolia is – powerful enough to lead such a fine brain as Carl Sagan’s astray.

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Michael Bok is a graduate student studying the visual system of mantis shrimp.

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