The Washington PostDemocracy Dies in Darkness

It’s little crabs vs. big sea stars in the Pacific. Winner gets the coral.

Trapezia flavopunctata is the largest species of coral-guard crabs. (Courtesy of Seabird McKeon)

On the technicolor coral reefs of tropical French Polynesia, crabs are locked in an ancient and epic battle with  sea stars to protect their turf.

It’s hardly a fair fight. The sea stars are as big as a Frisbee and go by frightening names such as “Crown of Thorns” for the sharp spikes on their backs. The crabs are the size of marbles, and many look the part, with red polka dots decorating their little white bodies.

They both have everything to fight for. Sea stars crawl on coral and eat it. Crabs rely on coral for their abode, and the organisms that live on it are their buffet. Needless to say, a house is not a home if it gets eaten, dinner table and all. So the crabs fight the giant invaders sometimes to death, jabbing red claws that resemble little boxing gloves.

A new study released Tuesday describes the crabs as fierce fighters that protect the coral. It's not just bigger crabs who challenge the monstrous sea stars, but little ones do, too -- an entire community of crabs pinching stubby sea star legs, grabbing their bodies, tugging at their hides and making such a nuisance of themselves that the sea stars lumber off to other coral.

Previous studies have shown that crabs engage in a behavior best summed up by an Under Armor sports commercial -- “We must protect this house!” -- but this study is the first to show, the authors say, that an entire neighborhood of crabs that vary in size get into the act in an attempt to repel sea stars and smaller predators.

Bigger crabs take on the biggest sea stars, but ignore smaller sea stars and other predators, such as snails. The smaller crabs engage those animals, a coordinated defense that has likely saved some of the world’s most gorgeous coral from extinction.

“I’d say the major importance of our study is it demonstrates you need a diversity of crabs on the reef,” said Jenna M. Moore, a co-author on the study who’s now a graduate student studying marine zoology at the University of Florida.

“The main message is we need to protect a lot of different kind of diversity of life we might not think about. Basically reef diversity is way more complicated than we understand,” she said. Science shouldn’t be simply concerned about the fall of a few lives that are in danger of extinction for whatever reason, but an entire web of life, particularly in coral, where the community dynamic works in ways no one realized.

Echoing other scientists, Moore says more studies are needed. “Where are the stories that show us how the reef is surviving threats? It’s important to know. Individual coral species, they’re the place where these crabs have a nursery. This little symbiosis of the coral and crabs is semi-well studied, but no one has looked at diversity” of animals on the reef.

The study, “Species and size diversity in protective services offered by coral-guard crabs,” was published Tuesday in the journal PeerJ, and was co-authored by C. Seabird McKeon, who’s currently somewhere in the field studying marine animals. McKeon is a researcher for the Smithsonian Institution Marine Science Network in Fort Pierce, Fla.

Under the crystal blue waters of Mo’orea, French Polynesia, are ghostly formations of dead coral. That graveyard is generally where crabs didn’t used to live and sea stars of various sizes and shapes descended upon.

The scary way sea stars eat might prompt some parents to cover the eyes of children who like to pull them out of the ocean to decorate their bedrooms and bathroom sinks. They crawl on coral, lay their stomachs on it, stick the stomach out and let their digestive enzymes dissolve it.

For some of the smallest crabs on populated reefs, that is the last thing they see. They are no match for the great Crown of Thorns sea star. But the bigger crabs, the largest of the Trapezia species, rise to the challenge and throws a few haymakers to the gut.

Like so many prizefights, the action happens at night. It is gruesome, and it is beautiful, a symphony of sea stars just trying to grab a meal, and crabs trying to not be a meal, as well as trying to protect their homes. Crabs nest in male and female pairs, and protect their eggs and young with ferocity.

As things happened during the study in 2008 and 2009, there was an event that occurs once about every 30 years -- an outbreak of “Crown of Thorns,” also known as Acanthaster, rhymes with disaster. By September 2008, most of the coral in that area of Polynesia was eaten.

But not all. A section of coral called the Pocillopora eydouxi that “occurs on the fore reef and hosts...a suite of...symbiotic species” was spared because gangs of big sea stars were repelled. A community of mismatch crabs working as one is called symbiotic.

Researchers jumped into deep water to observe the sea star wars, but they also duplicated the action at the Richard B. Gump Research Station of the University of California. They wanted to look at how the Trapezia crab species used the coral, look at how they fought for the coral, and observe if their roles tended to vary by size.

They pitted them against the Crown of Thorns, the Culcita novaeguineae that resembles the sand dollar, and Drupella cornus, a fairly large sea snail. The predators were starved about two days and released.

In one test, the coral was unprotected. In another, the coral was populated. In general, the unpopulated coral was greedily consumed. The populated coral had fewer feeding marks, a result of a spirited defense of crabs working together.

In one lab study, the removal of defenders from a clump of coral “led to an increased rate of attack and tissue loss,” the study said. “Over a two-week experimental period, 64 percent of corals with symbionts removed were attacked,” compared to 18 percent of those full of fierce and angry crabs.

The crabs are only three inches across, Moore says, and the combatants are “not evenly matched. The sea stars are really, really big.” It’s not unusual for crabs to get gobbled with the coral.

On the other hand, she said, “the crabs are aggressive and the sea stars are slow moving.” And when the sea stars ooze up on the coral, “the crabs grab and pinch with their claws. They basically pinch the feet, shaking the sea star and basically annoy it off the coral.

“If the crabs are doing their jobs and being annoying, the sea stars will move off to something else,” Moore said. “They’ll go to another coral without crabs. There are a whole bunch of dead coral and the live ones will be full of crabs.”

Read more: Sounds of predators cause mud crabs to cringe