Australia - February 25, 2025 Australia’s Great Barrier Reef — Amid rising ocean temperatures and mounting environmental threats, a surprising ally has emerged for the beleaguered corals of the Great Barrier Reef: a tiny, hoof-clawed crab. Researchers reporting in Proceedings Biological Sciences have discovered that the branching coral Acropora aspera benefits from the presence of the reef-dwelling crab Cyclodius ungulatus, particularly when the coral is wounded and heat-stressed. This finding highlights the power of nature’s partnerships in bolstering coral resilience against multiple stressors, offering fresh hope for conservation efforts. Corals, often described as the backbone of reef ecosystems, provide shelter, modify environments, and sustain a vast web of marine life. As global change accelerates, understanding what protects these vital species is more urgent than ever. “Foundation species like corals create the base of an ecosystem,” explained Julianna Renzi, the study’s lead author and a former graduate student at Duke University Marine Laboratory. “Learning how they respond to stress can help us design better strategies to conserve them and the species that rely on them.” Now pursuing a Ph.D. at the University of California Santa Barbara, Renzi’s fascination with mutualism — relationships where both species benefit — drove this groundbreaking research. The study unfolded during a heat wave in Australia, where Renzi and her team collected Acropora aspera samples from a designated research zone in the Great Barrier Reef. In controlled tank experiments, they subjected the corals to various combinations of stressors, including high temperatures, physical wounds, harmful algae, and the presence of crabs. Over a month, they tracked tissue loss as an indicator of coral health. The results were striking. Heat stress triggered tissue decline, but the crabs proved to be a game-changer. While algae increased the likelihood of severe tissue loss six-fold, crabs slashed the risk by more than 60%. Wounded corals, in particular, thrived with crab companions, losing less tissue than either uninjured corals with crabs or injured ones left alone. Out on the reef, the team observed crabs feeding along the edges of tissue loss, a behavior captured in video by Renzi. Lab experiments confirmed that crabs avoided healthy coral tissue, targeting instead areas of recent damage — possibly drawn by the nutritious mucus corals release when injured. This feeding appeared to clear away harmful algae, effectively pruning the coral’s habitat. “Corals probably experience small wounds from fish bites or abrasion fairly often,” Renzi noted. “These might not be devastating, but they could release enough mucus to attract crabs like C. ungulatus.” The findings challenge long-held assumptions about coral vulnerability. “The temperature at which corals succumb to heat stress is generally thought to be fixed,” said Brian Silliman, a distinguished professor at Duke’s Nicholas School of the Environment and Renzi’s former advisor. “But this work shows that a biological partnership can greatly boost their ability to resist heat stress by cleaning wounds.” The crabs don’t alter heat tolerance directly — they lighten the load of injury, giving corals a fighting chance. This discovery could reshape coral reef restoration. Introducing crabs to newly transplanted colonies might curb tissue loss from diseases, a common hurdle in such efforts. “We often assume survival is all about competition,” Renzi reflected. “But in cases like these, positive interactions may be key. Evolutionarily, looking out for yourself might mean looking out for someone else too.” As coral reefs face an uncertain future, these clawed custodians could prove to be unlikely heroes in the fight to save them. Funding for the research came from the National Science Foundation, Duke University, and other partners. [Credit: Video by Julianna Renzi]