Guardian Crabs: Coral Reefs Protected by Vigilant Crustaceans

While coral reefs may be under attack from climate change and coastal development, there may be one animal that's protecting them. Scientists have found that some species of corals in French Polynesia have a guardian in the form of coral guard-crabs, which offer different kinds of effective protection against various threats.

"We found that diversity in both species and size of coral guard-crabs is needed to adequately fend off coral predators," said Seabird McKeon, one of the researchers, in a news release. "It is an example of how biodiversity is crucial to conserving reef environments and the essential resources they provide for thousands of species, including humans."

In order to better understand the role of these crabs, the researchers examined four species of coral guard-crabs known to protect coral from predator sea stars in 2008 and 2009. At the time, crown-of-thorns sea stars experienced a population boon, threatening the entire reef community.

In one experiment, the researchers selectively removed the largest species of coral guard-crab from corals in the path of an army of sea stars. They found that corals without guard-crabs, or with other species of guard-crab, were eaten-usually overnight.

"Seemingly small differences among crabs guarding their coral homes have big effects on coral survival," said Jenna Moore, one of the researchers. "Not only does the level of protection provided vary by species, but the smallest crabs were defending the coral from coral-eating snails, a threat that larger crabs ignored."

The researchers reveal that multiple lines of defense are a direct result of guard-crab diversity. This diversity is needed in order to keep coral reefs safe in the long-term. As climate change continues to affect reefs, it's important to take steps in order to ensure the long-term survival of these corals.

The findings are published in the journal PeerJ.