It’s a common knowledge that hermit crabs live inside the empty sea snail shells. Whenever it gets too tight for them as they grow, they find a new shell and upgrade their home. However, hermit crabs are not the only tenants of their shells. They live in symbiosis with anemones, the hermits provide food and transportation while the anemones provide protection.
Moving out of her old shell meant that this crab would be left without any protection in her new home. Her way of dealing with that is stunning. I’ve been to a lot of aquariums and they never told me this stuff.