A thriving coral reef is a noisy place, just like a bustling city. Fish call out to each other, crabs clatter along, and snapping shrimp fire their oversized bubble-popping pincer to stun prey.
And just like a city, the reef soundscape is punctuated by daily rush hours and lull periods. Scientists listened in on three flourishing coral reefs in Hawaii over the course of a year and uncovered a dramatic change in reef noise that unfolds as the Moon rises and sets.
“This is an important development in understanding what a healthy reef sounds like,” said Lauren Freeman, an oceanographer from the Naval Undersea Warfare Center and a coauthor of the new study. “The more we know about reef ecosystems, the better chances we have of protecting them,” she said. Freeman and her colleagues published their findings in PLoS One.
Reef Rhythms
A flourishing reef has the rich and vibrant timbre of a full orchestra, Freeman explained, “but degraded reefs sound just as drab as they look.” Struggling reefs have reduced biodiversity and animal activity, and that makes for a subdued and monotone soundscape. Deep tonal fish calls and the loud crackle of snapping shrimp, both hallmarks of a healthy reef, are replaced by the high-pitched bubbling of algae that overgrow ailing reefs.
“Reef soundscape is a key indicator of ecosystem health.”
“The reef soundscape is a key indicator of ecosystem health,” said Freeman. With reefs the world over endangered by overfishing, pollution, and climate change, monitoring their changing state is critical, she added.
Acoustic monitoring is often more effective than visual, diver-led biodiversity surveys, which can’t be conducted at nighttime, when many denizens of the reef are most active. Listening to reefs is also nonintrusive; an acoustic sensor placed on the seafloor can capture ambient noise from a roughly 100-meter radius, giving information on changing animal numbers and activity levels over time.
Freeman and her colleagues have been returning to the same Hawaiian reefs for several years, capturing the soundscapes around the clock and throughout seasonal cycles. That long-term familiarity with background noise from weather, tides, and boat traffic is what allowed them to distinguish finer-scale moonlight-driven rhythms, Freeman said.
Scientists have known for a while that reefs are louder at night and when fish chorus at sunrise and sunset, just like birds and insects on land. And now Freeman and her colleagues have shown that the rising and setting of the Moon are other major prompts for the reef ecosystem.
Moonlight Cues
The reefs they studied are in areas where the tides are not synchronized with the rising and setting Moon, so they are sure that any changes in animal activity at these times aren’t related to water currents.
Their recordings contained hundreds of fish calls and thousands of invertebrate noises per minute. “These critters are all vocalizing at once, creating this ambient blur, like if you have a hundred people in a room chatting,” said Daniel Duane, an oceanographer also based at the Naval Undersea Warfare Center and lead author of the study.
The researchers split the complex audio into groups of animals that vocalized in three distinct frequency bands, or pitches. Each group responded to moonlight differently, they found. High-pitched fish calls were louder and more numerous when the Moon rose, and their raucous calling continued until the Moon went down. The reverse was true of fish calls that had more bass and high-pitched invertebrate noises, which grew quieter as the Moon rose.
“The strength and speed of the response [to the moonlight were] really surprising.”
Previously, researchers linked acoustic changes to phases of the Moon. Certain fish have been shown to be loudest when the Moon has waned, whereas others are noisier during a full Moon. But those studies tended to average noise levels throughout the night and look for changes over the course of a month, Duane said. “Instead, we analyzed our sounds minute by minute.”
Zeroing in on the data meant they could pinpoint the changes to within 10 minutes of the Moon’s rising or setting. “The strength and speed of that response was really surprising,” he said. “These reef animals are clearly taking their cues from the moment the Moon rises and sets.”
Better Baselines
The researchers can’t fully explain this moonlight-driven behavior yet. But they think certain fish might be more willing to snack and forage on brighter nights, so the invertebrates on which they feed hide quietly. To check this interpretation, Duane said they need to identify which species are calling, allowing them to establish feeding relationships between the herbivores and the predators.
“This study shows the value of exploring the fundamentals of how reef ecosystems function,” said Aaron Rice, an ecologist at Cornell University who was not involved in the study. Knowing that daily moonlight-controlled rhythms are a characteristic of a healthy reef could aid monitoring, he added. “This gives us a more complete baseline against which to measure ecosystem change, as well as a target for restoration efforts.”
Reef acoustics aren’t being used only as a metric of reef health. Scientists have shown that playing healthy reef soundscapes through underwater speakers can attract fish and coral larvae back to barren reefs.
According to Duane, reef monitoring and restoration efforts that use acoustics should consider daily variations in their design. With certain species more active than others when the Moon is up, the time at which those recordings are captured could affect interpretations of biodiversity or even determine the type of coral and fish larvae recruited to ailing reefs.
—Erin Martin-Jones, Science Writer
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