Sound bathing isn’t just a human wellness trend: It might be good for corals, too. Researchers at the Woods Hole Oceanographic Institution (WHOI) in Massachusetts have developed an underwater audio system that helps coral larvae to choose their permanent home. They’ve found that playing recordings of a healthy reef encourages the larvae of two different coral species to rebuild in areas that have been degraded by climate change-related stressors.
But it wasn’t as easy as hitting “play.” Although the team had a basic concept of how they wanted their playback to function, they faced a series of challenges in engineering an underwater system. Every part—electrical acoustic signals, power system, waterproofing, buoy, and mooring—had to operate seamlessly in the field.
Coral reefs are among the ocean’s most imperiled ecosystems. Since the 1950s, the world has lost half of its coral reefs; the UN recently said that every one of the world’s reefs could be bleached by the end of the century unless we drastically reduce greenhouse-gas emissions. Scientists and engineers are seeking novel ways to restore these fragile habitats, and one intriguing approach looks to underwater soundscapes.
“We know what a healthy reef and a degraded reef sound like,” says Ben Weiss, marine robotics engineer at WHOI. Under the auspices of biologist Aran Mooney, who has been at the forefront of building a global library of underwater biological sounds, and along with colleague Nadège Aoki, the team asked: If you serenade corals with the sounds of a healthy seascape, will they rebuild?
Subsea Soundscapes
Mooney had found previously that a playback of healthy reef soundscapes helped to attract certain families of larval reef fish. But, as Aoki says, the early sound system was “fairly basic.” It consisted of an MP3 player in a waterproof box with small batteries plugged into an amplifier and an underwater speaker on a basic mooring. But it didn’t float particularly well, so it tended to flood in rough seas, and it drained the battery quickly. “You had to go out to the buoy every 12 hours or so,” she says.
To change sound sequences and create needed experimental variability, the user had to upload or select new sound files with each daily deployment. If there was a problem—for example if the speaker turned off—it was difficult to tell without detailed analyses. The user effort was high.
Varying soundscapes is key, both experimentally and to better reflect natural conditions. As Weiss explains, reef soundscapes undergo “a crescendo as the sun sets and rises each day.” Further, fish sound patterns can change daily, weekly, and seasonally. He was tasked with building a more robust and flexible system that could be programmed to play files that matched the day-time and night-time cycles of a healthy reef. “We have a bank of recordings from the entire night, in hour-long chunks. We wanted to ensure that on the hour the right file was playing for days or weeks at a time,” he says.
The Woods Hole underwater acoustic system includes off-the-shelf components in a watertight Pelican case.Woods Hole Oceanographic Institution/MIT/Royal Society Publishing
To orchestrate the sound files and their timing, Weiss designed a solar-powered system that used a microcontroller. He started with off-the-shelf speakers and components—all in a watertight Pelican case—and wrote code to turn the speakers on, log data that reported system health, and control how power was allocated. He also designed a custom flotation system for attaching the waterproof case to a buoy. A line extending through the water connected the electronics to the speaker, which was fixed to a concrete mooring on the seafloor. Before deployment, the team loaded audio files onto an SD card, and installed software that told the microcontroller when to play each file. They monitored the system remotely via Bluetooth.
For calibrating the output of their Reef Acoustic Playback System (RAPS), the team used a four-channel SoundTrap recorder attached to four hydrophones placed on moorings at progressive distances from the source. Importantly, the hydrophone array captured both the emitted pressure and the particle motion. “Coral larvae are themselves little particles in the water,” explains Weiss. Like most marine invertebrates, they sense the vibrations induced by the sound, rather than detecting the pressure itself. “Our calibrations need to account for particle motion, so we could figure out how the larvae experience the sound,” he says. To do this, they measured pressure gradients across small distances between the hydrophones and calculated the particle motion from those gradients.
Good Vibrations
Finally, the researchers were ready to test how sound exposure affected larvae behaviors at reef sites in the U.S. Virgin Islands National Park. The researchers transferred Porites astreoidesand Favia fragum larvae into sound-permeable cups, anchored to stakes on the seafloor, so the organisms wouldn’t be washed away. These samples were placed at progressive distances from the RAPS speaker.
While exposing the larvae to playback of recordings from the healthy nearby Tektite Reef, the researchers measured how many larvae settled onto a ceramic stilt within the cup during the first 24 hours. The larvae showed increased settlement rates in the enriched sound environment—up to seven times more compared to control versions without the playback. This increase was most pronounced in samples closest to the source—corresponding to where speaker-induced particle motion was most pronounced. “Replayed sounds differ from the natural soundscape, but our sound field met whatever threshold of sensitivity these larvae needed,” says Aoki.
The results suggest that sound could be valuable for rehabilitating and building resilience within imperiled reef communities. But there’s more to do. The two species the WHOI team has investigated so far are both “brooding coral.” Their early developmental stages happen internally, resulting in fully-fledged larvae that reach the water column.
Other species, including many that are the focus of restoration projects, use “broadcast spawning.” In these species, gametes fertilize in the water column before becoming larvae, so sound may have a very different effect on their settlement. “There’s plenty of nuance in the timing and sensory needs of different corals,” Aoki says. Experimentally, the next step is testing how different sound conditions lead to different effects: for example, whether certain frequencies or sounds work better, and how natural currents affect larval trajectories.
On the engineering side, the plan is to design a better speaker, targeting the ideal frequency response for a turbid ecosystem, and doing so in a way that drives down the cost. The goal is a sophisticated, reliable, user-friendly, and cost-effective model that would allow for more experiments and scientific testing—and, ultimately, for restoration projects at damaged reefs. “We want this system to be used around the world,” Weiss says.
