Both photos above show a crab in the process of molting by backing out of its old shell.

Jeff Cox (Bering Sea Fisheries Research Foundation) is checking for molted crab in condos.

Top: new shell (after molt). Bottom: old shell (before molt). Note that the crab post molt is missing one leg, you can see it got stuck in the old shell during the molting process. By losing the leg the crab can still survive molting, and the leg can be regenerated through future molting events.