On the one hand, it seems that if any crabs were going to dissolve away part
of a mollusk shell with acid, it would likely be the terrestrial ones.  I
think this would be relatively difficult underwater, where any acid produced
would immediately be diluted and/or washed away.

On the other hand, a couple of new questions come to mind . . .

How could the crab produce enough acid to eat away the thickest, heaviest
part of the shell (the columella), without destroying, or at least eating
holes in, the thinner parts of the shell body, thereby making itself more
accessible to predators?

Once the inner structure of the shell has been dissolved away, how does the
crab hang onto the shell?  A hermit crab is difficult to remove from a shell
precisely because it's abdomen is specifically curved to fit the inner
spiral structure of a gastropod shell.

Paul M.