Back in the early 90's I had a 29gal tank with three large crabs (they were about the size of your hand). I would come home and find them on top of the tank or other places around the room. I finally had to put a wire mesh top and tape it down to keep them from escaping. I can't remember what they were called and I haven't seen them anywhere in years. Watching them catch goldfish was really cool.
They looked like
these but mine were: yellow, orange, and a one was orange and dark blue.