Hey there MRnR,
That's a great pic
I have one of these crabs too, he's been with me for over six years now (wow has it really been that long!). Is that a good age for a crab? I've got no idea. Anyhow he's been very well behaved, munching on algae and such like. I did catch him polishing off a
Turbo snail once but I think that it had already died and he was simply on clean up duty

Fortunately my snails don't have a habit of disappearing, hehe.
Here's a pic of mine cleaning up the shell of a
red legged hermit 
Infact this incident was rather amusing. The left-handed hermit cleaned up the little red leg's shell and then the red leg had a go at cleaning the left-handed hermit's shell. Sort of I'll clean your back if you'll clean mine
BTW These hermits have got THE most fantastic blue eyes don't you think.... they've just got to be contact lenses
Regards
Lisa