The hunt is on to uncover just what, exactly, this creature is. The Cambodians call it a “kapas,” which sounds like a localisation of carapace. The dish is called “Balang Kas,” and the one pictured here is from Andart Rorm (Dancing Tongue) Restaurant. The animal looks a little bit like an Asian paddle crab, but those are not native to the gulf, where all the seafood served at the restaurant comes from.
UPDATE: Veteran Asia hand Ian Taylor has the answer: Horseshoe crab, http://www.youtube.com/watch?v=5O0lqKc7Ob4