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Tuesday, March 19, 2024
March 19, 2024

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Tiny crabs take on big sea stars in battle over South Pacific coral

Scientists find crabs have kept some coral from extinction

The Columbian
Published:
2 Photos
Courtesy of Seabird McKeon
A study describes crabs as fierce fighters that protect the coral reefs of tropical French Polynesia.
Courtesy of Seabird McKeon A study describes crabs as fierce fighters that protect the coral reefs of tropical French Polynesia. Photo Gallery

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 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.

Value of diversity

“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.

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