The hermit crabs form a sort of conga line ordered from largest to smallest crab. As the largest crab enters its new home, the next crab in line takes the vacated shell, leaving an open shell for the crab behind him. The shell swapping continues down the line until everyone has upgraded. This chain reaction is called a vacancy chain, and it’s an ingenious way for the creatures to survive while sharing limited resources.
Consider the hermit crab.
CUTE!
You can find these little guys in tide pools in places like Hawai'i.
They take up residence in snail shells, so you'll see a clump of little shells like this and some will be snails, others will be hermit crabs.
Here's a half dozen of the really common ones and one big chonker. I think this is West Coast USA but they look pretty much the same
https://www.youtube.com/watch?v=hYBCw0p43ik
Cutest lil dudes, I had a few as pets when I was a kid. They really liked limes and also peanut butter, didn't mind being handled, and even seemed to enjoy running around the little obstacle courses I made for them
They remind me too much of spiders which I fear, but they are also very cute. I am deeply conflicted by this