While most people are familiar with whale songs, they’re often surprised to learn that many fish and invertebrates use sounds to communicate, too. Scientists recently created an inventory of species that produce sounds underwater, a big step forward in our understanding of aquatic animals.
Oceans pulsate with underwater songs, coral reefs crackle and croak, and the sounds from stars tell us just how far away they are. It seems the whole universe absolutely vibrates with music.
Now, science is verifying that notion. An international team of scientists has just issued an inventory of species confirmed to produce or suspected of making sounds underwater. More than 700 of them do, and thousands more are thought to. Not only are we discovering the multitude of underwater sounds that come from unexpected sources, the purpose of some of them are mind-blowing. For example, healthy coral reefs have rich soundscapes that larval corals use as cues to identify the best places to settle and grow.
And overhead, a team of astronomers is now using asteroseismology, the study of stellar oscillations, to accurately measure the distance of stars from the Earth by turning those tremors into sound waves.
Though silent most of the time, red-bellied piranhas can make an array of impressive noises, including croaks and drumbeats, each of which is thought to convey a different message. The fish break into aquatic barks at the first sign of trouble, such as when confronting other fish over food.
Everywhere, newly discovered soundscapes are transforming what we thought we knew of the natural world.
Acoustic aquatic ecosystems
More than 70% of the Earth’s surface is covered in water, and most of the planet’s habitats are aquatic. Yet there is a misconception that most undersea organisms are silent. Now, however, a recently published, comprehensive digital database on what animals are known to make sounds underwater is the first of its kind and may, say researchers, revolutionize aquatic and marine science.
Teams from the University of Florida’s Fisheries and Aquatic Sciences Program, the Global Library of Underwater Biological Sounds and the World Register of Marine Species collaborated to document 729 aquatic mammals, fish, invertebrates and other tetrapods (vertebrates with two pairs of limbs) that produce active or passive sounds. In addition, the inventory includes another 21,911 species that are considered likely to make sounds. The inventory was published in the journal Scientific Data in December 2023, and the data is available for free.
Seahorses can produce clicking or popping sounds by rubbing the edges of their own skull bones together.
The creators of the database state that eavesdropping on underwater sounds revealed a wealth of information about the species that make them and resulted in findings that will be useful in a variety of applications, including fisheries management, invasive species detection, improved restoration outcomes and assessing human environmental impacts.
Chordal coral reefs
As adults, corals are immobile, so the larval stage is their only opportunity to select a good habitat. They drift or swim with the currents, seeking the right conditions to settle out of the water column and affix themselves to the seabed. Previous research has shown that chemical and light cues can influence that decision.
But healthy coral reefs are also noisy; full of the croaks, grunts and purrs of various fish and the crackling of snapping shrimp. A reef that has been degraded—whether by coral bleaching, disease or direct human impacts—can’t support the same diversity of species and has a much quieter, less rich soundscape.
Research suggests that larval coral animals use a symphony of sounds to help them determine where they should live and grow. Broadcasting the soundscape of a healthy reef at a degraded reef caused coral larvae to settle there at significantly higher rates.
That caused researchers at the Woods Hole Oceanographic Institution in Massachusetts to wonder if soundscape also plays a major role in where corals settle. Recently, they traveled to the U.S. Virgin Islands to conduct an experiment, running it twice, in June and July 2022.
The scientists collected larvae from Porites astreoides, a hardy species commonly known as “mustard hill coral” due to its lumpy shape and yellow color. They distributed the corals in cups at three reefs along the southern coast of St. John. One of those reefs, Tektite, is relatively healthy. The other two, Cocoloba and Salt Pond, are more degraded with sparse coral cover and fewer fish.
At Salt Pond, an underwater speaker system was installed, and the cups of larvae were placed at distances of 3.2, 16.4, 32.8 and 98.4 feet from the speakers. Healthy reef sounds—recorded at Tektite in 2013—were broadcast for three nights. Similar installations were set up at the other two reefs, but sounds were not played there.
In the U.S. Virgin Islands, coral reefs are found around the three main islands of St. Croix, St. John and St. Thomas, as well as in most offshore cays.
When the cups were collected, the researchers found that significantly more coral larvae had settled in the cups at Salt Pond than the other two reefs. On average, coral larvae settled at rates 1.7 times (and up to seven times) higher with the enriched sound environment. The highest settlement rates were at 16.4 feet from the speakers, but even the cups placed 98.4 feet away had more larvae settling to the bottom than at Cocoloba and Tektite.
The results of this study, which were published in March 2024 in the journal Royal Society Open Science, show that settlement consistently decreases with distance from the speakers, when all else is kept constant. That’s particularly important because it demonstrates that these changes are due to the added sound—and not other factors.
Surprisingly, the researchers didn’t find much difference between settlement rates at the more degraded Cocoloba and the healthier Tektite reefs. A previous study in 2017 had found higher settlement rates at Tektite than Cocoloba. Some of this could be attributed to natural variation, said the scientists, but the Tektite reef has also faced several destructive hurricanes, a significant bleaching event and an outbreak of coral disease in recent years. That could have caused the complexity of Tektite’s soundscape over the last decade to decrease.
The hardy mustard hill coral grows as a bumpy, rounded mass in the Atlantic Ocean and Caribbean Sea. Its appearance is often soft and fuzzy because its polyps are usually out during the day.
The drop in settlement rates at Tektite, however, does underscore the severity of the threats that coral reefs are facing and the need for rapid, scalable solutions, the researchers said. Coral reefs support more than a quarter of all marine animals, protect coastlines from strong storms and waves, and provide food and tourism opportunities for millions of people around the world. But half of all coral reefs have been lost in the last 30 years.
The researchers say that they hope that their work can help with coral restoration efforts. Enhanced soundscapes could be used to increase settlement rates in coral nurseries, for example, or be passively broadcast at reefs in the wild to improve or maintain existing coral populations. People would still need to monitor reef conditions to ensure that coral larvae are able to thrive after they settle, but this could be a significant step in the restoration process and one that would be relatively simple to implement—far easier compared to replicating the reef chemical and microbial cues that also play a role in where corals choose to settle.
Sonorous stars
For most of us, the countless bright spots in the nighttime sky all seem to be stars. But, in fact, some of those dots are planets, distant suns or even entire galaxies located billions of light-years away. Just what you’re looking at depends on how far it is from Earth. That’s why measuring the exact distance to celestial objects is such an important goal for astronomers—and one of the biggest challenges they’re currently tackling.
The bright spots you see in the nighttime sky might all seem to be stars. However, in reality, some of those spots are planets, distant suns or even entire galaxies located billions of light-years away.
It was with this in mind that the European Space Agency launched the Gaia mission 10 years ago. Data collected by the Gaia satellite is opening a window into the near universe, providing astronomic measurements—such as position, distance from the Earth and movement—on nearly 2 billion stars.
Gaia increased by a factor of 10,000 the number of stars whose parallaxes have been measured. Today, scientists use parallaxes to calculate the distance to stars. This method involves measuring parallax angles, with the help of the satellite, through a form of triangulation between Gaia’s location in space, the sun and the star in question. The farther away a star is, the more difficult the measurement because parallax gets smaller the larger the distance.
Despite the resounding success of Gaia, the measurement of parallax is complex, and there remains small systematic effects that must be checked and corrected in order for Gaia parallaxes to reach their full potential. Through calculations performed on more than 12,000 oscillating red giant stars—the biggest sample size and most accurate measurements to date—the scientists compared the parallaxes reported by the satellite with parallaxes of the same stars that were determined using asteroseismology.
A red giant is a dying star in the final stages of stellar evolution. It forms after a star has run out of hydrogen fuel for nuclear fusion. In about 5 billion years, our own sun will turn into a red giant, expand and engulf the inner planets—including Earth.
In the same way that geologists study the Earth’s structure using earthquakes, astronomers use asteroseismology, the stars’ oscillations and vibrations, to glean information about their physical properties. Stellar oscillations are measured as tiny variations in light intensity and translated into sound waves, giving rise to a frequency spectrum of these oscillations.
The frequency spectrum allows scientists to determine how far away a star is, enabling them to obtain asteroseismic parallaxes. To turn sounds into distance measurements, researchers start with a simple fact. The speed with which sound waves propagate across space depends on the density and temperature of the star’s interior. By analyzing the frequency spectrum of stellar oscillations, they can estimate the size of a star, much like you can identify the size of a musical instrument by the kind of sound it makes, similar to the difference in pitch between a cello and a violin.
Having thus calculated a star’s size, the astronomers then determine its luminosity and compare this figure to the luminosity perceived here on Earth. They couple this information with chemical-composition and temperature readings obtained from spectroscopy and run this data through sophisticated analyses to calculate the distance to the star. Finally, the astronomers compare the parallaxes obtained in this process with those reported by Gaia to check the accuracy of the satellite’s measurements.
Improving the “Gaia” satellite’s parallax measurements will help pinpoint our place in the universe and benefit many subfields of astronomy and astrophysics.
In a report that was published in the journal Astronomy and Astrophysics in December 2023, astronomers say that they have listened to the “music” of a vast number of stars; some of them 15,000 light-years away. Improving Gaia’s parallax measurements, they conclude, will help pinpoint our place in the universe and benefit many subfields of astronomy and astrophysics.
Unheard utterances
The world is full of sounds. Some of them—usually those of our own making—can drive us to search for a quiet escape.
Acousticians say that a soundscape includes three fundamental sound source types: 1) anthropophony, sounds associated with human activity; 2) biophony, sounds produced by animals; and 3) geophony, sounds generated by physical events, such as earthquakes, rain or waves.
The term “geophony” means the nonbiological, natural sounds produced in any given habitat, such as wind in the trees.
Sometimes, I tire of the first kind of din and long for more of the second and third types. But wouldn’t it be great to experience a fourth sort of sound that I would add to the list: those which we have yet to hear.
Here’s to finding your true places and natural habitats,
Candy
