Coral reefs have "halos" and they can be seen from the sky

Coral reefs in the Red Sea are surrounded by a halo of light-colored sand. CNES/Airbus, Copyright DigitalGlobe)

What’s the story behind the mysterious “halo” of red sand surrounding coral reefs When coral reefs are healthy, an unusual phenomenon occurs: a ring forms around the coral* ** of sand. These so-called halos, or circles of bright sand devoid of vegetation, are visible from satellites miles above Earth.

But until now, scientists didn't fully understand how they formed or why some were larger than others. Now, two new studies may help answer the mystery of how halos form and what conditions allow them to grow. [In Photos: Diving a Coral Reef at Twilight] Mysterious Auras

Auras occur when fish and invertebrate residents eat the algae and seaweed that grow near coral reefs. Over time, this area is cleared of all vegetation; this empty sand can measure from a few hundred square feet to hundreds of thousands of square feet and creates a footprint around the reef that is visible from space.

Researchers from both studies recently took a closer look at these halos. Not only did they observe that the activity of reef animals unexpectedly lengthened the halo's circumference, the researchers also determined that these haloes could serve as a barometer of reef health. Study authors report that learning how to interpret halos from satellite images could help scientists monitor inaccessible coral reefs. In a study published today (April 24) in the journal Frontiers in Ecology and Evolution, scientists have discovered a halo formed by complex species interactions on Australia's Great Barrier Reef.

Information captured by remote underwater cameras shows that herbivorous fish are not the only coral reef residents expanding their halos—fish that prey on burrowing invertebrates also play a role . By digging up prey in the sand, these predatory fish push beach-dwelling algae further away from the reef and expand the halo's boundaries, the researchers report. A pattern emerges

More clues about the halos emerged when scientists examined high-resolution satellite images of coral reefs, they report today (April 24) in the Proceedings of the Royal Society B published their findings. They reviewed 1,372 reefs on the Great Barrier Reef and measured the characteristics of 214 reefs; each reef contains hundreds to thousands of small reefs, isolated coral platforms that can be surrounded by a halo.

The study authors also conducted underwater surveys using camera traps at 22 halo locations. Over the course of three weeks,

The scientists were able to directly compare waters where fishing was allowed and where fishing was restricted. A halo pattern in protected waters, because fishing fish are less likely to be preyed upon, led researchers to investigate a reef halo in waters near Heron Island in the southern Great Barrier Reef. (Copyright DigitalGlobe)

The study authors suspect that in protected waters where predators are abundant, herbivorous fish will be more cautious and graze near coral reefs; therefore, the halos will be smaller. In reefs open to fishing and with fewer predators, scientists expect grazing fish to be bolder, with halos extending farther from the reef, or even overgrowing and disappearing. But the halo sizes in protected and unprotected waters were nearly the same, the study authors said.

However, they found that halos are more likely to form in protected ocean areas, particularly older reserves where predator populations have had longer to recover from previous fishing, the study's Lead author Elizabeth Martin, an assistant research professor at the Hawaii Institute of Marine Biology at the University of Hawaii at Manoa, told Live Science:

This demonstrated to researchers that haloes are reliable indicators of stability. is an indicator of a healthy coral reef ecosystem," Madin said.

Their findings provide new evidence that laws banning fishing near coral reefs can improve coral reef health, according to the study.

Coral reefs are often monitored by divers who count species and assess the condition of corals and other life. However, because halos are visible from space, satellite images can complement these surveys by providing snapshots of reef halo changes over time, Madin explains. They can also provide insights into reefs that are inaccessible to divers. At a glance, she added,

"We can see them almost anywhere in the satellite imagery, so that will give us a much broader view than what we might hope to do with traditional monitoring methods." "This will complement and expand what we can see," Madin said. Photo gallery: The Great Barrier Reef travels through time in beautiful pink: Bubble coral photos: The secret life of corals "KDSP" Originally published in Live Science. superior.