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Why corals are always surrounded by a halo of sand

View a coral reef from above and you might spot something puzzling: dozens of underwater coral islands encircled by expanses of clean, white sand. Called reef halos, marine biologists have long theorized that these unusual structures were formed by fear, specifically the fear of fish and invertebrates that stray only a few feet from the protective coral patches to feed on algae and other food sources in the surrounding sand. Because the threat of predators remains uniform around the coral, a circle or halo of sifted sand is created.

According to two new studies, the seemingly simple explanation behind how reef halos form is only one part of a much deeper mystery — one that could someday allow scientists to more quickly gauge reef health from nothing more than satellite imagery.

Dozens of reef halos dot the underwater landscape off Heron Island in Queensland, Australia.

Dozens of reef halos dot the underwater landscape off Heron Island in Queensland, Australia. (Photo: Google Maps)

“We urgently need more cost- and time-efficient ways of monitoring such reefs,” study lead Elizabeth Madin, assistant research professor at the Hawai’i Institute of Marine Biology (HIMB) in the UH at Mānoa School of Ocean and Earth Science and Technology, said in a statement. “Our work couples freely-available satellite imagery, with traditional field-based experiments and observations, to start to unravel the mystery of what the globally-widespread patterns of ‘halos’ around coral reefs can tell us about how reef ecosystems may be changing over space and/or time due to fisheries or marine reserves.”

In one paper published in the journal Proceedings of the Royal Society B, Madin and her team explained how they initially believed the size of reef halos was governed by the density of predators in a given area. Working off the hypothesis that a coral reef located in a no-fishing reserve would feature significantly smaller halos than one where commercial fishing was permitted, the team conducted field surveys of reef halos around Heron Island off the Queensland coast in Australia and scanned satellite imagery of reefs in contrasting sites.

To their surprise, while the frequency of halos in protected no-take reserves was greater, there was no deviation in size in unprotected areas.

“The work took a long time to complete, but even as the results from a few reefs rolled in, we could see the pattern we expected wasn’t borne out,” Madin recalled in an article on New Scientist. “Halos seemed no different in size on reefs where predators could be fished or on those that were protected.”

Reef halos are a common sight within Australia's Great Barrier Reef.

Reef halos are a common sight within Australia’s Great Barrier Reef. (Photo: ikpro/Shutterstock)

Hoping that a better understanding of the ecosystem at work within these halos might shed light on their formation, Madin and her team returned several times to Heron Island to painstakingly document the species that dared venture out to scour the surrounding seafloor. In a second paper published in the journal Frontiers, the researchers revealed that, in addition to diurnal plant-eating species, the sand on the very outside of halos was disturbed each night by species digging for invertebrates.

Despite uncovering more about the complex relationship of predator and herbivore populations that help drive halo formation, Madin isn’t convinced that she’s fully solved the mystery.

“We have a number of clues,” she writes. “For one, we are starting to find that the overall number of fish of all types — not just the predators — in the vicinity of a reef seems to affect the halo size, but in surprising ways that we are struggling to grasp. If we can understand what these patterns are, and if they hold true at reefs in different locations, it might explain more of the riddle.”

The 72-acre Heron Island and its lagoon is located in the southern Great Barrier Reef off the coast of Australia.

The 72-acre Heron Island and its lagoon is located in the southern Great Barrier Reef off the coast of Australia. (Photo: Google Maps)

Much like peeling an onion, Madin adds that her team’s digging has uncovered a new mystery to the reef halo phenomenon possibly linked to environmental drivers.

“From time to time, halos blink on and off the reef, like lights on a Christmas tree, with no apparent relation to things like seasons, temperature, wind or water motion,” she writes. “Even stranger, we have seen that many halos in an area can change size at the same time, almost as though the reefscape is breathing, but again with no obvious relationship to environmental influences.”

As her team continues to unravel this mystery, Madin has high hopes that such research will one day enable scientists to determine reef health without ever getting their feet wet.

“This will therefore pave the way for the development of a novel, technology-based solution to the challenge of monitoring large areas of coral reef and enable management of healthy reef ecosystems and sustainable fisheries,” she adds.

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