Houdini Crab??

[BCWolfen]

So my brand new Arrow Crab was in the tank for all of 2 days and this morning I found him lying on the floor.

From the time I put him into the tank he prefered to stay on the rock work near the top. I did see him happily munching on a bristle worm which was the main reason I got him.

Has anyone else experienced this with arrow crabs? Is this normal behaviour or did I just end up with a suicidal wacko? I'm not sure that adding egg crate would stop a new one from doing the same as they can eaisly fit through that. Maybe I have to redesign my rockwork around my overflows. They are nice an hidden as they are but an easy climb for an arrow crab....