The boxer crab or pom-pom crab gets its name from the two living anemones it holds in its claws. When provoked by a predator the crab will wave the stinging anemones to ward them off, giving them a boxing glove- or pom-pom-like appearance.
Boxer crabs use the anemones’ stinging, poisonous nematocysts — harpoon-like, venomous appendages — to ward off or attack predators. The crab will also use the anemones as appendages to drag across the ocean floor, acting as little “food magnets.” Any food item touched will adhere to the anemones’ nematocysts. It’s classic symbiosis, with both animals benefiting from their association.
Obviously the crab benefits. The anemone benefits by being carried to food sources and protection by the swift moving crab. But beyond “simple” mutualism, this association demonstrates a crab utilizing the anemones as a tool.
Here are some other fun facts about the boxing crab:
- If a pom-pom crab loses an anemone and another isn’t easily accessible nearby, these crabs have been observed to “frag” their existing anemone (if it’s sufficiently large enough to do so). They physically split their existing anemone into two, allowing them to replace their lost glove by actively creating a new one.
- While their relationship is beneficial to each creature, it is not essential for the survival of either. There have been numerous observations of boxer crabs substituting anemones for other organisms, such as sponges or corals. On the flip side, anemones do not need to be held by a crab to survive.
- Because of their hard outer shell (exoskeleton), crabs, including pom-pom crabs, have to shed the outer layer of skin to grow. During molting, the crabs have to put down their anemones, leaving them more vulnerable than most crab species during molting.
- There are also freshwater pom-pom crabs, but they aren’t closely related to the ocean-dwelling ones. The freshwater crab gets its name from the pom-pom-like patches of hair on their claws.
- Boxer crabs hail from the Indo-Pacific. At only about an inch long, they easily hide underneath stones and coral rubble, giving them camouflage.
