With climate change and warming waters threatening to wipe out many reefs around the world, scientists may have discovered an unexpected nurse for stressed corals: crabs.
Researchers found that the presence of a type of crab (Cyclodius ungulatus) that lives in coral reefs improved outcomes for stressed corals during heatwaves, according to a new study published in Proceedings of the Royal Society B: Biological Sciences.
These little critters could help 'wounded' corals by acting like maggots on a wound: feeding on dying tissue and cleaning damaged areas.
“Coral-associated crabs – although often overlooked – can have strong impacts on coral health,” says the study’s author Julianna J. Renz, a researcher at the University of California, Santa Barbara.
These species can have a mutualistic relationship: when two organisms benefit each other. “Many of these coral-associated crabs will die without the shelter (and sometimes food) provided by corals and it's becoming increasingly clear that some corals will also die without their associated crab species,” says Renzi.
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During their experiments, the scientists tested how the presence of the crabs affected corals that were exposed to algae or had physical damage (or both) during a heatwave.
When they raised the water temperature, it triggered tissue loss among the corals, especially if they were in contact with algae. But the experimental corals were over 60% less likely to suffer significant tissue loss during the period of increased temperatures if there was a crab was present. Corals exposed to algae also lost less tissue if a crab was around, too.
“Macroalgae – a known coral competitor – drove dramatic increases in coral tissue loss in our tank experiment but crab presence dampened the effects of stress,” Renz says.
Surprisingly, being wounded could have a positive impact. Corals had the best outcome when there was no algae, they had a physical wound and had a crab with them.
“We expected to find an interaction between wounding and macroalgae, with the idea being that a wound would make it easier for pathogens associated with macroalgae to invade coral tissues and cause tissue loss in the same way a wound might predispose a person to an infection if they swam in dirty water,” she says. “Instead, we found that wounding actually reduced coral tissue loss, if it happened with a crab around.”
When corals were wounded, the crabs appeared to feed on the dead tissue and the edge of the injury but avoided living tissue. This may have helped the corals recover, in the same way maggots can be used to help heal a wound by removing dead tissue.
These findings could help scientists better understand coral survival in the face of different stressors.
"We see a lot of variability in which colonies die across a reef during stressful events like heat waves and I think species interactions are a missing component in our understanding of why some corals survive and others don't,” says Renzi.
Understanding how species interact and can help each other’s survival is important for conservation and restoration efforts. "Traditionally when we outplant corals we outplant them on their own, despite corals in the wild relying on a suite of mutualisms to buffer them from external stressors,” says Renzi.
"I hope studies like this one help us not only understand what factors make corals more resilient to stress but also provide potential avenues for improving restoration outcomes by incorporating these positive species interactions.”
Video credit: Renzi Julianna J., Morton Joseph P., Bergman Jessica L., Rowell Devin, Iversen Jr. Edwin S., Gaskins Leo C., Hoehne-Diana Juliana and Silliman Brian R. 2025. An abundant mutualist can protect corals from multiple stressors Proc. R. Soc.
Main image: healthy coral reef in Indonesia/Getty
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