If I recall Fenner's "Conscientious Marine Aquarist" (or maybe it was Fenner/Calfo "Reef Invertebrates"), porcelain crabs have one less set of visible legs. Not counting the big claws, only 3 sets of visible legs, versus 4 for all the other crabs.
I've got a picture posted here someplace (in a post, not in gallery) of a red emerald crab, which is a less desirable hitchhiker.