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Meet the coral guard crab

When we scuba dive in Hawai‘i we are always on the lookout for the large crown of thorn starfish. This venomous, foot-wide starfish eats live coral and is native to Hawai‘i. Sometimes the crown of thorns population gets out of control and can eat up an entire coral reef.

You can tell a coral that has been eaten by the crown of thorns because there will be a pure white circle on a brown coral. That is where the starfish ate the coral polyps, and only the white hard coral skeleton remains.

I have seen several coral reefs in Hawai‘i get devastated by the crown of thorns, but some of the branching corals survived the crown-of-thorns attack. I started to study the remaining live corals, and each one had several coral guard crabs living deep down in the branching coral.

These tiny, one-to-two-inch-wide crabs protect the large, branching corals from being eaten by predator starfish and snails. The tiny, bright-orange coral guard crabs are quite feisty, and have large, powerful pinchers, and they won’t let anything invade or eat their coral home.

The coral guard crab will bite off the tiny feet of the large crown of thorns starfish if it tries to invade the coral they are living in. The starfish, after losing a few of its tube feet to the aggressive crab, will go find another coral to eat that is not protected by coral guard crabs. The corals that house the crabs produce fats from their coral polyps that the guard crabs feed on. In nature this is called “mutualism.”

The coral feeds the crabs that live in their branches, and the crabs protect the coral from predators, and that is a win-win situation for both the coral and crab.

It seems like every branching coral on the reef would want to have coral guard crabs protecting it, but only certain corals house the bright-orange crabs, and we are not sure why. Maybe only certain corals produce food for the crabs to eat, as a coral guard crab won’t protect a coral if it is not getting fed by that coral. The next time you go on a snorkel and see a large, branching antler coral, look close down into the branches and see if that coral has its own coral guard crabs!

You can see the coral guard crabs in action in my marine life educational movie series on YouTube at Underwater2web.


Terry Lilley is a marine biologist living in Hanalei and co-founder of Reef Guardians Hawai‘i, a nonprofit on a mission to provide education and resources to protect the coral reef. To donate to Reef Guardians Hawaii go to 

Source: The Garden Island

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