(von Siebold, 1824)
Heikegani (平家蟹, ヘイケガニ) (Heikeopsis japonica) 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 spirits 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 science television show Cosmos: A Personal Voyage 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 resemble 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. As humans don't use heikegani for food, Martin posits that 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 species in many parts of the world, including fossil remains.