I don't know about the spider crab, but my guess is that there is nothing you can do...
About the hermits and snails, unfortanetly, that's all too common. You can try and prevent it by adding in larger, empty shells for the hermits (they are probably killing the snails for their shells). I had 6 or so hermits wipe out my entire tank (30 snails or so) in a few months...
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