Listening to the Reef: How Sound Reveals Coral Life

Along the shores of Bintan, some say the crackling sounds heard underwater come from octopus. Others believe it is the corals themselves that make these noises. But when scientists listen more closely, a different story emerges.

The sound, it turns out, is mostly made by tiny animals called snapping shrimp.

Coral reefs are not silent places. They are filled with clicks, pops, and crackles produced by fish and invertebrates. Among the loudest inhabitants are snapping shrimp, small creatures, only a few centimetres in size, that produce surprisingly powerful sounds.

Listen to the audio clip below to hear the distinctive crackling sound produced by snapping shrimp on a coral reef.

These shrimp create their signature ‘’snap’’ by rapidly closing one of their claws, generating a tiny bubble that collapses almost instantly. This collapse produces a sharp, loud sound so strong that it can dominate the acoustic environment of an entire reef. The instantaneous energy in each snap is so much that the temperatures in the bubble can momentarily reach that of the surface temperature of the sun. When thousands of snapping shrimp are active at once, their combined snaps form a constant crackling sound, a defining feature of reef life. Together, these sounds create what scientists call a reef soundscape, one that reflects the life and activity within the reef. Decades of well-tested sonar technology developed for other soundscapes, has been shown to fail in Singapore waters, simply because of the dominating effects of these shrimp snaps that are not accounted for.

For scientists, these sounds are more than just background noise. They offer a way to understand what is happening beneath the ocean’s surface.

Dr Hari Vishnu, together with his colleagues, Prof Mandar Chitre, and Teong Beng Koay, marine acoustics researchers at the Acoustic Research Laboratory at the National University of Singapore, studies the ocean through sound using a method known as Passive Acoustic Monitoring (PAM). Instead of actively sending signals into the water, PAM relies on underwater microphones, called hydrophones, that are placed underwater, sometimes on the seafloor, to continuously record the natural sounds of the marine environment over long periods of time.

In their work around Singapore’s reefs, these devices are deployed for months at a time, continuously capturing the reef’s soundscape: from fish calls to marine mammals to the persistent snapping of shrimp. Over time, the laboratory has built a repo of many years of recordings from Singapore’s waters. This approach is especially valuable in places where visibility is low or conditions make visual surveys difficult, and helps scientists read the reef through sound. By analysing these recordings, their research explores how sound can reveal patterns of life and activity beneath the ocean’s surface, including in coral reef ecosystems that are often difficult to study visually. Healthy reefs tend to be louder, filled with the activity of snapping shrimp, fish, and other organisms. In contrast, degraded reefs, affected by bleaching, pollution, or habitat loss, are often noticeably quieter.

Because hydrophones can record continuously over long periods, and sound data can be stored easily, passive acoustic monitoring allows researchers to track changes in reef activity over time, compare different reef sites, and detect disturbances that might otherwise go unnoticed. Patterns such as the intensity of shrimp snapping or the presence of fish or marine mammal calls can indicate shifts in biodiversity, coral cover, and overall ecosystem health.

Their sounds can even help other marine animals find coral reefs. Fish larvae and other reef organisms use reef soundscapes as cues when choosing where to settle. Loud, active reefs attract more life, while quieter, degraded reefs may struggle to draw new generations of marine species. In this way, the crackling sound of snapping shrimp is not just a by-product of reef life, but part of how reefs regenerate and sustain themselves over time.

Thus, what may sound like simple crackling noise to the human ear is, in fact, a complex signal of life. And by learning to listen carefully, scientists are beginning to understand coral reefs not just as visual landscapes, but as living, sounding ecosystems, revealing new ways to monitor and protect them in a rapidly changing ocean.