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You need to trap him. Could build a trap out of a plastic pop bottle.
http://www.youtube.com/watch?v=LuA97UniMd8

Lol, don't freak out too much. I had living in my tank for over 2 years....I never did figure out why he suddenly disappeared.

They will eat all your hermit crabs and shrimp so you might want to remove them while you try to trap it

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how big was it?

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sorry about your crab ...as I suspect it is done for, but hey I would love to be in your shoes. a free mantis sounds like a great deal.

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we will take him