“The sound of the Gion Shōja bells echoes the impermanence of all things; the colour of the sāla flowers reveals the truth that the prosperous must decline. The proud do not endure; they are like a dream on a spring night; the mighty fall at last, they are as dust before the wind.”
— The Tale of the Heike, 1.1, translated by Helen Craig McCullough
It seems ironic to me that the tradition from which the legend of the Heikegani emerged is embodied in the opening lines of The Tale of the Heike. Impermanence and the fleeting nature of things are among its central themes, and though that sentiment runs throughout the tale, the legend of the crabs has demonstrated the staying power of the best ancient stories.
If you’re familiar with Carl Sagan, you’ve probably heard of the Heikegani crabs. He showcased them in a segment of his original PBS series, Cosmos: A Personal Voyage. In that segment, Sagan described a theory proposed by Julian Huxley in 1952: that these peculiar crabs were the evolutionary products of unintentional artificial selection.
It’s a relatively simple idea. Human beings, deliberately or otherwise, favour the survival of individuals possessing a particular physical characteristic. Over enough generations, that preference can alter a population. But to understand how anyone could possibly have done that to a crab, we first have to go back nearly eight hundred years before Huxley—to a battle at sea, the destruction of a clan, and a legend about the faces of dead samurai.
In the year 1185, in the waters of Dan-no-ura off the coast of Japan, a battle raged between the two dominant powers of the time: the Taira and Minamoto clans. That battle became known as the Battle of Dan-no-ura. The Taira, also known as the Heike, suffered a decisive defeat, and rather than surrender to Minamoto warriors, many of the Heike chose death in the sea. Among them was the child Emperor Antoku, who, according to The Tale of the Heike, was taken into the arms of his grandmother and carried with her beneath the waves.
Over time, the defeated Heike samurai became associated with a small species of crab found in those waters, whose carapace bore what appeared to be the likeness of an angry samurai’s face. Those crabs became known as Heikegani, or Heike crabs.
You’re probably now wondering just how a crab could end up with the face of a samurai on its back, and there are some explanations. Though remember, things aren’t always what they seem.
One seemingly satisfying answer is that Japanese fishermen caused it. Well, sort of. It was said that fishermen would occasionally find Heike crabs in their nets and, upon seeing the face on their backs, throw the poor things back into the water, allowing them to survive another day.
As mentioned, the famous British zoologist and evolutionary biologist Julian Huxley (1887–1975) believed that the crabs had undergone a form of artificial selection. If fishermen preferentially threw back those crabs whose carapaces bore the most striking resemblance to a samurai’s face, they would unwittingly give those individuals a greater chance of surviving and reproducing than those whose resemblance was less pronounced. Over many generations, that selective pressure could have exaggerated the facial appearance throughout the population.
This answer is, on the surface, elegant, plausible, and deeply satisfying—which brings us back to Carl Sagan. When Sagan presented Huxley’s idea to millions of viewers on Cosmos: A Personal Voyage, he helped turn an intriguing evolutionary hypothesis into perhaps the best-known explanation for the samurai-faced crabs. Which is quite an endorsement, if I do say so.
There are some problems, however. Not the least of which is that Heike crabs are terribly small creatures and are not normally eaten. Huxley’s explanation also treats the samurai-faced crab as essentially a local phenomenon, tied to the fishermen and traditions surrounding the waters of Dan-no-ura. And this isn’t the case.
Heikeopsis japonica, the current scientific name for the species in question, inhabits a broad range extending from Japan and Korea through the coastal waters of China and Taiwan, and as far south as Vietnam. This has a potentially devastating effect on Huxley’s elegant explanation, for if the same carapace features are present throughout the species’ range, it can hardly be argued that Japanese fishermen imposed any appreciable selection pressure on populations living far beyond the reach of that tradition. And current scientific literature confirms that the face-bearing morphology is common in H. japonica throughout its range. In fact, Heikegani aren’t even the only species of crab to bear this strange feature.
So, if Huxley was wrong about artificial selection, why do these little crabs appear to bear the face of a samurai?
Perhaps we’ve been looking at it from the wrong direction. It’s now known that the features we see as a face are actually morphological structures related to the crab’s underlying anatomy—the arrangement of grooves, ridges, and depressions in the carapace reflecting, among other things, the attachment of muscles and the internal organization of the animal. The shell looks the way it does because the anatomy of the crab requires it to.
There is no face until we give it one.
There is, of course, a name for that: pareidolia. That is the human propensity for deriving familiar and meaningful patterns from ambiguous information. It’s what allows you to see animals in clouds or faces in coffee foam. Those famous stories of people seeing Jesus in their toast are examples of pareidolia at work. In the case of the Heikegani, the anatomy supplies the pattern. We supply the face.
When we combine two elements of this case, we end up with a more plausible explanation for the Heike crabs bearing the face of an angry samurai. Pareidolia tells us why we see a face, and the tradition of The Tale of the Heike tells us why we see a samurai.
Mystery solved, I suppose, and the impermanence highlighted in The Tale of the Heike seems to have come full circle. The Taira’s power vanished into the ocean along with their warriors. The Heike themselves faded into history, and ultimately into legend, and so too does Huxley’s explanation for their Heikegani.
We can probably forgive Huxley for seeing the influence of the legend in the evolution of the crabs, but ultimately, he was seeing a face where there was only the ordinary anatomy of a crab.
The crab isn’t carrying the face of a dead samurai. We are.
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