We had three hermit crabs. They ate celery, lettuce, and grapes. When they got too big for their shells, we got them bigger ones. But we didn't get to see them switch shells. They made sure to do it when we weren't looking.
We also had a few regular crabs. They were about an inch and a half across from claw to claw. If you've ever looked at a crab, you'd have noticed that their right claw is at least twice as big as their left. They use the bigger claw for eating, fighting, etc. In our crabs' case, they used them for something else.
We kept the crabs in an aquarium that they shared with some neon tetra fish (see photo). They are very small fish with blue and red streaks on their sides. Evidently the crabs thought that they would make a tasty meal, because they went fishing. One morning, I got up to do my schoolwork and I saw one of the crabs with a fish in his claw. He was happily eating. The crabs would wait motionlessly until one of the fish swam close enough to catch, then grab it with their claws. Soon all the tasty fish were gone, and they got into more mischief. As usual, I got up early, just in time to see both crabs scuttling sideways down the table. Fortunately, my dad was up, and quickly scooped up both crabs and put them back in their aquarium. After that, we made sure the lid was tightly shut!