Each time we feel we have an understanding of what animals are, of their nature and capacities, the goalposts get moved. The more we learn about them the less we find we knew. Old assumptions have to be let go. Animals we'd once dismissed as brute, have been shown to possess greater subtlety than we'd ever thought possible; from empathetic yawning dogs, to trainable goldfish.
So, in line with the ever-changing goalposts I give you the pain of the crab:
http://news.bbc.co.uk/1/hi/northern_ireland/7966807.stm
It certainly raises interesting questions about the humanity of our killing/cooking methods with crustaceans.