Photo by James W. Fatherree, M.Sc. (as published in his December 2010 Advanced Aquarist article: “Crabs in the Marine Aquarium”)
Here is an uncommon video of a common blue-legged hermit crab in the act of changing shells. Don’t blink … the swap transpires in about one second (a little after the 1:00 mark). Hermit crabs do not build their own shells, so make sure to provide your hermit crabs additional shells for them to move into as they grow.