Boxer crabs (or pom-pom crabs) go through their lives with an anemone in each claw. (The anemones have defensive stinging cells.) If you take one of their pom-poms away, they’ll induce asexual reproduction in the remaining one by tearing it in half. If you take both away, they’ll steal one off another crab and do the same thing, so they always have two at all times. Full article