I actually had this same thing happen to me a few years back. I had 2 skunks and they decimated one of my clams in the same manner. I'm not sure if the clam wasn't doing well and it just let them finish it off (it wasn't the healthiest when I got it a few weeks before) or what but I was pretty steamed about it. They weren't as big as yours tho. It's always neat to put your hand in the tank and let them pick at your hand, wish they didn't cost so darn much I really wouldn't mind to have a couple more (that is if my coral banded would't go rambo on them).