This is a Heikegani, a kind of crab. I'll let Wikipedia talk:

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 Heike Monogatari ("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. However, some experts have questioned this. The story goes that the crabs with shells resembling Samurai were thrown back, while those not resembling Samurai were eaten, giving the former a greater chance of reproducing.

samurai pizza crabs

Both stories of origin are bunk, but they still look a lot like japanese ceremonial masks.