Guard your pom-poms!
Those are a pair of tiny sea anemones in the claws of the “boxer” crab shown left. In that fascinating symbiosis, the crab uses the anemones as a kind of live net for catching prey. If left with one, the crab will divide it. If left with none, it will steal one from another crab. So far, biologists do not know where the anemones originate. (Cite.)