This mutualism benefits the hermit crab by giving it an additional layer of camouflage and protection (ie. anemones with stinging cells) while the anemone, Calliactis polypus, benefits by sharing food with a mobile hermit crab.

Many people have thought that these crabs abandoned the anemones during shell changes.  However, as this incredible video shows, some crabs choose to take their prickly partners along with them.

https://www.youtube.com/watch?v=dYFALyP2e7U