Samurai Crab

InvertebrateEast Asia

Feb 21

Written By Alexander Julius Jensen

Heikeopsis japonica

The samurai crab's shell resembles the face of a samurai warrior. A popular theory proposed that fishermen spared the crabs with the most face-like shells, throwing them back instead — selectively breeding the species to resemble a scowling samurai. While a neat idea, it's unlikely to be true.

Battle of Dan-no-ura Bay

The story of the samurai crab begins with the end of the Heike Empire.

It is April of 1185. Two armies face each other in Dan-no-ura Bay, a narrow strait of Japan's Inland Sea. On one side are the ruling Heike, led by their child emperor, Antoku. On the other, the Genji, who seek to take the throne. The battle is to the death. The Heike have the initial upper hand but soon, through bloodshed and betrayal, the tides turn. The Genji send their arrows like hailstorms unto Heike ships and the failing army recognizes their imminent defeat. The remaining Heike, along with their child emperor, leap into the sea, choosing the suffocating waves over the shame of capture. With their victory, the Genji usher in an age of military rulers that will last seven decades, with their leader, Minamoto Yoritomo, reining as the first Shogun. The Heike warriors lie at the bottom of Dan-no-ura Bay.

Their vengeful souls, however, are restless. They escape their dead human husks and seek something else to latch onto. But what is there at the bottom of the sea? With little choice, the warriors' souls enter into the little crabs that live along the seafloor. Confined inside these crustaceans, consumed by vengeance but unable to take it, the souls twist their shells into fierce, grimacing faces, full of hate. But all they can do is scuttle and scowl.

Samurai Selection

The Heikegani, literally translating to the "Heike crab," is a species of crab whose shell bears a more-or-less striking resemblance to an angry face or samurai mask — a resemblance striking enough to capture the attention of curious minds across the world. The evolutionary biologist Julian Huxley wrote in 1952 that "the resemblance of Dorippe to an angry Japanese warrior is far too specific and far too detailed to be accidental; it is a specific adaptation which can only have been brought about by means of natural selection operating over centuries of time. It came about because those crabs with a more perfect resemblance to a warrior's face were less frequently eaten than the others." Later, in 1980, Carl Sagan used these crabs as an example of artificial selection.

Artificial selection is how we got our motley of dog breeds, our uber-productive livestock, and our bulging crops. The process is the same as natural selection — those individuals who have the most offspring, pass on the most genes, and thus traits, ultimately shaping the future generations — with one critical difference; the selector. In natural selection, individuals are "chosen" for their survivability and fecundity. Death is the ultimate sieve, and of those that survive, the most reproductively successful disproportionately influence the future of their species. The latter holds true for artificial selection, but instead of death and mate selection deciding who is "successful," it is the human hand — or, more accurately, the human eye or stomach.

Most of our domesticated animals were shaped by our needs — altered from their wild ancestors through artificial selection — long before Charles Darwin coined the concept in the first chapter of On the Origin of Species (1859). We chose the individuals with traits we desired and we bred them, over generations, to be more fat, or furry, or friendly, all without even knowing how those traits were passed from one generation to the next (discovered by Gregor Mendel in 1865, but only widely recognized decades later). We've been changing animals (and plants) to suit our needs for over 10,000 years. But not every case of artificial selection is intentional. Even preferential feeding could be considered a type of selection — early humans, simply by feeding the boldest or most amicable wolves, likely contributed to the increased survival of those individuals and thus their unique traits. Long before we intentionally bred for chihuahuas and Great Danes, we — at least initially — unintentionally helped their ancestors, the wolves friendliest to humans, survive and proliferate.

The samurai crab is a product of unintentional artificial selection, or so the theory goes. It wasn't feeding them, which has shaped them, but not eating them. The idea goes like this; Japanese fishermen noticed that some crab shells resembled human faces and, whether due to superstition or respect, refused to kill those that looked most human, instead throwing them back to sea. As a result, the crabs with the most face-like shells had a higher chance of surviving and having offspring — offspring that share this same face-shell trait. Over the following generations, crabs with the most pronounced human features enjoyed a consistent survival advantage, while those with little resemblance were caught and killed, their like removed from the genepool, and the species as a whole eventually became dominated by crabs with strikingly humanoid shells. This story is a neat example of artificial selection in action. There's only one problem with the particular case of the samurai crab. It's probably not true.

Ancient Myths and Modern Folklore

The Heike crab isn't confined to Dan-no-ura Bay, but can be found along Japan's coasts and the seas surrounding Taiwan and southern China. It seems the samurai souls have, in the past 840 years, taken to wandering far from the site of their last battle. In reality, these crabs have long been present outside of that singular, legendary bay. This poses a problem for the artificial selection theory; how could such disparate populations of this species, from the Sea of Japan to the Yellow and East China Seas — all of which resemble samurai faces — have been bred by superstitious fishermen? Perhaps it's possible, if unlikely, that an aversion to eating human-faced crabs was more widespread. Regardless, another fact all but damns this scientific folk story. The Heike crab is small — the width of its carapace measures less than 3 centimetres (1.2 in) — too small to be worth catching and it isn't commonly eaten in Japan. All crabs are thrown back, not just the ones with samurai faces.

Clockwise from top; A dorippid crab (*Paradorippe granulata*), a fossil of a dorippid crab (*Dorippe sinica*), and a masked crab (*Corystes cassivelaunus*).

The samurai crab's "face" is just a coincidence of its physiology, combined with our tendency to see human faces everywhere we look. The grooves of its carapace are the result of supportive ridges called apodemes, which serve as sites for muscle attachments, while the bulging parts which form the angry "eyes" and "nose" of a samurai are pockets that provide additional space for the crab's organs. This same carapace pattern appears throughout the Dorippidae crab family, to which the samurai crab belongs, and is present in at least 17 species from two different families — such as the so-called "masked crab" (from the family Corystidae), distantly related to the samurai crab. Many don't live within range of human fisheries. And it's not just modern crab species. Fossils of dorippid crabs, and other closely related species, feature human-like faces — crabs that lived long before the Battle of Dan-no-ura Bay and long before the first humans even evolved.

Pareidolia

Clockwise from top; a face on the moon, a death's-head hawkmoth (*Acherontia*), and an orca (*Orcinus orca*).

Our brains are wired to be especially sensitive to human faces. We are a very social species and, for a long time, our individual survival depended on being part of a tribe or family — being ostracised likely meant death. Creating conflict or breaking a social taboo was to be avoided at all costs. For that, it helped to be keenly observant of others' facial expressions. Are they sad or angry or happy? If you could figure out their internal state from their expressions, you could better respond in a way that kept the peace, and ultimately, your place within the group. We became so good at reading human faces that we now see them everywhere; in cars, electrical sockets, and on the moon. This is called pareidolia — perceiving meaningful images in random patterns — and those meaningful images are often faces. We create a cartoonish face for an orca, seeing its large, white spots as its eyes. We see a human skull on the back of a death's-head hawkmoth. We see a warrior's face within the ridges and grooves of a samurai crab's carapace.

The samurai crab has been the focus of folklore both supernatural and scientific. Is it possessed by the souls of a drowned samurai army or an unintentional experiment in artificial selection? The answer, perhaps a disappointing one, is that this crab's shell is more like the Man in the Moon, than an ancient revenge tale or evolutionary lesson.

**Where Does It Live?**

⛰️ Shallow coastal waters.
📍 Dan-no-ura Bay, along Japan's coasts and the seas surrounding Taiwan and southern China.

Size // Tiny
Length // 3 cm (1.2 in) carapace width
Weight // N/A
Activity: Nocturnal 🌙
Lifestyle: Solitary 👤
Lifespan: N/A
Diet: Omnivore
Favorite Food: Other crustaceans 🦐
Phylum: Arthropoda
Class: Malacostraca
Order: Decapoda
Family: Dorippidae
Genus: Heikeopsis
Species: H. japonica

Legend has it that samurai crabs, also known as the Heikegani ("Heike crabs"), are inhabited by the souls of soldiers from the Heike Empire who drowned during the battle of Dan-no-ura Bay in April of 1185 — their vengeful spirits twisting the crabs' shells into scowling faces.
Another piece of folklore — this time scientific — suggested that this crab's shell was artificially selected to look like a human face. The theory goes like this; fishermen refused to kill the crabs with the most human-like shells, throwing them back to sea instead, and over many generations, the fishermen unintentionally bred the entire species to look more humanoid — since the crabs with the most human-like shells had a better chance of surviving and reproducing.
The artificial selection theory is debunked by a few facts.
- The first is that the Heike crab isn't confined to Dan-no-ura Bay, but can be found along Japan's coasts and the seas surrounding Taiwan and southern China.
- The second is the crab's size. With a carapace less than 3 centimetres (1.2 in) wide, the crab is too small to be worth catching and it's not commonly eaten in Japan.
- Thirdly, it's not the only crab with a face-like shell. The same pattern appears in seventeen species from two different families, and even in fossil crabs that lived long before the Battle of Dan-no-ura Bay — and long before the first humans even evolved.
The grooves on a Heike crab's carapace are the result of supportive ridges called apodemes, which serve as sites for muscle attachments, while the bulging parts that form the angry "eyes" and "nose" of a samurai face are pockets that provide additional space for the crab's organs.

The samurai crab's “face” is just a coincidence of its physiology combined with our tendency to see human faces everywhere we look.