Exploring the Role of Acoustics in Coral Larval Settlement: Mechanisms, Effects of Habitat Degradation and Potential for Reef Restoration

Recruitment of coral larvae to reefs is vital for both individual survival and ecosystem health. Coral larvae exhibit remarkable sensitivity to various biotic and abiotic cues, including acoustic signals, which guide them to suitable settlement sites. Despite this, our understanding of the acoustic ecology of coral larvae, the impact of soundscape degradation on settlement, and the mechanisms underlying auditory detection in these organisms remains limited. In this study, we adapted established choice chamber experimental designs in combination with recordings of both healthy and degraded coral reef sounds to investigate the settlement behaviour of captively spawned Acropora millepora larvae in an ex situ setting. Additionally, we explored a novel application of nano-scale scanning laser Doppler vibrometry (LDV) to quantify the mechanical responses of putative acoustically sensitive epidermal microstructures in A. millepora larvae. Our findings reveal that captively spawned planula larvae of the scleractinian coral A. millepora exhibit phonotaxis, a directed movement in response to sound, towards both healthy and degraded reef sounds in an ex situ environment. This marks the first recorded instance of phonotaxis in captively spawned coral larvae. Furthermore, a significantly higher proportion of A. millepora planula settled closer to the sound source when exposed to healthy reef sound playbacks compared to degraded reef sounds. Our innovative use of LDV technology provided preliminary insights into the frequency bandwidth (100?400 Hz) at which epidermal microstructures of A. millepora larvae likely resonate and respond to sound. This research underscores the significance of acoustic cues in coral larval settlement and presents the first evidence of phonotaxis in captively broadcast spawned coral larvae. Moreover, it offers a valuable experimental framework for future investigations into the auditory perception mechanisms of corals, setting a benchmark for methodologies and result interpretation in this field.<p></p>