The Moon influences life here on Earth more than you might realize – and that includes shifting the sounds coming from coral reefs, which indicates changes in ecosystem activity, according to a new study.
Every coral reef has its own soundscape, created by the activities of fish and other organisms as they move about the reef. Scientists can use this background hum to keep a check on what's happening around the coral.
Here, researchers from the Naval Undersea Warfare Center (NUWC), and the Advanced Research Projects Agency – Energy (ARPA-E) in the US, found that the underwater din changed rapidly as the Moon rose and set.
"High frequency pulse train sounds from fish are found to increase during moonlight hours, while low frequency fish vocalizations and invertebrate sounds are found to decrease during moonlight hours," write the researchers in their published paper.
"These discoveries suggest that the rising and setting of the Moon triggers regular shifts in coral reef ecosystem interactions."
Oceanographer Daniel Duane of NUWC and colleagues took their sonic monitoring equipment to three reefs off the west coast of Hawaii, measuring sounds across 2020 and 2021. Hydrophones were used to capture one-minute snapshots of the soundscapes at 10 or 15 minute intervals.
The researchers didn't go into detail about what the sound variations they picked up might mean. Generally speaking though, a louder coral reef is a busier and a healthier one, as it indicates support for more organisms and more feeding.
Acoustic monitoring is a helpful way for scientists to measure coral reef health without interfering too much with the creatures that live there – and it means long-term ecosystem activity in remote locations can be tracked, where this may be impractical using conventional methods.
"The observed changes in biological sound between moonrise and moonset are likely a response to changing lunar light levels rather than tidal variations, since high tide and low tide are not synchronized with the rising and setting of the moon in the environment studied," write the researchers.
The main benefit here could be in conservation efforts. These moonlight patterns will likely affect most reefs worldwide, and the team suggests that playback of reef sounds could even be played to help attract more fish towards a reef, a practice that has shown promise in Australia.
It might also give scientists a way of comparing different reefs against each other, as we try and protect them from global warming. One next step could be to map how the audio variations match up to other indicators of coral reef health.
"Future acoustic monitoring of reef health may be improved by comparing soundscapes during moonlight and non-moonlight hours, which may provide early indicators of shifts in the relative abundance of separate reef communities," write the researchers.
The research has been published in PLOS ONE.