I bought a "pet" hermit crab. It was all OK until he pinched my daughter's finger and wouldn't let go. She was terrified! But we enjoyed him quite a while, feeding him and watching him change shells.
There's a Don Knotts' movie, "The Incredible Mr. Limpet" where Don Knotts character falls into the ocean and becomes a fish. There's a hermit crab, named Crusty, that uses an old tin cup for a shell.
Reminds me to restrict all of my swimming to a pool where I can see the bottom. I practically lived in the ocean for many many years. What was I thinking of? Eeek!
Went to the Caribbean once, collected shells from the beach, put them on the outside patio to dry in the sun. Went back to check on them a little while later, one shell was moving away by itself. Guess who? Hermit crab taking one of my shells. I just watched him go enjoying his new home. LOL :)