Coral collections and spawning
Eight colonies of the brooding coral Porites astreoides were collected on St. John, U.S. Virgin Islands (18.31384N, 64.76439W) on 22 June 2017. The colonies were maintained in a shaded outdoor ambient seawater-supplied aquarium. Corals spawned overnight 22-24 July and larvae were collected each morning and maintained in 0.2m filtered seawater. On 25 July, larvae from all colonies and spawning nights were pooled, and groups of 55 actively swimming larvae were selected. Groups were randomly assigned to one of 9 light or 9 dark 140ml polyethylene chambers (preconditioned with reef water for one month) filled with 0.7m filtered seawater (remove grazers, retains microbes). Each chamber contained two preconditioned settlement surfaces: a clay stilt (3.8cm diameter) and a red cable tie (10.2cm; chosen from previous findings, Mason et al. 2011). Light chambers were transparent, allowing ambient light ingress, while dark chambers were externally covered with black tape to prevent light transmission.

Settlement experiments
Following larval addition, three light and three dark settlement chambers were each affixed to a vertical pole deployed at three sites: Tektite Reef (18.30962N, 64.72218W), Cocoloba Reef (18.31528N, 64.76065W), and a sand site with no reef structure within 100m (18.31789N, 64.75059W) (Table 1). Sites differ in biophysical habitat characteristics (Table 1) known to influence soundscape properties (6). The experimental set-up included acoustic recorders (SoundTrap ST-300, Ocean Instruments NZ), recording continuously at 48 kHz, and temperature/light loggers (HOBO Pendant UA-002, Onset Corporation). The chambers and instruments were secured 0.20.5m above the seafloor in 710m water depth (see Figure 1C). Larvae were completely isolated within settlement chambers, allowing exposure to ambient sounds (polyethelene plastics have high acoustic transparency) while preventing exposure to other water-borne habitat cues (e.g., reef water chemicals).
Chambers were recovered after 62 hours and maintained in seawater tables during the 6-hour processing period in which settled corals were enumerated. Some actively swimming larvae were still present, suggesting that oxygen remained sufficient for larval survival.