Healthy Reef Sounds Help Coral and Fish Larvae Settle on Artificial Reefs

A sandy patch of seafloor off Moku o Lo'e, a small island in Hawai'i's Kāne'ohe Bay, held 37 artificial structures 4.5 meters underwater. An underwater speaker among them played the crackle of snapping shrimp and the calls of damselfish, sounds recorded at a healthy reef 300 meters away, from sunset to sunrise for two weeks. Around the July 2024 new moon, drifting coral larvae settled, meaning they attached to a surface to start growing, most densely on the structures closest to the speaker. Beyond 21 meters, where the playback blended into the reef's ambient sound, settlement fell to a background level.

The result comes from two field studies led by UC San Diego's Scripps Institution of Oceanography, published October 8, 2026. One, in Communications Biology, measured how sound affected coral larval settlement. The other, in Scientific Reports, tracked fish larvae with custom-built autonomous cameras. Both ran inside the Rapid Resilient Reefs for Coastal Defense (R3D) consortium, a DARPA-funded project testing nature-based ways to protect coastlines, with the University of Hawai'i and the Hawai'i Institute of Marine Biology among the partners.

Why Sound Works on Larvae

Healthy reefs are loud with the snaps of shrimp and crustaceans and the calls of fish. The Scripps team's reading is that drifting larvae detecting a lot of sound take it as a signal that the reef below is a good place to settle. Lead author Aaron Thode also said the analysis gave strong evidence that synthetic chemical cues attract larvae to reefs.

How the Coral Experiment Ran

The coral study ran over a two-week spawning event of Montipora capitata, a native Hawaiian stony rice coral. The team placed 19 3D-printed clay settlement modules with crevices of varying depth, six cinder blocks fitted with small volcano-shaped microhabitats, six more of those blocks coated with BRINK, a bioactive "reef ink" carrying settlement-inducing microbes developed by former Scripps postdoctoral researcher Natalie Levy in Daniel Wangpraseurt's lab, and six fish habitat modules. Divers then counted settled larvae one and two weeks after the new moon, using a handheld blue light and yellow filter to spot each one.

The pattern held up under statistical testing. Settlement density fell with distance from the speaker, with the modeled average halving about every 7 meters. Sound alone did not carry the result: the plain blocks collected no larvae, while the BRINK-coated ones did. The creviced 3D-printed modules settled at roughly twice the density of the cinder blocks on the same date. The team's regression marked both speaker distance and the BRINK coating as strong effects. "The acoustics help, and with the living biofilm, it's a lot better," lead author Aaron Thode, head of the Scripps Environmental Acoustics Lab, said in the Scripps release published on phys.org. The authors recommend that future trials pair acoustic enrichment with the most complex 3D-printed structures coated in BRINK.

Fish Larvae Followed the Sound Too

A companion study tested the same idea on fish across three spawning seasons: August 2023, June 2024, and July 2024. Scripps Ph.D. candidate Océane Boulais built open-source autonomous cameras, low-power, that photographed the fish habitat structures from dawn to dusk for weeks without divers in the water. Two sites received identical structures and speakers. One speaker played the reef recordings; the other stayed silent. Between deployments, the team swapped which site was which, so the results could not reflect a better spot.

The treated sites drew four to 14 times more fish larvae than the controls, measured through the camera counts. Larval counts at both sites peaked around the new moon. The cameras detected no change in mature fish presence, only larvae, and the extra larvae stayed for days rather than weeks. Absolute numbers were small in the July 2024 deployment, roughly 25 larvae at the sound site against 2 at the control.

From a Sandy Flat to a Living Breakwater

The trials ran on bare sand, but the next step is built. DARPA plans to install the $22 million Kalaeloa Hybrid Reef off O'ahu this fall, a 50-meter prototype living breakwater using 60 engineered porous-concrete structures developed with the University of Hawai'i. The installation draws on the R3D toolkit: Scripps microstructures, biomaterials that suppress competing algae and feed juvenile coral, and an acoustic enrichment system broadcasting recorded reef sounds to draw algae-grazing fish to the site. In large-scale wave-flume tests, the breakwater design cut incoming wave energy by more than 80 percent, and separate Kāne'ohe Bay trials of the helix-shaped 3D-printed ceramic modules showed an 80-fold increase in baby coral settlement over flat surfaces.

Corals are likely to be outplanted in late 2026 or early 2027, after which ownership of the structures passes to the Hawai'i Department of Transportation. The state will treat Kalaeloa as a living laboratory, with a monitoring partnership of researchers, agency staff, community organizations, and cultural practitioners guiding the assessment of its engineering, environmental, and cultural effects.

What Is Known and What Is Still Open

Known: playing a healthy reef's sounds at night raised coral settlement near the speaker across 37 structures during a two-week field trial, and the effect combined with creviced surfaces and a living microbial coating. Fish larvae visited treated sites at four to 14 times the control rate across three spawning seasons. The tools, speakers, cameras, coated structures, and 3D-printed modules, all worked in real ocean conditions.

Still open: whether the larvae that arrive stay, survive, and build reef; whether the effect holds for other coral species and other bays; and how much of the coral settlement owes to the sound itself versus the fish grazing that the sound attracted. The fall deployment of the Kalaeloa Hybrid Reef is where those questions start getting answers at scale.