Effects of acoustic enrichment on settlement of a critically endangered broadcast spawning coral.

Coral reefs are in global crisis, with larval recruitment occurring at historically low rates. Soundscapes provide important recruitment cues for reef larvae, but the effect of sound on settlement is understudied, particularly in broadcast spawning corals. We investigated effects of sound playback on settlement of the endangered reef-building coral, Diploria labyrinthiformis. In a 2023 field study, we placed D. labyrinthiformis larvae at six sites spread across two reefs on St. Croix, U.S. Virgin Islands. We exposed larvae in-situ to 72-h playbacks of recordings from acoustically active reefs. When larvae were held in static cups of filtered seawater, settlement rates were 1.79x higher at acoustically enriched sites. In comparison, larvae enclosed in flow-through mesh tents showed minimal responses to sound playback but exhibited a tenfold difference in mean settlement between the two reefs. This is the first study to our knowledge to demonstrate settlement responses to replayed sound in broadcast spawning corals. Our results suggest that archival recordings of healthy reefs can be used cross-regionally to support coral breeding efforts, while emphasizing that corals are sensitive to multiple cue types in addition to sound. The broader sensory environment must therefore be carefully considered when selecting sites for potential acoustic enrichment.