Hermit crabs change shells when they outgrow their current one or when they find a more suitable one. To change shells, a hermit crab will first inspect the new shell to see if it’s a good fit and suitable for them. They use their antennae and legs to feel the inside of the shell and make sure it’s smooth and clean.
If they decide the new shell is suitable, the hermit crab will then back up to the new shell and pull themselves out of their old one. They will then twist and turn their body to position themselves inside the new shell, making sure their abdomen is snugly fit inside.
It’s important for hermit crabs to find a shell that fits them well, as their shell serves as their protection from predators and the environment. In the wild, hermit crabs will often compete for shells, and some species of hermit crabs have been known to form “shell lines,” where they will form a line from largest to smallest and each crab will take the next size up shell when it becomes available.