On the origins of ambient biological sounds in shallow water tropical ecosystems

Although discovered more than 60 years ago, the origins of much ambient underwater biological noise remain unclear. Snapping shrimp sounds dominate some environments but elsewhere the shallow-water biological sound field is often heterogeneous. Here we show that dominant components of underwater ambient noise recorded on coral reefs around the Line Islands archipelago in the central Pacific are spectrally consistent with the interaction of hard-shelled benthic macro-organisms with the coral substrate. Acoustic recordings taken from shallow coral reef environments show a consistent, nightly 4.7 to 6.9 dB increase in estimated pressure spectral density level in the 11 to 17 kHz band with a spectral peak centered between 14 to 15 kHz. Intensity-filtered recordings of an example species, the hermit crab Clibanarius diugeti, in quiet aquarium conditions reveal that transient sounds produced by the interaction between the crustacean's carapace, shell, and coral substrate are structurally consistent in spectra with the dominant components of the recordings from the central Pacific. Passive acoustic monitoring of such ambient noise may be useful as a complimentary ecological survey technique to SCUBA-based visual observations, which are typically poor in estimating the abundance and diversity of cryptobenthic organisms.