What are the details on hairy crabs and why you don't want them?

The title pretty much sums up what I'd like to know. I believe I have been losing some of my cleanup crew so far due to some sort of murderous activity in the tank. I thought I might have a mantis because of the excessive popping sounds I hear coming from the tank, but I'm thinking that's most likely the pistol shrimp h.h. that came on my rock, but since I really don't have any other real predatory animals in the tank I'm thinking maybe the hairy crabs are the problem.

What's the deal with these things? How capable are these things of doing some serious damage to snails and hermits I have in the tank?

I've been trying to snag them as often as I see them. I've pulled out quite a few over the last few days that are probably about the size of a nickel.

For some reason I woke up in the middle of the night and so I decided since I was up I'd look in the tank. I noticed one of my brittle stars was about half out from under one of the rocks and I noticed that a few of its legs were much longer and thicker than one (or maybe two) of the other legs. I'm thinking something got a hold of one of the legs and so it's been slowly regenerating, but I can't prove that.

I think I need to devise a trap for the crabs. Maybe use the bottle method, which I can't seem to find any info. on....