Underwater shades show promise protecting vulnerable corals from heat damage

In the summer of 2024, when the Florida Keys experienced the second-hottest water temperatures on record, a team of marine scientists conducted an experiment that sounds almost absurdly simple: they built umbrellas for corals. Twenty custom-made shades—constructed from pipes with nylon cloth stretched across them—were anchored to the seafloor and floated about a foot above ten different colonies of brain coral at Newfound Harbor. The shades blocked 60 percent of incoming light. Every two weeks, divers descended to measure whether the corals were bleaching, and to collect tissue samples that would reveal the health of the symbiotic algae living inside the corals' bodies.

The experiment emerged from desperation. In 2023, Florida's coral reefs had endured a catastrophic bleaching event—the kind of thermal devastation that turns living reef into a graveyard of white skeletons. Branching corals were essentially eliminated from many areas. Brain corals, which are more robust, still suffered severe damage at some inshore sites. Karen Neely, a researcher at Nova Southeastern University, and her colleagues wanted to know whether they could develop a practical tool to protect the most vulnerable corals when the next heatwave arrived. The shades were their answer: not a solution to climate change itself, but a way to buy time for specific corals in specific places.

The logic behind the shades is straightforward but important. Heat stress alone damages corals, but ultraviolet radiation and intense light make that damage worse. The shades couldn't lower the water temperature—nothing short of addressing the climate crisis itself could do that—but they could reduce the light exposure that compounds thermal injury. When corals experience extreme heat stress, they expel the algae living in their tissues, a process that turns them white and leaves them starving. The algae, called zooxanthellae, provide the coral with most of its nutrition and give it color. Lose the algae, and the coral begins to die.

What Neely's team found was encouraging but modest. The shaded corals—two species of brain coral, Pseudodiploria clivosa and Colpophyllia natans—lost less color than the unshaded control corals during the most intense periods of heat stress. Some of the shaded corals even recovered some of their color after the shades were installed, a sign that the symbiotic relationship was beginning to heal. The differences were most pronounced during the worst thermal conditions, exactly when protection mattered most.

But the results also came with caveats. The overall bleaching in the experiment was less severe than the researchers had anticipated. No corals died, even among the unshaded controls. This may have been because the 2023 bleaching event had already selected for a particularly heat-tolerant strain of algae called Durusdinium, which had colonized most of the corals in the area. That algae is more resistant to heat, but it's also associated with slower coral growth and possibly greater vulnerability to disease. The shades reversed the observed bleaching, but they didn't prevent mortality because mortality didn't occur.

There were also practical limitations to the study itself. The research permits required that tissue samples be taken only from the edges of the corals, which naturally receive less light and experience less bleaching than the exposed surfaces. This meant that even the control corals were receiving some incidental shade. Additionally, the shades couldn't be installed until after the water had already warmed enough to trigger bleaching. If they'd been deployed earlier, they might have prevented the damage entirely rather than merely reducing it.

Neely is clear-eyed about what the work does and doesn't show. The shades worked on two highly susceptible coral species, and she believes they would likely be effective on most others, though additional studies would be needed to confirm that. But shading is not a cure-all. It works only on small scales, protecting individual colonies or small groups of corals. Scaling it up would require making difficult choices about which corals are worth saving and which are not. There's also a threshold beyond which no amount of shade would help—a point of thermal stress so extreme that even protected corals would die. That threshold remains unknown, waiting for future experiments to define it.

The real value of the work may lie not in the shades themselves but in what they represent: a proof of concept that targeted, temporary interventions can reduce damage to vulnerable corals during extreme heat events. As climate change makes such events more frequent and more severe, having even small-scale tools in the conservation toolkit becomes more important. The shades won't save the reefs. But in the window before the climate crisis is addressed—and Neely emphasizes that addressing climate change is the only real solution—they might save some corals that would otherwise be lost.