Fat Crab?

[GMGQ]

Last night I saw the infamous Fat Crab front and center in my tank. I reached over to get my digicam, but he scootched behind a rock in a flash.

His body is FAT and ROUND, like a jawbreaker. Looks like it has a belly. About the diameter of a TWOONIE.

Anybody have any idea what kinda crab this is?? All the over ones I've seen are black with hair legs. This guy was sorta white/grey, non-hairy legs.