Heikegani is a species of crab native to Japan, with a shell that bears a pattern resembling a human face which many believed to be the face of an angry samurai hence the nickname Samurai Crab. It is locally believed that these crabs are reincarnations of the Heike warriors defeated at the Battle of Dan-no-ura as told in The Tale of the Heike.
Heikegani were used by Carl Sagan in his popular sciencetelevision series as an example of unintentional artificial selection, an interpretation published by Julian Huxley in 1952. According to this hypothesis, the crabs with shells resembling samurai were thrown back to the sea by fishermen out of respect for the Heike warriors, while those not resembling samurai were eaten, giving the former a greater chance of reproducing. Thus, the more closely the crabs resembled a samurai face, the more likely they would be spared and thrown back. This idea has met with some skepticism, as noted by Joel W. Martin. He posits that humans do not use heikegani for food, and as such there is no artificial pressure favoring face-like shell patterns, contrary to Sagan's implication. The pattern of ridges on the carapace serves a very functional purpose as sites of muscle attachment. Similar patterns are found on the carapaces of other species and genera throughout the world, including numerous fossil taxa.
Battle of Dan-no-ura
The Battle of Dan-no-ura was preceded by an immense struggle between the imperial rulers of Japan, the Taira clan, who the Heikegani crabs are named after, and the Minamoto clan, who were fighting for control of the throne at the end of the 12th century in the Genpei War. On the 24th of April, 1185 AD, two powerful Samurai clans fought to the death on the Dan-no-ura bay of Japan's Inland Sea. The ruling Taira clan was led by their child-Emperor, Antoku, and his grandmother, Tokiko Taira. The Heike had ruled Japan for many decades, but now, massively outnumbered, they faced defeat at the hands of the Minamoto. During the battle, Tokiko took the seven-year-old Emperor Antoku and leaped with him into the water in the Shimonoseki Straits, drowning the child emperor, rather than allowing him to be captured by the opposing forces, and most members and generals of the Taira clan followed them in despair. Antoku came to be worshipped as Mizu-no-kami. This crucial battle was a cultural and political turning point in Japanese history: Minamoto Yoritomo became the first Shōgun, or military ruler, of Japan. Dan-no-ura marked the beginning of seven centuries during which Japan was ruled by warriors and Shōguns instead of Emperors and aristocrats.
