Sonic youth: Healthy reef sounds increase coral settlement

Researchers at the Woods Hole Oceanographic Institution (WHOI) recently demonstrated that replaying healthy reef sounds could potentially be used to encourage coral larvae to recolonize damaged or degraded reefs. A reef that has been degraded -- whether by coral bleaching, disease, or direct human impacts -- can't support the same diversity of species and has a much quieter, less rich soundscape. In a paper published on Wednesday, March 13, in Royal Society Open Science, the researchers showed that broadcasting the soundscape of a healthy reef at a degraded reef caused coral larvae to settle at significantly higher rates.

"What we're showing is that you can actively induce coral settlement by playing sounds," said Nadège Aoki, a doctoral candidate at WHOI and first author on the paper. "You can go to a reef that is degraded in some way and add in the sounds of biological activity from a healthy reef, potentially helping this really important step in the coral life cycle."

Corals are immobile as adults, so the larval stage is their only opportunity to select a good habitat. They swim or drift with the currents, seeking the right conditions to settle out of the water column and affix themselves to the seabed. Previous research has shown that chemical and light cues can influence that decision, but Aoki and her colleagues demonstrate that the soundscape also plays a major role in where corals settle.

The researchers conducted their work in the U.S. Virgin Islands, running the same experiment twice in June and July 2022. They collected larvae from Porites astreoides, a hardy species commonly known as mustard hill coral thanks to its lumpy shape and yellow color and distributed them in cups at three reefs along the southern coast of St. John. One of those reefs, Tektite, is relatively healthy. The other two, Cocoloba and Salt Pond, are more degraded with sparse coral cover and fewer fish. At Salt Pond, Aoki and her colleagues installed an underwater speaker system and placed cups of larvae at distances of one, five, 10, and 30 meters from the speakers. They broadcast healthy reef sounds -- recorded at Tektite in 2013 -- for three nights. They set up similar installations at the other two reefs but didn't play any sounds.

When they collected the cups, the researchers found that significantly more coral larvae had settled in the cups at Salt Pond than the other two reefs. On average, coral larvae settled at rates 1.7 times (and up to 7x) higher with the enriched sound environment. The highest settlement rates were at five meters from the speakers, but even the cups placed 30 meters away had more larvae settling to the bottom than at Cocoloba and Tektite.

"The fact that settlement is consistently decreasing with distance from the speaker, when all else is kept constant, is particularly important because it shows that these changes are due to the added sound and not other factors," said Aran Mooney, a marine biologist at WHOI and lead author on the paper. "This gives us a new tool in the toolbox for potentially rebuilding a reef."

Surprisingly, the researchers didn't find much difference between settlement rates at the more-degraded Cocoloba and the healthier Tektite reefs. A previous study in 2017 had found higher settlement rates at Tektite than Cocoloba. Some of this could be attributed to natural variation, Mooney said, but the Tektite reef has also faced several destructive hurricanes, a significant bleaching event, and an outbreak of coral disease in recent years.

"We seem to have lost some of the complexity of Tektite's soundscape over the last decade," Aoki said. "It could be that conditions there are not as good as we thought they were, but we don't know for sure."

The possible drop-in settlement rates at Tektite underscores the severity of the threats that coral reefs are facing and the need for rapid, scalable solutions, the researchers said. 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. But researchers estimate that half of all coral reefs have been lost in the last 30 years.

Aoki, Mooney, and their colleagues hope that their work can help with coral restoration efforts. Enhanced soundscapes could be used to increase settlement rates in coral nurseries, for example, or be passively broadcast at reefs in the wild to maintain or improve existing coral populations. People would still need to monitor reef conditions to ensure that coral larvae are able to thrive after they settle, but this is a significant step in the restoration process, and one that would be relatively simple to implement.

"Replicating an acoustic environment is actually quite easy compared to replicating the reef chemical and microbial cues which also play a role in where corals choose to settle," said Amy Apprill, a microbial ecologist at WHOI and a co-author on the paper. "It appears to be one of the most scalable tools that can be applied to rebuild reefs, so we're really excited about that potential."

This research was supported by the Vere and Oceankind Foundations and the National Science Foundation.