Dead hermit crab

[Minireeftank]

Last week I added two tiny hermit crabs to my nano reef- I spotted one of their shells upside down near a rock and left it a few days assuming it was moulting but today found it was empty and a few crumbs came out- definitely hasn’t moved shells as it wasn’t near the new ones- what killed/ ate him? Could it have been a bristleworm as at night I do see some giant ones and he was a teeny hermit crab