Lybia edmondsoni, also known as the “boxer crab” or “pom pom crab”, employs a rather unique anti predator defense. With the aid of minute sea anemones in each claw, these crabs can attack oncoming predators by delivering a jab and electric shock. This relationship is mutualistic, meaning that the anemones get something in exchange for their help: food. It is also known that boxer crabs will not use their anemone boxing gloves when in conflict with one another, opting instead to use longer distance signals to settle their disputes.
Sketch contributed by Victoria Farrar. Fact contributed by Adrian Perez.
Source: Karplus I, Fiedler GC, Ramcharan P. The intraspecific fighting behavior of the Hawaiian boxer crab, Lybia edmondsoni-Fighting with dangerous weapons?. SYMBIOSIS-REHOVOT-. 1998 Jan 1;24:287-302.
Watch a PBS video of the behavior here