Introduction

At first glance, corals present as little more than colorful rocks — piles of lobes, stalagmites, and branches poking out from the seafloor.

They are anything but. Corals are complex creatures that form enduring colonies, and just like other animals they need oxygen to live. Across the living surface of coral, a frantic dance of survival takes place, invisible to our eyes and unknown to science before 2014. The tiny dancers are hairlike cilia, and new research into these microscopic structures is revealing just how active corals are in determining their own fate.

Corals aren’t fortunate enough to have a consistent supply of oxygen, and they can’t change location to seek it out. During the day, the tiny polyps that make up a coral colony get plenty of oxygen from the symbiotic algae that photosynthesize within their tissues. But at night that process stops, and a coral polyp’s only source of oxygen is the water around it. Then it’s do or die for the cilia. Using mechanisms scientists are still trying to understand, the cilia wave around to generate fast-moving vortices of water that circulate oxygen to the coral’s outer tissues, in addition to helping keep the colonies free of sediment.

A study published in Science in May 2026 provides new insight into how organisms with no brain or musculoskeletal system can generate and regulate this process — and what happens when the water around them warms up. Warmer water naturally carries less oxygen, which prompts corals to move their cilia faster and faster, as if gasping for breath. Above a certain temperature, the system starts to work against itself; the furious beating of cilia uses up any oxygen the coral’s tissues can absorb, and then the polyps can suffocate in the less oxygenated water. Biophysicists, marine biologists, mathematicians, and modelers are now collaborating to better understand the physiological and hydrodynamic forces at work, and how they correlate with bleaching patterns, coral disease, and mass die-offs.

This dynamic picture is somewhat new to scientists, who have long used corals’ symbiotic algal partners as indicators of their health. Cilia may serve as a more direct signal, said Rachel Alderdice, a marine biologist who studies coral stress biomarkers and genomics at the University of Konstanz in Germany and was not involved in the research. “It’s these finer details that could help us understand why some corals bleach and others don’t, [even when] they sit right beside each other.”

Swirling To Survive

Every coral colony is cushioned by a thin boundary layer of water whose movement is slowed by friction at the coral’s surface. Researchers assumed that corals were passive with respect to the slow-moving boundary layer, simply relying on natural diffusion through it to provide nutrients and oxygen.

National Marine Sanctuaries

Then, in 2014, a team from the Massachusetts Institute of Technology and the Weizmann Institute of Science published a groundbreaking study showing that coral cilia interact with the boundary layer by rapidly whipping about to generate swirls of fresh, oxygenated seawater. Until a decade ago, scientists thought of these cilia merely as brooms that move mucus and sweep away waste particles and other debris. The research not only modeled the tiny vortices created by the cilia for the first time, but also revealed the cilia’s importance for survival and metabolism.

At first, the microbiologist and environmental engineer Orr Shapiro, who led the 2014 work at MIT as a postdoctoral fellow, was interested in how microbes that infect corals and cause disease follow concentration gradients, a process called chemotaxis. Under the microscope, he noticed something weird: In the boundary layer, particles were swirling around and mixing together — not at all like the passive diffusion he had been expecting.

“That was to me, and I think later on to the entire field, sort of a paradigm shift,” said Shapiro, now a researcher at the Volcani Institute in Israel. It became clear that the boundary layer wasn’t static, but rather a dynamic zone, and one where cilia were creating their own turbulence. The realization inspired Shapiro’s team to go off on a tangent, mapping the flow of oxygen to coral tissues via cilia. “It really transformed how we understand this [micro]environment, because suddenly the diffusion is no longer really important,” Shapiro said.

Diffusion is the default route for nutrients traveling through water, but it’s painfully slow. It can take as long as four minutes for oxygen to travel just 1 millimeter. That’s why the fast-moving flows created by cilia are so important: because naturally flowing water slows down near the coral’s surface, and corals consume oxygen faster than diffusion can supply it.

It is a system that delivers enough oxygen, despite the cilia’s energy consumption. But there’s a downside: Oxygen dwindles as temperature climbs. That’s when corals run into trouble.

Hyperventilating Under Water

Scientists have a clear understanding of one thing that happens to corals when water gets too hot: bleaching. As water temperatures rise, a coral’s symbiotic algae become stressed and release molecules that are toxic to the coral in large quantities. To protect itself, the coral expels its own algae — a primary food, energy, and oxygen source — and soon loses its color. It’s a slow death and an increasingly common occurrence as heat waves sweep across the world’s reefs.

But sometimes, some corals on a reef bleach while others don’t, and in other cases corals under heat stress die without expelling their algae.

An international team of microbiologists, engineers, and physiologists was eager to understand how heat affects cilia, and whether this could explain different types of coral death.

Courtesy of Cesar Pacherres

“We’re living in a world right now of extreme scenarios,” said Cesar Pacherres, a co-author of the new study and a marine biologist at the University of Copenhagen who studies fluid dynamics. As marine heat waves become more common, and as a particularly strong El Niño year threatens to catalyze a fifth global bleaching event in late 2026, “corals can experience an increase in temperature of several degrees in a time frame of a few hours,” Pacherres said.

To explore heat’s impact on cilia, the team ran a series of 24-hour experiments. They exposed aquarium-raised stony coral called Porites lutea to incrementally higher water temperatures up to 39 degrees Celsius (more than 102 degrees Fahrenheit) — an extreme scenario, but one that could occur. They kept the tanks dark to better observe how cilia transport oxygen when algae aren’t producing any.

Every hour, the researchers recorded the microscopic cilia with a high-speed camera to capture how frequently they moved as they were exposed to warmer and warmer water. The team also used a technology called SensPIV to track oxygen flow. Fluorescent, oxygen-reactive nanoparticles “traced” the water swirls, creating a vivid map of how oxygen concentrations corresponded to the cilia’s vortices.

Visualizing the movement of oxygen was key to the team’s findings. In warmer water, corals burned energy faster, which increased their demand for oxygen — prompting the cilia to dance faster. But there’s only so much dissolved oxygen available in the surrounding water, and as temperatures climbed, the cilia started to send oxygen-deficient water toward the coral in their frenzy. “The oxygen demand of the coral increased faster than the increased swirling of the water,” said co-author Michael Kühl, a marine microbiologist at the University of Copenhagen.

When the water approached 37 degrees Celsius (the temperature of the human body), the cilia started to slow down. Past 39 degrees Celsius, they shut down altogether, and the coral died. These were the findings they reported in Science in May 2026.

Courtesy of Cesar Pacherres

Pacherres cautioned against interpreting these temperature limits as a standard threshold; each species of coral is adapted to its own range of daily temperature fluctuations. Even so, climate change has started to push many corals toward their respective limits, with record-high surface water temperatures nearing 38 degrees Celsius in places such as Florida.

Why the cilia move faster in hotter water remains a mystery. Perhaps seawater’s viscosity — which decreases as temperature increases, making the water thinner —affects ciliary movements, Shapiro said. Corals lack a central nervous system and brain, but they do have neurons — inside what’s called a nerve net — to process sensory information. Perhaps the coral senses heat or lack of oxygen, and some biological mechanism then triggers the faster beating. “It’s physics, biology, engineering, all mixed up together, which is what makes it really interesting,” he said.

A Polyp Puzzle

Scientists are now investigating cilia physiology and the boundary layer as a whole. “It’s much more complex than we thought,” Shapiro said.

In another paper co-authored by Pacherres and Kühl in May 2026, they found that cilia’s vortices resemble corkscrews. The turbulence pushes unwanted particles away from the coral’s surface while redirecting nutrients toward polyps’ mouths.

Courtesy of Cesar Pacherres

The cilia on each polyp, they found, are arranged in hexagonal units that keep ciliary movement streamlined, which helps explain how cilia can coordinate their movement to produce predictable swirls. “It demonstrates that coral skeletal architecture and living tissue are functionally integrated,” Pacherres said.

Combined, the studies recontextualize how ciliary movement, previously overlooked, gives corals some stability and buffers them against environmental change, he said.

The relationship between cilia and bleaching remains ambiguous, said Alderdice, who was not involved in the studies. But testing cilia under bleaching conditions without heat — by using red light, for example, which can also trigger bleaching — would help answer this question.

**Playing Catch-Up  **

Scientists knew from the 2014 work that cilia help corals self-ventilate when water flow is low, said David Suggett, a marine biologist at King Abdullah University of Science and Technology who wasn’t involved in the research. “But until this point, we had been focusing on molecular and metabolic machinery” to understand how corals evolved to deal with low-oxygen conditions, he said. “We hadn’t really appreciated that there are these behavioral-physiological mechanisms at play.”

For Suggett, the research raises a critical question: What does this say about how corals will deal with future climates?

Sometimes, corals of a single reef don’t bleach evenly, with some individuals dying off while their neighbors persist. In new research led by Suggett’s colleague Tadd Truscott, marine biologists are finding that these patchy bleaching patterns are correlated with areas of reduced water flow — areas therefore receiving less oxygen.

Next, Kühl and Pacherres’ team wants to test corals under different conditions — especially under normal light-dark cycles — and to better understand the mechanism driving ciliary beating. Is it a molecular process that activates their movement? The physics of warmer seawater? A bit of both, or something else?

It’s only in the past decade that scientists have realized the role of deoxygenation in coral health, Suggett said. Ocean acidification took center stage as the primary concern for corals in the early 2000s. In hindsight, deoxygenation was a much bigger issue, he said. The new research suggests that deoxygenation, not just bleaching, is a fatal threat to corals resulting from a hotter climate.

“We’re playing massive catch-up,” Suggett said. “The amount of information we’re gathering quickly is demonstrating just what a problem for corals it is — so much so that we’re starting to really revisit long-standing paradigms of the role of other environmental factors, like temperature and light, where in fact, it could be oxygen that’s been the smoking gun all along.”