Red Sea study finds heat limits for clownfish-anemone partnership

A new study has observed marine temperatures too hot for clownfish and anemones to survive, providing vital insight for conserving reef ecosystems. 

The study, published September 12 in npj Biodiversity, tracked 168 clownfish and 143 host anemones across three known hotspots in a small part of the central Red Sea from 2022 to 2024.  

During the study, the region experienced an unprecedented heatwave, where accumulated thermal stress reached about 22 degrees heating weeks, a climate metric developed for coral reefs and used to assess exposure. The U.S. National Oceanic and Atmospheric Administration defines coral bleaching as likely at 4 degree heating weeks and significant mortality at eight. 

The extreme heat — along with the ongoing, long-term monitoring of the area — allowed the researchers to document a clear sequence of events. First, the anemones ‘bleached’, then 94–100% of the clownfish died, then the anemones themselves died, with mortality ranging from 66–94% across the reefs and reaching about 78% cumulatively by May 2024. 

Co-led by Professor Raquel Peixoto and Morgan Bennett-Smith, from King Abdullah University of Science and Technology (KAUST) and the University of Boston, the research offers vital insights into the issues that marine ecosystems worldwide will increasingly face as ocean temperatures rise. 

“The Red Sea is a living laboratory, a testing ground of what’s to come in the ever-changing climate crisis,” Peixoto said. “At KAUST, we act as first responder to these crises and developments, working non-stop to document these losses, understand their implications and develop solutions. Specifically, KAUST teams are proactively expanding and building new reefs, developing probiotics and investigating genes of resistance in certain Red Sea species.” 

Although restricted to a small area, the investigation highlights the often-overlooked heat risks faced by non-coral reef organisms, which remain critical to reef ecosystem health. The study also stresses the need for broader monitoring within and beyond the Red Sea and calls for taxon-specific thresholds, points at which a specific group of organisms reaches its tolerance limit, to better predict when reef symbiotic relationships are at risk. 

The findings come as Saudi Arabia accelerates its reef conservation. Through KAUST’s Coral Restoration Initiative (KCRI), the Kingdom is restoring a 100-hectare coral reef area, in part by constructing the world’s largest land-based coral nursery. The project integrates state-of-the-art coral propagation approaches with digital monitoring, driven by AI and machine learning, to deliver ecological recovery at scales not previously possible. KCRI’s ultimate strategy is to enhance coral reef resilience under current and future climate scenarios. 

Alongside her team at KAUST, Peixoto has also built the first coral probiotics village in the Red Sea — a permanent underwater laboratory in which researchers use probiotics to enhance the heat resilience of corals. Earlier this year, the team discovered that for corals which can tolerate rising temperatures, their resilience is determined by the type of microorganisms that live inside them. The findings give valuable clues for formulating coral medicine to rejuvenate dying coral reefs.  

“We’re committed to remaining at the forefront of this research, sharing our findings with the global scientific community to guide policy decisions and conservation priorities, before it is too late,” Peixoto said. “We encourage policymakers and the global scientific community to invest in an urgent, coordinated effort to combat marine decline.”