Mysterious Rings Around Reefs Have No Simple Explanation

These strange, barren halos are thought to be the work of fearful fish—but the reality is far more complicated.

Halos in the Red Sea
Halos in the Red Sea (CNES/Airbus; DigitalGlobe)

The last decade of Elizabeth Madin’s work began with one day of terrible weather.

In 2010 she and her husband, Joshua, both ecologists, flew to Heron Island in Australia to study how fishing influences the creatures of the Great Barrier Reef. But they arrived to find strong winds and rough seas that constrained them to the beach. While trying to work out how to spend her time while the elements calmed down, Madin happened across a large satellite image of the island and its surrounding lagoon. That’s when she noticed the rings.

The lagoon is full of “patch reefs”—isolated chunks of coral that can be as small as a melon or can span acres. Regardless of size, these chunks are often encircled by barren rings of sand, which separate them from the algae and seagrass that lie further afield. “It’s a teeming city on the reef and then when you move away, it’s like a desert,” says Madin. These bare circles are obvious to swimmers, but they’re also visible on satellite images like the one Madin found.

First described in the 1960s, the circles have since become known as grazing halos. The idea is that fish and sea urchins that live within the reefs gobble up anything that grows nearby, leaving bare sand behind. But these grazers are loathe to venture into the open, where they could be easily picked off by sharks, barracuda, snappers, and jacks. Their fear keeps them close to the reef, and their hunger keeps that zone free of greenery. Hence: grazing halos.

At least, that was the theory. No one had ever truly tested it, so the Madins decided to do so. As they waited for the weather to calm down, they waded through the waist-deep lagoon and sowed clumps of seaweed in various locations. Anything they placed within nine meters of a patch reef was quickly eaten; everything else was largely untouched. The grazer hypothesis was right. “We thought it would be a quick open-and-closed study,” says Madin, who is now at the Hawaii Institute of Marine Biology. “But I fell down a rabbit hole. These halos are far more complex than we originally thought.”

In subsequent trips to Heron Island, Madin and her colleagues carpeted the lagoon with GoPro cameras to work out exactly which fish are creating the halos. To their surprise, they learned that grazers such as surgeonfishes never ventured out to the very edges of the halos. These fish were clearing some of the greenery away, but not all of it.

Their accomplices weren’t active during the day, so to find them, the team had to jury-rig a network of night-vision cameras and infrared lights. That’s how they learned that the outer rims of the halos are the work of emperors and sweetlips—fish that forage for buried shellfish by rootling through the sand “much like pigs,” Madin says. Bigger, less vulnerable, and less fearful than the grazing species, they’re more likely to stray from the reefs. They widen the halos that the grazers create.

But even that can’t be the whole story. If the halos are entirely driven by the presence of fish, they should be more common in waters where fishing is forbidden. They should also be smaller, since fishing bans preserve big predators, which force fearful grazers to stick closer to the reefs. But only the former is true. By comparing 214 patch reefs through the Great Barrier Reef, the team showed that halos are more common within marine reserves—and especially within mature ones that are at least 8 years old. But they were neither bigger nor smaller. “That was a big surprise,” Madin says. “We’re surely missing something.”

The team also can’t explain why the halos sometimes grow and shrink, disappear and reappear. These changes don’t correlate with obvious environmental conditions such as temperature or wind speed. Are they related to other reef dwellers, such as invertebrates living in the sand? Could they be influenced by forces beyond grazing and digging, such as the poop from local fish? Are the microbes that live in the reefs involved? Halos seemed like a simple matter of hungry predators and fearful grazers. But “to truly understand the mechanisms that drive them, we’ll likely need to integrate most of what we know about reef ecology,” says Randi Rotjan from Boston University.

Other mysterious patterns are similarly testing our understanding of nature. Take the “fairy circles” of Namibia and Australia—bizarre spots of barren soil that pockmark grassy landscapes. Some scientists think they’re the work of termites. Others say that they’re caused by plants, whose battles for water self-organize the land into patches of drought and moisture. Whatever the answer, the surprisingly vigorous debate between these two camps highlights how little we know about the forces that shape landscapes—or seascapes.

By understanding those forces, Madin hopes that scientists could eventually use the halos to quickly assess the health of coral reefs, and the fish populations within them. Rather than visiting these sites directly, they could simply measure the presence and extent of the halos on satellite images.

“We would never completely replace the traditional methods of sending divers down to count fish and corals,” she cautions. But in remote areas, “it’s expensive, difficult, and time-consuming to send diver teams, and in some places, you can’t even get a boat in. With satellite images, we can see those places and get a first-cut idea about what might be changing in their food webs. We can also go back in time, which you can’t do on a swim.”