You might notice something peculiar about Heikea japonica.
In his 1980 documentary series Cosmos, Carl Sagan (who was absolutely brilliant) used this species as an example of evolution by artificial selection. Thomas Henry Huxley, one of Charles Darwin's greatest advocates, apparently did the same. The two claimed that Japanese fisherman tossed back crabs that had human, or more specifically, samurai-like faces. Over time this selection pressure led the entire population to this phenotype. It's really a very cool example, but also contentious. Here's another picture:
As a side note: personally, I'm not a fan of 'artificial selection' in this case; is any selection pressure exhibited by humans on another species artificial? Where do we draw the line, perhaps domestication? But that's a different and perhaps more boring semantic conversation. Here's a giant Japanese spider crab (nearly 13 foot leg span!) to bring you back.
Joel Martin argues in 'The Samurai Crab' that only natural selection played a part in shaping this animal's appearance. He points out that the contours on the crab carapace are indications of internal muscle attachment points(apodemes), which is useful information, but not nearly as useful as I think he thinks it is.
Martin's strongest claim is that the crabs, not more than 1.5 inches across, were too small to be eaten. He also cites other species which show human/samurai-esque faces (pictures below), some of which live in the same waters off of Japan as H. japonica. These other species are so similar that locals don't differentiate between them. Perhaps all of the crabs in the region experienced similar human induced selection?
The jury is still out on this one; proponents of the Sagan/Huxley version say that villagers would have eaten these crabs historically, especially before mass-fishing practices became popular. Also, just because other species exhibit similar characteristics does not mean selection didn't occur to put the finishing touches on the modern H. japonica appearance. I submit that eye/mouth mimicking shapes occur often in nature to scare off visually guided predators, perhaps another reason for the face shape on this intriguing crustacean.
Pictures 1 and 4 comes from The Samurai Crab, linked above.
The giant crab comes from wikipedia.
Picture two can be found here.