Researchers use the sounds of healthy coral reefs to encourage growth of a new species of coral larvae

The researchers found that now a second species of coral larvae responded to the sounds of a healthy reef played through a speaker, indicating 'acoustic enrichment' encourages coral to settle and has the potential to be a widely applicable method among coral species for reef restoration.

Golfball coral larvae settled at significantly higher rates when exposed to the sounds of a healthy reef during their first 36 hours in the water. After that window, the sound had little effect.

Coral reefs support more than a quarter of all marine animals, protect coastlines from strong waves and storms, and provide food and tourism opportunities for millions of people around the world. Researchers estimate that 25% of all coral reefs have been lost in the last 30 years.

Coral reefs worldwide are in trouble. These ecosystems support a billion people and more than a quarter of marine species. Still, many have been damaged by unsustainable fishing and tourism, coastal construction, nutrient runoff, and climate change. Now, researchers have shown that broadcasting the sounds of healthy reefs is a way to encourage larval corals to repopulate degraded sites and help revitalize them.

A recent study done by researchers at the Woods Hole Oceanographic Institution (WHOI) showed that golfball coral larvae can be encouraged to settle when they hear the sounds of a vibrant, healthy reef. This is the second coral species to demonstrate a responsiveness to sound, indicating that this technique has the potential to be a widely applicable tool for reef restoration.

"Acoustic enrichment is continuing to show promise as a technique in the field and in the lab to enhance coral settlement rates," said Nadège Aoki, a doctoral candidate at WHOI and first author of the recently published paper in JASA Express Letters. "There is a very limited pool of species that have had any kind of acoustic work done with them so far, and this is the second one where the corals have responded to replayed sound and settled."

During the larval stage of their life, corals drift or swim through the water looking for the right place to settle. To decide where they should attach to the seabed and mature into their stationary adult forms, coral larvae may rely on cues from chemicals, light, and -- as Aoki and her colleagues demonstrated previously and in this study -- sounds. Healthy coral reefs echo with a chorus of purrs and grunts from fish feeding, looking for mates, or defending their territories, underscored by the persistent crackling of snapping shrimp. Damaged or degraded reefs are much quieter, and it appears that some coral larvae can tell the difference.

In July of 2022, Aoki and her colleagues collected larvae from Favia fragum -- commonly known as golfball coral -- in the U.S. Virgin Islands. They divided the larvae into cups and set them up in two quiet, sandy bays off the southeastern coast of St. John: Great Lameshur Bay and Grootpan Bay. At Great Lameshur, the researchers placed the cups of larvae one meter away from a solar-powered speaker playing sounds recorded at the nearby Tektite reef, which is considered relatively healthy and noisy. The researchers used the same setup in Grootpan Bay, but the speakers only played silence or sounds recorded in Grootpan.

At each site, half of the larvae cups were in the water for 24 hours and half for 48 hours. After 24 hours, none of the larvae at the control site had settled to the bottom of their cups, but about 30% of the larvae hearing the sounds of a healthy reef had settled. After 48 hours, the settlement rates at both sites were much higher and roughly equivalent -- around 73% at Great Lameshur and 85% at Grootpan.

The sample sizes at both time intervals were too small for the results to be statistically significant. However, the researchers also conducted a similar experiment in fiberglass aquarium tanks. In the tanks, they checked for larval settlement after 24 and 72 hours of sound exposure. Combining these results, they found that golfball coral larvae settled at significantly higher rates when exposed to the sounds of a healthy reef during their first 36 hours. After that window, the larvae settled at basically the same rate, regardless of what they were hearing.

"Acoustic enrichment worked for 36 hours or so," said Aran Mooney, a marine biologist at WHOI and senior author of the paper. "After that, they seem desperate to settle, and healthy cues become less important."

Golfball coral have a relatively short window of viability in their larval stage. They don't have the resources to float around for weeks searching for the ideal spot; they want to settle in 8 to 36 hours after they are released into the water, Mooney said. The researchers found that sound cues are most effective while the larvae have the resources to be picky -- once they run out of time, they'll settle just about anywhere.

"We're getting at some of the nuances of coral biology," Aoki said. "There's a huge range of reproductive strategies that corals use and different species have different larval periods. We're opening up this broad realm of questions about how responsiveness to sound will vary between species."

The work also demonstrates that corals will respond to auditory cues even in tanks, where sound reflections, aerators, and water filters make the acoustics less than ideal. It can be tricky to get corals to reproduce and settle in tanks, sometimes taking months to get everything just right. Adding healthy reef sounds might facilitate that process in land-based nurseries. There isn't likely to be a single solution that works for every coral species in every part of the world, but the researchers hope that acoustic enrichment, applied with an understanding of the local ecology and coral biology, will prove to be an effective tool for coral restoration.

"Finding a second species settling in response to sound shows that this isn't just a one-off, and maybe we can really scale this up," Mooney said. "But we can't just throw a speaker over the side of a boat and think it's going to work. We have to know the system and it has to be integrated with other conservation and restoration efforts."

This research was supported by the Vere and Oceankind Foundations, the National Science Foundation, and WHOI's Reef Solutions Initiative.