Red-clawed crabs

Ok, this isn't so much cichlid-specific, but it does involve a cichlid tank
I was at Petsmart the other day, picking up a couple Eureka peacocks for
my tank (LFS didn't have these). They had a tank of red-clawed crabs, and I
picked up a couple, just for grins. One completely disappeared, but the
other one is doing fine. Eating, roaming, etc. Anyone ever have one of
these? Do I need to provide a way for him to get air occasionally (girl at
store didn't know)? Will my larger peacocks eventually id him as a food
source?