The Heikegani Crabs and the Problem with Pareidolia

If you’re a regular reader here, you no doubt caught the exchange that took place between myself and a commenter named A.J. Kitt on my recent post about pattern recognition.  If not, you can read it at your leisure.  The post in question was an explanation of the various cognitive concepts involved in human pattern recognition; of prominent position in the discussion were the concepts known as apophenia and pareidolia.  Suffice it to say, Kitt and I disagree on the correct definition for those terms, though as has been pointed out, we are both right, to a degree.

The issue is muddied by several popular misconceptions about the process, and by some relatively famous misrepresentations of natural items that give a skewed image of the issue.

One such item is the Heikegani crabs of Japan.

“The sound of the Gion Shōja bells echoes the impermanence of all things; the color 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 Heiki. Chapter 1.1, Helen Craig McCullough’s translation

It seems ironic to me, that the tradition from which the legend of the Heikegani crabs comes is embodied by the above quote from the opening line of the epic prose of the Tale of the Heiki.  Impermanence and the fleeting nature of reality seem to be its central themes, and though the same sentiment is woven throughout the tale, the legend of the crabs has proven to have the staying power of the best ancient morals.

If you’re familiar with Carl Sagan, you’ve probably heard of the Heikegani crabs.  He showcased them in a segment on his original PBS science show, Cosmos: A Personal Voyage.  In that segment, Sagan was expounding on a theory put forward by Julian Huxley in 1952, whereby these crabs are the evolutionary products of unintentional artificial selection.  It’s a relatively simple concept – that some artificial mechanism selects certain members of a species for survival, based on some apparently arbitrary physical characteristic of the species in question.  This particular example, however, requires some in-depth backstory to be fully understood.

Native to the Bay of Japan (or Tokyo Bay, Edo Bay), Heikegani crabs are most often found in the small bay called Dan-no-ura, which was the location of the famed battle of the same name in the Genpei War (1180-1185).  That war, or more specifically, that battle, were instrumental in the creation of the Heikegani Crab Legend.  The Battle of Dan-No-Ura was a crushing defeat for the Heike (or Taira by some translations) and their child emperor Antoku.  They were the ruling samurai clan at the time, but many samurai were killed in the bay, and ultimately, Antoku was drowned by his caretaker to ensure that he would not be captured by their ruthless enemy, the Kyōto and their leader, Minamoto Yoritomo.

Following that battle and the subsequent loss of the war, the former subjects of the child emperor noticed something strange happening in the waters of their now infamous bay.  The crab seemed to be taking on a carapace design that showed a resemblance to an angry samurai warrior.  It was said that the noble Heike samurai who were felled in the battle were being reincarnated as crabs, and were showing their allegiance to their clan by donning a fierce mask on their shells.

It’s fairly easy to see how such a legend comes about.  It would take only a few instances of crabs with a carapace that even remotely resembled the grimaced face of a warrior to convince a superstitious people that their beloved samurai protectors were reborn and still, somehow, watching over them.

It’s not as far-fetched as one might think either.  The crabs exist, and they do, uncannily, look like they wear a carved mask of an angry samurai warrior.  This has been well documented and it’s quite baffling to a good number of people.

Now, this isn’t to say that anyone actually believes they are truly reincarnated warriors from The Battle of Dan-No-Ura.  Or, well…not many people do.  And this is where Sagan and Huxley come into it.

Such a strange thing requires an explanation, and though some would like to simply declare the Heikegani crab an example of pareidolia and be done with it, it’s really not that simple.  Huxley believed that the crabs were a cultural artefact of the local people.  He believed that at some point in the past, fishermen began to see crab that only scarcely resembled the samurai masks of the modern crabs, and that upon that discovery, their superstition and reverence for their cultural heritage caused them to begin throwing back any and all crabs that showed such an image on their backs.

Anyone familiar with the process of evolution can already see where this is going.  The fishermen were inadvertently giving the crabs with the samurai images a drastically better chance for survival than those without, thereby artificially selecting for whatever genetic happenstance occurred to give them that appearance.  Ultimately, Huxley believed that the remaining Heikegani crab are all descended from those early crabs who were given a permanent reprieve from human fishing activity and ensured their survival.  Not only that, but as the practise continued, those same fishermen eventually began to refine the selection process, unintentionally selecting those crabs which more and more resembled their current form.  Not only did they give them a reprieve, but they made them better at creating the masks on their shells.

There’s a problem with this theory though, and it’s one that’s kind of embarrassing for Sagan and his fans.  While the above is a plausible explanation, it doesn’t bear a little fact checking and knowledge of history.  The main problem is that the Heikegani crab were never on the menu for any Japanese culture.  No one was fishing for them, thus no one was throwing back the ones which resembled their beloved samurai.  It’s a simple problem, but it’s also a devastating problem for Huxley’s theory.

Heikegani crab are quite small, and though they are abundant in the waters of Southern Japan, history knows of no instance of them being a part of any Japanese diet, though they were likely eaten occasionally in some regions.

The features of the namesake masks on their shells are in fact the result of connection points for muscle and ligament tissue.  As you’re well aware (or at least you should be), crabs – belonging to the phylum arthropoda crustacea – employ an exoskeleton.  That means that they wear their bones on the outside, but they still have muscles and connective tissues inside that make their limbs move.  Just like us (endoskeletons), those tissues have to have a strong connection to the bone in order to exert leverage over their limbs.  Those connection sites tend to require specific shapes and locations for the system to work properly, and in the case of the Heikegani crabs, that means that their carapace looks like an angry samurai mask.

So, while it’s clear that the face we see on the backs of these crabs is not actually a face, but is in fact the refined musculature of a well-adapted crustacean, the Heikegani crab serve as an excellent tool for demonstrating that the concept of pareidolia is not the end of the story when we see faces where none exist.  It’s important to understand that even though pareidolia or apophenia may be the reason we see what we see, it’s not generally the reason why what we see is there to begin with