Healthy reef soundscapes can help degraded coral reefs grow

Healthy reefs are known as  vibrant homes for colorful corals and fish.. As with any bustling ecosystem, they have their own sounds and can be quite noisy. The purrs, croaks, and grunts of fish and crustaceans that live there and the sounds of healthy coral growing can echo through the water. Larval animals may use some of this sound to help them determine where to put down roots or when it’s time to grow. Broadcasting these healthy coral reef sounds may encourage coral larvae to recolonize degraded or damaged coral reefs. The findings are detailed in a study published March 13 in the journal Royal Society Open Science.

One shot to settle down

As adults, corals are immobile. Their larval stage is their only chance to move around and find that perfect habitat. They swim or drift with the currents to  find the right conditions to settle down and then anchor themselves to the seabed. Earlier studies have shown that chemical and light cues can help influence that decision, but this new work looked at the role that sound may have. They likely can sense these vibrations, since corals don’t have traditional ears. 

[Related: Google is inviting citizen scientists to its underwater listening room.]

“What we’re showing is that you can actively induce coral settlement by playing sounds,” Nadège Aoki, a study co-author and a doctoral candidate at Woods Hole Oceanographic Institution (WHOI), said in a statement. “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.”

Reef soundscapes

To look closer, a team of researchers conducted experiments in the US Virgin Islands in June and July 2022. They collected larvae from a hardy coral species named Porites astreoides. It is more commonly known as mustard hill coral, due to its yellow color and lumpy shape. They distributed the larvae along three reefs along the southern coast of St. John. Of these reefs, Tektite is relatively healthy. Cocoloba and Salt Pond are more degraded, having fewer fish and less coral cover.  

Mustard hill coral that was replanted at Carysfort Reef in the Florida Keys. CREDIT: Greg McFall/NOAA

The team installed an underwater speaker system at the Salt Pond reef and placed cups of larvae at distances of 3.2, 16.2, 32.8, and 98.4 feet from the speakers. For three nights, they then played healthy reef sounds at Salt Pond that were recorded at Tektite in 2013. They also set up similar installations at Tektite and Cocoloba, but did not play any of the recorded reef sounds.

After collecting the cups, they found that significantly more coral larvae had settled in the cups at Salt Pond than the other two reefs. The larvae settled there an average of 1.7 times higher in the enriched sound environments than in the ones that were not. The cups that were about 16 feet from the speakers saw the highest rate of larvae settlement, but even the cups that were almost 100 feet away had more larvae settling at the bottom than those where the sounds were not played.   

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