How shrimp sounds can diagnose the health of coral reefs

The ocean is far from quiet at night – especially near coral reefs, where a steady crackle rolls through the water. A new study shows that this natural sound can help scientists judge whether reefs are bouncing back after heat stress.

Researchers report that the clicks of small crustaceans known as snapping shrimp reveal how much living coral remains on the reef. The louder and higher-pitched the crackle, the healthier the coral cover is likely to be.

A team led by Xavier Raick from Cornell University’s K. Lisa Yang Center for Conservation Bioacoustics lab placed underwater recorders around Moorea, a volcanic island in French Polynesia.

The researchers compared acoustic data gathered before the severe coral bleaching episodes of 2016 and 2019 with recordings made two years after the second heatwave.

Listening for coral recovery

Each snap from these shrimp is incredibly loud, and when many of them are together, their clicking becomes the main sound heard on the reef at night.

Because sound travels farther than light underwater, the team could “hear” reef structure long after dusk had forced divers back to shore.

“The high frequency sounds, mainly during the night, of snapping shrimp can be used as a real indicator of coral resilience,” said Raick.

His group analyzed thousands of five-minute audio clips with specialized software, looking for peaks in the nightly shrimp chorus.

Sites inside marine protected areas produced stronger high-frequency crackle after bleaching than nearby unprotected reefs. That pattern matched diver surveys showing more surviving coral colonies within protected boundaries.

Shrimp sounds and coral health

Snapping shrimp live in crevices formed by large coral heads, and their numbers increase as three-dimensional habitat grows.

Earlier work in Moorea found that habitats rich in branching Acropora corals had the loudest high-frequency sound signatures.

When warm water kills coral tissue, those hideouts erode, and shrimp populations decline within months. The resulting dip in acoustic energy gives scientists a rapid, noninvasive way to track losses that might otherwise require dozens of daylight dives.

Reef sound also matters to the next generation of organisms. Experiments have shown that free-swimming coral larvae steer toward recordings of healthy reefs, choosing them over silent or degraded habitats.

If the shrimp chorus fades, fewer larvae may settle, slowing recovery and locking reefs into a downward spiral. That feedback makes acoustic monitoring a practical early-warning system for managers.

Quiet reefs signal damage

Raick’s team focused on hours when human observers rarely work. At night, visibility drops, but the shrimp chorus grows, offering a clean signal free from boat traffic and diver bubbles.

Two years after the bleaching, protected reefs on Moorea still had stronger, higher-pitched shrimp sounds, while unprotected reefs had weaker, lower-pitched ones.

This suggests there were fewer or smaller shrimp in the damaged areas.

Protection status mattered even though the bleaching heat stress hit the whole island. Global analyses show that well-enforced marine protected areas with at least five key design features can increase fish biomass eight-fold compared with lightly regulated zones.

By giving corals a break from anchor damage and overfishing, MPAs may preserve the structural complexity shrimp need, allowing reefs to rebound faster after thermal events.

Low-cost reef monitoring tool

Traditional reef assessments rely on scuba teams that sample a few quadrats during daylight, leaving vast areas and all nighttime hours unobserved.

Passive acoustic monitoring runs recorders continuously for weeks at a fraction of the cost of repeated dive campaigns.

A single hydrophone package can log more than 100 days of data on one set of batteries, and the hardware fits in airline luggage. Analysis pipelines, once limited to specialists, now use open-source algorithms that flag shrimp snaps in minutes.

Nighttime audio also bypasses shallow-water hazards such as currents and low visibility that can limit diver safety.

Managers get a continuous record that reflects both biological change and illegal activity like dynamite fishing, which shows up as low-frequency blasts.

For Caribbean reserves already stretched thin, deploying acoustic recorders during the warmest months could reveal early bleaching impacts, long before divers notice paling colonies or dead fish.

Shrimp chorus offers hope

Listening alone will not stop coral decline. Average intervals between mass bleaching events worldwide have shrunk from once every 27 years in the 1980s to less than six years today.

Climate-driven heat remains the root cause, and no amount of acoustic monitoring can compensate for rising temperatures. Yet a cheap sensor that tracks resilience can guide triage, steering limited restoration funds to reefs that still crackle at night.

“It’s a story on one island and needs to be repeated in other parts of the world,” noted Raick. He wants to expand the approach beyond Moorea. 

Future work will test whether regional shrimp species differ in acoustic signatures and whether artificial soundscapes could lure coral larvae back to damaged reefs. 

The study is published in the journal Royal Society Open Science.

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