Thanks for all the info. He does seem to keep to himself. He's made a hole in one of the rocks and hides out there. The past few months I've heard the clicking noise in the tank. I thought I must have had a mantis or pistol shrimp. But, maybe it's this crab. And if it is the crab, what's the noise he's making? I guess I'll leave him be. Kinda cute lil' guy.