Well, I feel really stupid. Two days ago I bought a couple of emerald crabs for my tank. Well one day later I saw one of them was on it's back at the bottom of my tank so I was pretty upset, I thought I didn't aclimate them properly or my water parameters were off.
So I checked EVERYTHING, even though I'd checked them only 3 days earlier. I took out the dead crab and threw it away.
Well, last night I was removing my rock to re-aquascape everything and low and behold, I found a well hidden crab in one of the rocks (the other one I had removed and put aside in a small bowl). It would appear that the crab had molted and I had found it's dead shell.
I knew that other people had done the same thing, but I really thought it had died since I had only had the crabs for 2 days.
So, good news is the crab isn't dead, bad news is I'm a moron.
This crab almost doubled in size with it's molt. That's amazing.