|Slightly blurry; footing is precarious in the seaweed and wobbly rocks.|
Bees have bee mimics; crabs have crab mimics. The porcelain crab is not a crab, but is more related to the squat lobsters, which are not lobsters, but, as Wikipedia says, "more closely related to the porcelain crabs."
The difference is obvious, once you know what to look for. Porcelains have only 3 walking legs. True crabs have 4. Of course, that is a maximum. All crabs lose legs and replace them after a molt or two, but the porcelains are especially prone to drop appendages when threatened, which is what gives them their name; as fragile as a porcelain cup.
Like a crab, the porcelain folds his abdomen down underneath the carapace, but in this case, it is free to move. In the true crabs, the abdomen unfolds to molt, or fans slightly open and shut to fan the eggs when a female is in berry. Otherwise, it stays firmly attached to the upper body.
The porcelain crabs are small. The two we saw (Yay!) were adult-sized, about an inch across the carapace.