Underwater speakers playing the crackles and pops of a healthy reef drew more baby coral to artificial structures in Hawaii and attracted 4 to 14 times more fish larvae than a silent control site, according to two new studies led by Scripps Institution of Oceanography at UC San Diego. The coral study was published Oct. 8 in Communications Biology, according to a UC San Diego release.

The method is simple. Researchers recorded reef sounds over one lunar cycle, then played them through an underwater speaker from sunset to sunrise for two weeks in Kāneʻohe Bay, off Oʻahu.

The work is part of a larger effort to build reefs that protect coastlines. Healthy reefs break waves before they reach shore, and the results feed into a hybrid reef of engineered structures and living coral planned off Oʻahu.

Building the Sound Experiment

Larvae of many reef fish and corals drift in open water before settling on a reef, and they use cues such as sound to find suitable homes. Degraded reefs are quieter than healthy ones — fewer snapping shrimp and fish make less noise — which can make it harder for young animals to find them.

The team placed 37 artificial structures at a depth of about 4.5 meters (15 feet), at distances from 1 to 42 meters (3 to 138 feet) from a speaker on a flat, sandy area near the small island of Moku o Loʻe. The structures varied in design and surface. Some were coated with BRINK, a bioactive reef ink containing living bacteria developed by former Scripps researcher Natalie Levy and colleagues. Bacterial films on natural reef surfaces are one of the cues coral larvae use when choosing where to attach, and the coating was designed to mimic that signal on artificial structures.

The researchers ran three experiments over two years, during coral spawning events in 2023 and 2024. Divers counted settled coral larvae one and two weeks after the new moon using a blue light and yellow filter that make the tiny animals glow.

Coral Settled Closest to the Sound

Structures nearest the speaker had the highest coral settlement, and those coated with BRINK performed best of all. Structures with crevices also worked well alongside the sound, and 3D-printed structures from the Hawaiʻi Institute of Marine Biology performed well with sound, though they were not tested with BRINK.

"The acoustics help, and with the living biofilm, it's a lot better," said lead author Aaron Thode of the Scripps Environmental Acoustics Lab in the release. Daniel Wangpraseurt, the Scripps scientist leading the broader project, described hybrid reefs as a new kind of living coastal infrastructure that blends engineering and biology.

The fish results come from a companion study in Scientific Reports led by Scripps doctoral candidate Océane Boulais. Low-power autonomous cameras counted fish larvae for up to three weeks at a site with an active speaker and a control site with an identical silent speaker. Larvae counts peaked near the new moon at both sites, but the site with reef sound drew 4 to 14 times more overall, and the results held when the two speakers swapped locations.

An Evidence Check on the Results

The 4-to-14-times figure is a range across the fish study, not a single result, and it counts larvae, not adult fish. The release did not report exact coral settlement counts or sample sizes per treatment, and the fish cameras are a relatively new tool. Those details are in the full Communications Biology paper.

The experiments measured where young coral and fish arrived, not whether they survived to adulthood or formed a lasting reef. Long-term outcomes still need study.

The studies were funded by the Defense Advanced Research Projects Agency, or DARPA, which supports the broader hybrid reef effort that includes Scripps' R3D project. More than 50 researchers and consortium members are co-authors, including collaborators at the University of Hawaiʻi.

No independent scientists were quoted in the release, and the findings have not yet been tested at other reefs or in other seasons.

From Kāneʻohe Bay to Coastal Protection

The research is moving toward real-world use. DARPA plans a $22 million, 50-meter (about 164-foot) hybrid reef off Oʻahu, the Kalaeloa Hybrid Reef, this fall, with coral likely to be outplanted in late 2026 or early 2027. Ownership is expected to transfer to the Hawaiʻi Department of Transportation after installation.

That connection matters for coastal communities. Reefs act as natural breakwaters, reducing wave energy that would otherwise reach beaches, roads, and harbors, and many U.S. coastlines in Hawaii, Florida, and the territories rely on them as storms intensify and seas rise. Living reefs can also grow over time, unlike concrete seawalls.

For Hawaii residents and visitors, the work does not change everyday reef rules. Avoid touching or standing on coral, use reef-safe practices, and follow state guidance on protected areas, since healthy reefs are the source of the very sounds that drew larvae in these experiments.

Other U.S. reef regions, including Florida, Puerto Rico and the U.S. Virgin Islands, already run programs that grow and transplant coral. If the sound results hold up at more sites and over longer periods, playback could become one more tool to help those efforts attract natural recruits rather than relying only on nursery-grown coral. The next step is testing combined approaches, such as pairing 3D-printed structures with BRINK and sound. The bottom line is that a simple speaker helped draw young coral and fish to new habitat, a promising tool for reef restoration that still needs to prove it builds lasting reefs.

What Readers Want to Know

What did the researchers do?

They recorded sounds from a healthy reef and played them through an underwater speaker at night for two weeks near artificial reef structures in Kāneʻohe Bay, Oʻahu.

How much did sound help?

Coral larvae settled most on structures closest to the speaker, and a companion study found 4 to 14 times more fish larvae at the site with reef sounds than at a silent control.

What is BRINK?

BRINK is a bioactive reef ink containing living bacteria. Structures coated with it attracted the most coral larvae.

Does this mean the reef will survive?

Not yet. The studies measured where larvae arrived, not whether they grew into a lasting reef.

Who funded the research?

The Defense Advanced Research Projects Agency funded the studies as part of its hybrid reef program.

What happens next?

DARPA plans a $22 million hybrid reef off Oʻahu this fall, with corals likely outplanted in late 2026 or early 2027.