In thriving coral reefs, a symphony of fish sounds—including grunts and purrs from feeding, courtship, and territorial disputes—is accompanied by the constant crackling from snapping shrimp. These aquatic sounds help larval corals find suitable habitats for settling down.
Recent findings reveal that another species of coral larvae reacts positively to the sounds of a healthy reef played back through speakers. This suggests that using sound, or ‘acoustic enrichment’, could be an effective strategy for encouraging coral settlement and may be applicable to various coral species for restoration efforts.
Golfball coral larvae showed a notably higher settlement rate when they were exposed to the sounds of a healthy reef during their first 36 hours in the water. After that period, the impact of sound diminished.
Coral reefs play a critical role in marine ecosystems, housing over a quarter of all marine species, safeguarding coastlines from strong waves and storms, and offering food and tourism opportunities for millions globally. Alarmingly, researchers estimate that 25% of coral reefs have vanished in the last three decades.
The survival of coral reefs is under threat due to factors like unsustainable fishing practices, tourism, coastal development, nutrient runoff, and climate change. Recent research indicates that broadcasting sounds from healthy reefs can motivate larval corals to repopulate damaged areas, aiding in their restoration.
A study by the Woods Hole Oceanographic Institution (WHOI) demonstrated that golfball coral larvae are encouraged to settle when they hear sounds from a vibrant, healthy reef—a second species displaying this sensitivity suggests that this method could widely benefit coral restoration.
“Acoustic enrichment shows ongoing promise both in field studies and laboratory settings for enhancing coral settlement rates,” said Nadège Aoki, a doctoral candidate at WHOI and the primary author of a newly published paper in JASA Express Letters. “Only a few species have been studied related to sound, making this finding notable.”
During their larval phase, corals drift in the water, searching for an ideal spot to settle and grow into their adult fixed forms. To choose where to attach to the sea floor, coral larvae respond to various cues—chemical, visual, and, as shown by Aoki and her colleagues, sound. Healthy coral ecosystems create a rich soundscape, while damaged reefs are much quieter, allowing larvae to differentiate.
In July 2022, Aoki and her team collected larvae from Favia fragum, commonly known as golfball coral, in the U.S. Virgin Islands. They placed the larvae in separate cups and set them in two quiet, sandy bays off St. John’s southeastern coast: Great Lameshur and Grootpan Bay. At Great Lameshur, cups were set one meter from a solar-powered speaker playing recordings from the nearby Tektite reef, known for its health and sound activity. In contrast, Grootpan’s setup only played silence or local sounds.
At both locations, some cups were exposed to water for 24 hours, while others were left for 48 hours. After 24 hours, none of the larvae at the control site settled, while about 30% of those exposed to healthy reef sounds did. After 48 hours, settlement rates at both sites increased significantly, reaching about 73% at Great Lameshur and 85% at Grootpan.
Although the sample sizes were small, limiting statistical significance, the researchers replicated similar experiments in fiberglass tanks. In these controlled settings, they monitored larval settlement after 24 and 72 hours of sound exposure. Combining all findings, it was evident that golfball coral larvae settled at much higher rates when they heard the sounds of a healthy reef during their first 36 hours, after which their settlement rates became similar regardless of sound.
“Acoustic enrichment showed efficacy for about 36 hours,” noted marine biologist Aran Mooney of WHOI, who led the project. “After this initial period, the urgency to settle appears to overshadow the importance of healthy cues.”
Golfball coral larvae have a limited larval stage viability; they prefer to settle within 8 to 36 hours after being released into the water. The study’s findings suggest that sound cues are particularly influential while larvae are still capable of discerning their options—beyond this timeframe, they tend to settle anywhere.
“We are uncovering the complexities of coral biology,” Aoki remarked. “Corals exhibit various reproductive strategies, and different species have distinct larval durations. This research opens up numerous questions about how sound responsiveness varies among species.”
The research also illustrates that coral can react to sound even in less-than-ideal tank environments, where factors such as sound reflections and aerators complicate acoustics. Getting corals to settle and reproduce in tanks can be challenging, often requiring months of optimization. Incorporating reef sounds may streamline this process in nursery settings. There likely won’t be a universal solution for every coral species worldwide, but researchers are optimistic that marrying acoustic enrichment with an understanding of local ecology and coral biology will enhance coral restoration efforts.
“Identifying a second species settling in response to sound indicates that this phenomenon isn’t an isolated case, suggesting we might scale this approach,” Mooney stated. “However, we can’t just deploy a speaker casually—we must comprehend the ecosystem and coordinate this with broader conservation and restoration initiatives.”
This research was funded by the Vere and Oceankind Foundations, the National Science Foundation, and WHOI’s Reef Solutions Initiative.
