I've got CRABS!

I also got rock from florida and it was packed full i different corals and critters. But it also came with gorilla crabs there were 3 that i found in the box when i opened it. About a week after i had put it in my tank i started seeing a couple dead ones here and there than i saw what was killing them. There were the little mantis shrimp that were about 1 1/2 to 2 inches long. I left them in my tank for about 4 months because every time i put hermit crabs in they would pick them off one by one. But they were very interesting to watch zoom in one hole of the rock and out another somewhere else. I didnt have any trouble catching them either.