The story that coral reefs are telling us (and why we should listen)
Written by Giuliana Vomero, edited by Rachel Small
Today, I’m going to be your DJ and guide you through a different kind of playlist. So put on your headphones, find a quiet place, and take a deep breath — relax for a moment. Then press play on the audio clip below.
Audio credit: Tim Lamont, University of Exeter
What are your impressions? Did it sound chaotic to you? Maybe it felt familiar in some way, as if someone were laughing while rain poured heavily in the background. Did you wonder, What in the world am I listening to?
I’m guessing you probably didn’t imagine you were hearing an underwater carnival.
In other words, a healthy coral reef.
Coral reefs, located in the tropics, are among the most biologically complex ecosystems on earth. They are the foundation of many coastal communities, providing livelihoods, medicine, protection from storms, and more.
Photo credit: Beth Watson, Ocean Image Bank
Coral reefs teem with life and sound
Coral reefs are one of the largest and most ancient, diverse, and threatened animal-built structures on the planet. Around 800 species of hard corals, the building blocks of this ecosystem, are home to 25 percent of marine life. In other words, thousands of organisms—including algae, crustaceans, fish, sharks, rays, sea turtles, and marine mammals—rely on coral reefs to reproduce, grow, feed, and survive.
And the sounds of a coral reef are as diverse as its residents.
Snapping shrimps, for example, from the family Alpheidae, are major musicians. When it’s feeding time, this crustacean creates a network of bubbles with its unique and characteristic claw, which can stun or kill its prey. When many are hunting at the same time, their claws create a sound similar to that of rain, as you heard in the audio clip—a sound so loud, it can compete with the sound produced by the blue whale, the loudest animal in the ocean.
Fish also like to gather and communicate with each other within coral reefs. Some of their sounds include popping, croaking, grunting, whooping, and trumpeting. They tend to make these sounds while hunting, arriving in new territory, or mating, or when predators are near. Put your headphones back on and immerse yourself in the world of fish.
Audio credit: Tim Lamont, University of Exeter
Audio credit: Eric Parmentier, University of Liège
Audio credit: Tim Lamont, University of Exeter
Fish can produce sounds by moving their “sonic muscles,” which makes the swim bladder vibrate, or by swimming, changing speed or direction very quickly, or rubbing their teeth or hard skeletal parts together.
Photo credit: David P. Robinson, Ocean Image Bank
The story of a coral reef soundscape
We once thought the ocean was silent, but as she has so many times, Mother Nature has proven us wrong. In fact, the ocean abounds with soundscapes. Coral reef soundscapes are more than simply animal noises combined. With the help of science and technology, scientists have discovered that they tell detailed stories.
Beneath the surface of the vast ocean, light disappears, so when looking for places to reproduce and grow, fish and other invertebrates rely on sound waves to detect the best places to do so. Water can move sound quickly and carry it far away from its source, which means that fish far from a particular reef can hear it and receive information about the presence, behavior, diversity, and abundance of the organisms living in it.
The sounds produced by marine life in a healthy coral reef indicate that the reef is a vibrant place to live. Coral soundscapes are like big billboards in the middle of the ocean declaring, “I’m the most suitable coral reef for you in the area!”
Sound also plays a critical role in coral growth. Corals of the same species in a particular area synchronize to liberate eggs and sperm into the water column in an event called coral spawning. When these gametes find each other and fertilization occurs, the larvae will search for a surface on which to settle and grow—hopefully into a colony. In a recent study, scientists explain that the larvae of the coral species Porites astreoides tend to prefer settling in places with a soundtrack environment; a soundscape can improve a baby coral’s settlement in a reef.
And coral soundscapes don’t just help marine life. Scientists can use them as tools to measure the health and biodiversity of coral reefs.
As you read this, the fourth global coral bleaching event on record (and the second in the last ten years) is taking place, according to NOAA scientists. Climate change and marine heat waves increase water temperature, which puts stress on corals. This stress breaks their vital relationship with zooxanthella, the microalgae living within their tissues, and causes them to become white and weak. As they lose their colors and strength, coral reefs stop being the nourishing and vibrant places they once were, so their residents leave. In the space left behind is a heartbreaking sound.
Audio credit: Ben Williams
Gone is the diversity and complexity we heard earlier. No more laughing or whooping—only a few shrimps snapping. When comparing soundscapes, scientists from the University of Exeter at the Great Barrier Reef heard a reduction in sound following the massive bleaching events of 2016.
Coral bleaching event in the Maldives. Some coral species can recover from bleaching events once the temperature has fallen. In other cases, corals don’t survive and the reef never recovers, becoming a silent, white desert.
Photo credit: The Ocean Agency, Ocean Image Bank.
The power of sound
The loss of coral reefs isn’t just devastating for marine life. Reefs also play a crucial role in the lives of the nearby communities. Approximately 1 billion people rely on coral reefs for food, coastal protection, culture, and tourism. Silence mode isn’t an option. And so, all around the world, numerous efforts are being made to grow coral fragments in order to restore and bring coral reefs back to life.
As part of these efforts, scientists have been using the power of sound. At the Great Barrier Reef, an international team of scientists installed loudspeakers playing the soundtrack of a healthy coral reef on patches of degraded corals. Afterwards, twice as many fish were attracted to (and stayed in) these sites than to those without sound. Fish of all trophic levels were present—some that eat algae and plankton and some that eat other fish, for example—helping to make the ecosystem complex and dazzling once again. This technique, called acoustic enrichment, is a hopeful tool for professionals working on coral reef restoration all around the world.
Located in South Sulawesi, Indonesia, the Mars Assisted Reef Restoration System (MARRS) is one of the world’s largest reef restoration projects. Coral fragments are attached to hexagonal structures called Reef Stars and installed in degraded areas to stabilize loose rubble and kick-start rapid coral growth.
Photo credit: The Ocean Agency, Ocean Image Bank
A restored reef three years after the installation of Reef Stars. Scientists monitor the success of restored sites by comparing the diversity of fish sounds (i.e., the phonic richness) on these sites with that of healthy reefs and degraded reefs. After two years, MARRS sites have the same level of phonic richness as healthy natural sites.
Photo credit: The Ocean Agency, Ocean Image Bank
It’s time to start listening
Rachel Carson’s revolutionary book Silent Spring (1962) starts with the story of a fictional village grappling with the effects of pesticides:
And there was a strange stillness . . . It was a spring without voices. In the mornings, which had once throbbed with the dawn chorus of robins, catbirds, doves, jays, wrens, and scores of other bird voices, there was now no sound; only silence lay over the fields and woods and marshes.
More than 60 years later, our reality isn’t far removed from Carson’s town. Coral reefs are bleaching as well as being degraded by illegal fishing and killed by bacterial diseases. We might very well wonder if “a spring without voices” is the next season for the ocean.
But we don’t have to let this be the case.
As the clips in this article reveal, sound is so much more than vibrating waves traveling through a medium. It can carry information, guidance, and HOPE.
This World Ocean Day, let the sounds of the ocean spark curiosity and joy in you. Let them remind you of how marvelous and important our ocean is. Let the coral reef soundscapes play on repeat in your mind. Let them become a memory that moves you to action.
Support your local ocean conservation organizations, volunteer for coral reef restoration programs, join the field as a professional, learn more about your local ecosystems, or share this story with your friends and family.
The ocean is telling a story with every sound it makes. And if there is no sound, there is no life. Silence mode isn’t an option.
Want to hear more?
The first time I heard a coral reef soundscape, I had many questions: Are we really silencing the ocean? What’s the science behind acoustics? What type of research is being done? You may wonder the same. These are topics for other articles, I promise!
In the meantime, check out some of the other books, articles, and videos that inspired and informed this post:
Fish Sounds: Do fish talk to each other? | Earth Unplugged (watch)
The Soundscapes of Restored Reefs , Mars Coral Reef Restoration (read)
One Hour Real Coral Reef Sounds | HD Coral Reef Soundscape (listen and relax)
World faces ‘deathly silence’ of nature as wildlife disappears, warn experts, The Guardian (read)
Author’s Note
This post was inspired by science communicators, marine scientists, friends, and educators passionate about the ocean, coral reefs, and conservation.
Special thank-you to Rachel Small, of Rachel Small Editing, for her respectful, powerful suggestions and edits, which helped bring this article to life. And to Benjamin Williams, for sharing with me his research on coral reef soundscapes and his sound recordings.