The book “Continuum Encyclopedia of Animal Symbolism in World Art” by Werness contain the lists of interesting stories about the crabs:
1. “The Inca thought of the crab as a devourer and connected it with the waning moon”
2. Crab is known as lunar symbol. This belief is based on the fact that “crabs cast off their shells when they outgrow them” this symbol also related with the phase of the moon.
3. “In the Medieval period, the crab was seen as evil, standing for deceit, cruelty, inconstancy, and greed.”
Source:
Continuum Encyclopedia of Animal Symbolism in World Art, by Hope B. Werness
Crabs Folklore 