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Listening for the sounds of a healthy coral reef

by CIFAR May 2 / 14

Image above: Divers swim around a busy coral reef as a hydrophone records the sounds of ocean life. Photo courtesy of Senior Fellow Forest Rohwer

The chorus of clacking shells and pattering feet that sea creatures make as they move around under water could help researchers assess the health of coral reefs, according to new research.

“Right as soon as the sun goes down, all these little invertebrates, like hermit crabs and little shrimp and everything, come out of the reef structure and start eating things on the surface of the reef,” says CIFAR Senior Fellow Forest Rohwer (San Diego State University), a co-author on the paper published in the Journal of the Acoustical Society of America.

Rohwer and his team witnessed this bustling nightlife using time-lapse photography and a hydrophone at five sites in the Line Islands Archipelago, located in the Pacific Ocean south of Hawaii. They compared the sounds of day and night at all locations and found that after dark, the noise level increases by about 5 decibels, then quiets down just before sunrise.

“All that sound is actually just the little animals crawling along and tapping their little feet and their little shells against the coral structure,” Rohwer says.

The researchers also found the sounds were more diverse than previously realized. Snapping shrimp, which have a large claw that makes a pistol-loud popping sound, are typically thought to dominate the ocean conversation, but Rohwer says the main sounds and images the researchers captured were from other animals, such as hermit crabs.

The researchers found they could use this large variety of sounds as a clue about whether or not a reef was active and healthy.

“When the reef was degraded, there was a difference in the change in sound at the nighttime signal,” Rohwer says. He says that damaged coral reefs might be quieter because they have fewer cracks and crevices where organisms can live and therefore a smaller population. By gathering more recordings of coral reefs from around the world, researchers could identify in more detail the soundscape changes that signal trouble in ecosystems.

Rohwer, a member of the Integrated Microbial Biodiversity program, says that listening to the activity of the wildlife is useful because it helps researchers track the tiniest life forms keeping coral reefs healthy — microbes. Researchers already understand how viruses and bacteria change as a sick coral reef breaks down, but it is difficult to monitor those populations over time.

“It’s the microbes that determine the health of the reef. So we want to see how the microbes are correlated to the sound fields,” Rohwer says.

Rohwer says his team will record sounds from many different animals off the coast of Indonesia this summer to form a reference library. They may also use their recording techniques to track how often boats pass remote reefs, assessing the impact of human activities on ocean life.