Pom pom crab: The crustacean that uses anemones as boxing gloves

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The pom pom crab grows to only around half an inch (13 millimeters) wide, and its soft exoskeleton means its armor is pretty useless. Yet despite its minuscule size, this pugnacious little crustacean likes to fight and eat while clutching dangerous weapons — sea anemones.

The pom pom crabs, also known as Hawaiian boxer crabs, carry tiny sea anemones in each claw and use them to spar with competitors. This anemone species, Triactis producta, is venomous — and the crabs wave the anemones around as a way of defending against predators and catching food.

But they also carry them during mini battles with each other.

In a study of the behavior published in 1997, researchers selected 12 pairs of crabs — six males and six females — and pitted them against each other in a tiny crustacean gladiator arena. The winner was the crab that retreated or fled the least.

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Finally, the anemones may be so valuable that it wouldn't be worth it for the crabs to risk damaging the anemones. They do know the crabs use the anemones to collect food particles and eat from them. When they lose one of the anemones, the crabs split the remaining one in two, so it always has one in each claw.

Scientists are still sorting out why the boxer crabs cling to the sea anemones. What the anemone gets out of it is still unknown.

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