Between the anemone and the hermit crab there is a mutualistic symbiosis: an interaction in which both organisms in play get benefits.
The anemone, like the jellyfish, is part of the phylum cnidari; therefore, it presents the cnidocysts or stinging cells, distributed throughout the body and mostly in the tentacles. These act as active protection to the hermit crab, which for this reason "transplants" the anemone even when it grows and is forced to change the shell in which it lives. The hermit crab in return offers movement and food. The movement allows better oxygenation to the anemone and also the possibility of capturing microplankton. In addition, when the hermit crab feeds, small particles of food still reach the anemone.
The crab shell can also accommodate more than one anemone.