A subset carried BRINK, a bioactive "reef ink" loaded with living bacteria. Former Scripps postdoc Natalie Levy and colleagues developed the coating. Other pieces were 3D-printed tiles built by collaborators at the Hawaiʻi Institute of Marine Biology.

After each new moon, divers returned with handheld blue lights and yellow filters to count settled larvae under fluorescence.

Structures closest to the speaker won. Within that group, BRINK-coated pieces attracted the most coral larvae.

"The acoustics help, and with the living biofilm, it's a lot better," Thode said. "When combined with structures that had crevices, acoustic enrichment worked very well for the coral larvae settlement."

The team recommends combining sound with the most complex 3D-printed tiles coated in BRINK for the best results.

How did the fish study work?

The companion paper appeared in Scientific Reports. Lead author Océane Boulais, a Scripps Ph.D. candidate, built custom low-power cameras to count fish larvae.

Divers scare fish, which makes hand counts unreliable. "By developing these noninvasive cameras, we can essentially spy on the fish and observe a lot of their natural behavior," Boulais said.

Her autonomous cameras watched the entrances of 3D-printed "fish hotels" for up to three weeks at a stretch. The team deployed the camera array twice: once near a speaker playing reef recordings and once at a control site where an identical speaker stayed silent.

Larval counts peaked around the new moon at both sites. The sound-enriched site drew 4 to 14 times more fish larvae overall. The result held even after the team swapped the real and silent speakers.

Why do fish matter for reef recovery?

"Fish are important components of healthy coral reefs, because certain species feed on smothering microalgae that would otherwise make it difficult for coral larvae to settle and grow," Boulais said.

Attracting young fish could make it easier for coral to take hold. The authors frame their work as a step, not a finished recipe.

What's next for the project?

Both studies sit inside a larger consortium called Rapid Resilient Reefs for Coastal Defense, or R3D. The group tests nature-based ways to break waves and shield shorelines.

DARPA plans to install a $22 million, 50-meter hybrid reef called the Kalaeloa Hybrid Reef off Oʻahu this fall. Corals should follow in late 2026 or early 2027. DARPA will then hand the prototype breakwater to the Hawai'i Department of Transportation.

Wangpraseurt, R3D's UC San Diego lead investigator and a co-author of the coral study, called the field results "a major step forward."

"It brings us closer to the vision of hybrid reefs, a new class of living coastal infrastructure that combines engineering and biology to protect our shores while supporting the growth and recovery of reef ecosystems," he said.

What are the limits?

The 3D-printed structures from Hawaiʻi were tested with sound alone, not BRINK. Larval counts capture settlement, not survival into adulthood. And the experiments ran at a single bay, so effects may differ on other reefs.

Still, the authors point to acoustic enrichment as another tool that reef builders can layer on top of better-known fixes.