Whos Killing the Crabs?

[seashells]

Had a good look into the tank on Saturday, & to my surprise I saw a large crab with its claws open, not moving. Pointed a light at it ...still not moving. Took a better look around, and wow theres another one. We got them both out, but not sure if we crushed the heads getting them out or something else did. Also noticed one of are Purculas is missing. Everything else is fine. Havent heard any clicking or snapping noises. Any ideas?