The year was 1185, the place a tiny bay called Dan-no-ura. Two great fleets faced one another; on once side, the Heike clan, imperial rulers of Japan, and on the other the Minamoto, upstarts fighting to control the throne. At stake was control of all Japan. After a half-day of fighting, the Heike were routed, and their 6-year old emperor drowned to keep him out of Minamoto hands. Minamoto Yoritomo went on to become the first Shogun, or military ruler, of Japan.
A strange story arose in the wake of the battle. Locals told a legend about crabs in the area with strange patterns on their shells, said to resemble samurai masks. Legend held that the crabs were the reincarnations of samurai slain at the Battle of Dan-no-ura.
The crabs do bear an uncanny resemblance to samurai masks. Carl Sagan speculated in his show Cosmos that the resemblance was due to artificial selection. Basically, people would throw back crabs that resembled samurai masks, and eat the ones that didn’t. So that put selection pressure on the population to grow shells that resembled masks.
While it sounds good and it does fit the mold for how selective pressures tend to work, there’s a problem–nobody eats Heikegani. They’re too small. Plus, crabs with this kind of shell pattern aren’t confined to only that small bay, but they can be found all over the Bay of Japan. And there are other species of crabs with similar patterns, although maybe not as pronounced.
The folds and creases are points where muscles attach to the carapace. Humans just happen to think they look like faces–or masks–because of a phenomena called pareidolia, where we see faces in random patterns. It’s not quite as cool as reincarnated samurai ghosts, but then again, not many things are.