Convex Crab, Convex Reef Crab, Variable Coral Crab

This Convex Crab has a worried look which is rather endearing, reminding me of a puppy or kitten. It was about 3" across, found in the surf on the northern shore of Kauai, and ha a puffed-up look, as though slightly over-inflated. This crab is wide-spread, found over a major part of the world, but there isn't much information about them available. With all its names, Convex Crab, Convex Reef Crab, Variable Coral Crab, you'd think someone would have done a study of it.