According to this BBC article:

"crustaceans have decentralised nervous systems, meaning that unlike fish, they can't be rendered unconscious with a single blow to the head.

To "spike" a lobster to death, again unlike fish, you have to pierce the body in more than one spot."

From: What's the kindest way to kill a lobster? (Yes, I'm aware of the irony of the headline) http://www.bbc.com/news/world-europe-42647341