In the abyss, everyone can hear you scream.
You might know what a hydrothermal vent looks like: black plumes billowing from deep-sea pillars encrusted with tube worms, hairy crabs, pouting fish. But do you know what it sounds like?
To the untrained ear, a hydrothermal vent — or more precisely, one vent from the Suiyo Seamount southeast of Japan — generates a viscous, muffled burbling that recalls an ominous pool of magma or a simmering pot of soup.
To the trained ear, the Suiyo vent sounds like many things. When asked during a Zoom call to describe the Suiyo recording more scientifically, Tzu-Hao Lin, a research fellow at the Biodiversity Research Center at Academia Sinica in Taipei, Taiwan, took a long pause, shrugged and laughed. People always ask him this, but he never has the answer they want to hear. "I usually tell people to describe it with their own language," Lin said. "You don't need to be an expert to say what it sounds like to you."
Lin adores acoustics; in his official headshot, he wears a set of headphones. He has listened to the sea since 2008 and to the deep sea since 2018. He has deployed hydrophones, which are microphones designed for underwater use, in waters off Japan to eavesdrop on the noises that lurk thousands of feet below the surface. He published these recordings in August at a conference of the Deep-Sea Biology Society.
Lin is not interested in focusing on the song of a singular whale or the clatter of ship traffic, but rather on the habitat's soundscape — the totality of all its sounds, human, animal and geological — to glean an area's biodiversity. Think of it as a hydrothermal vent's acoustical calling card.
Lin joins a growing field of acousticians who believe that sound may be the quickest, cheapest way to monitor one of the most mysterious realms of the ocean. A database of deep-sea soundscapes could provide researchers with baseline understanding of healthy remote ecosystems, and singling out the sounds of communities or even individual species can inform scientists when populations are booming.
"You need to know what the habitat sounds like when it's healthy," said Chong Chen, a deep-sea biologist at Japan Agency for Marine-Earth Science and Technology, or JAMSTEC. "When the soundscape has changed, the habitat may have changed, too."
The allure of deep-sea sound
Light holds little power in the ocean; it is so easily absorbed and scattered by seawater that anything deeper than 656 feet (200 metres) is essentially shrouded in darkness. But sound reigns supreme underwater, where it travels five times faster than in air.
If this statistic seems abstract, several acousticians laid out a helpful scenario in a 2018 paper in Acoustics Today. Imagine staring down at a city on a clear day from atop a mountain, the highest point within 60 miles (96km). You can see far into the horizon but only hear the sounds nearby, perhaps a chirping bird or a gust of wind.
In the deep sea, the rules are reversed. Standing on a ridge several thousand feet underwater, peering out to the ocean's abyssal plain, you would see almost nothing. But if you listened through a hydrophone, you could detect sounds from hundreds of miles away: echolocating whales, chattering fish, even occasional pulses from seismic surveys for oil and gas.
Scientists have long listened in on the sounds of the oceans but only recently have they turned to the deepest, darkest parts of the sea, where sound holds promise as a portal into an unknown world. Here, specialized creatures occupy habitats that would be fatal to surface dwellers; when Lin's colleagues retrieved the hydrophone from Suiyo, the vent's heat had melted part of the cables. "We got too close to the orifice," Lin said with a sigh.
The deep sea is difficult to visit and expensive to observe; underwater robots do not come cheap. But it's fairly easy to drop a hydrophone overboard — along with a baited camera, to see if anything bites. The hydrophone can pick up not just the noisy clicks of bickering dolphins but also the ambient hum of the deep sea.
Snooping on soundscapes
Lin became interested in underwater acoustics around a decade ago as a graduate student at National Taiwan University, on a project observing Indo-Pacific humpback dolphins. Although the project seemed exciting, he found the work anticlimactic, working long hours and seeing very few dolphins. But as Lin listened to the recordings, he heard a chorus of other creatures — the sounds of snapping shrimp and choruses of fish — as well as noise pollution from industrial development. "People are still really crazy about marine mammals," he said. "They do not really care about soniferous fish or invertebrates."
When he joined JAMSTEC in the spring of 2019 for a yearlong stint as a postdoctoral research fellow, he was surprised by the diversity of deep-sea life and even more surprised that few had tried to capture the sounds of deeper-living creatures and their often volatile, volcanic habitats. The work felt even more pressing as international interest in deep-sea mining continued to rise. In 2019, he proposed the use of deep-sea soundscapes as a conservation tool in a paper in Trends in Ecology & Evolution.
Research cruises are expensive, and Lin did not have time to develop a dedicated cruise for deep-sea soundscapes. So he and other researchers at JAMSTEC dropped hydrophones on already scheduled cruises, collecting daylong recordings from coastal areas near Japan and the Suiyo vent, and an even deeper recording from more than 18,000 feet (5,486 metres) below waters by the isosceles-shaped island of Minami-Tori. He found that the shipping traffic drowned out the coastal soundscapes, but the Minami-Tori Shima recording picked up noise from dolphins, humans and the tectonic grumblings of the seafloor itself, as well as a potpourri of as-yet untraceable sounds.
Lin's recordings reveal a medley of shrill beeps, distant whistles and an underwater chorus of fish that sounds almost like wind gusting through a mountain pass. But what is it all? Lin and his lab at Academia Sinica are developing a software algorithm to separate the elements of the soundscape into categories: biophony (creatures), geophony (weather, earthquakes, volcanic eruptions) and anthropophony (human noises like seismic tests, ships and mining). Then the program will isolate individual sounds, such as dolphin whistles or chattering fish, and could even discover the sounds of new species.
Although the researchers are still poring through the data, some soundscapes have already provided insight into life in the deep sea. The Minami-Tori Shima recording revealed a chorus of fish that began right after sunset and ended after midnight at depths with no visible light. "It's really amazing," Lin said. He suspects the chorus may be timed with some fish's daily vertical migration toward the surface at night, although he said he would need to conduct more surveys to confirm this connection.
Deep-sea detective work
Whereas the clicks and songs of marine mammals are well-documented, the identities of smaller deep-sea noisemakers are still shrouded in the dark.
At face value, deep-sea fish would not appear to be the most competent vocalists. "Many fish sounds require hard parts like bone or dense muscles," said Rodney Rountree, an adjunct professor at the University of Victoria who specialises in fish acoustics.
But some fish, such as the sailfin catfish, make sounds by rubbing their body parts together like crickets do. Then their air-filled swim bladders act like a drum to amplify the sound. This movement can create sounds most often described as rasps, creaks or grunts — but these terms vary. "It's a big headache," Rountree said. "Even in the same study, I might call one thing a groan, and when I process it the next day I may call it a grunt."
Other fish, such as cusk eels, have dedicated muscles that push on these bladders to bang like a drum or croak like a frog. "It's really loud, like a jackhammer," he said, clearing his throat before demonstrating: "AH-AH-AH-AH-AH." But many more gelatinous deep-sea species are bereft of such bladders, as well as the musculature needed to press against them. A blobfish, after all, is more water than muscle.
Researchers have observed sonic muscles or have recorded sounds from five families of deep-sea fish, including grenadiers and sablefish, according to Marta Bolgan, a marine biologist at the University of Liege in Belgium. "It is a very new field," she said. Bolgan recently published a paper in the journal Fish and Fisheries highlighting the importance of listening to deep-sea fish.
Some researchers are working to improve current listening technology. At the Woods Hole Oceanographic Institution in Woods Hole, Massachusetts, Ying-Tsong Lin is building a starfish-shaped contraption of hydrophones that can tune into certain sounds hundreds of miles away, like a telescope for sound.
Bolgan's strategy involves attaching video cameras to recorders to capture fish vocalising on screen. But even this is no sure thing. A video that captures a fish and a fish sound in the same frame still doesn't prove that fish made that sound. Researchers have to sleuth out whether that fish could physically make that sound, either by listening to existing recordings or speculating how the fish's sonic muscles might produce noise. "It is a combination of deduction and luck," Bolgan said.
Fish often make sounds alongside particular behaviours, such as spawning, which can be difficult to replicate in a lab, although some researchers have succeeded. In 2016, Eric Parmentier, Bolgan's supervisor at the University of Liege, recorded cusk eels growling in fiberglass tanks after sunset. Floating egg masses the following morning indicated the fish had spawned.
Fish may represent researchers' best bet at parsing deep-sea biodiversity, as many key deep-sea animals are not known to make sound, according to Chen. "Snails don't vocalise," he added as an example. There are exceptions. In 2019, researchers recorded loud snapping sounds from small, mouth-fighting marine worms that dwell in glass sponges. And a 2017 study revealed that glass sponge reefs possess a distinct soundscape entirely their own.
A sonic library
Lin of Academia Sinica wants to make all his soundscapes available online. This way, researchers such as Bolgan can sort through the recordings to single out a particular fish chorus or any other particular sound.
"Once the data is digitised, it can be used over and over again," Lin said. "Future generations will be able to see what biodiversity was like decades ago. " He uploaded all his recent recordings to SoundCloud and invites any would-be acousticians to listen in.
Lin's eventual goal, the Ocean Biodiversity Listening Project, is an international, open-access database of underwater recordings that can establish a baseline of healthy, deep-sea ecosystems. He knows he's working against the clock. "Deep-sea mining is about to start anytime now," Chen said.
In 2017, Japan successfully extracted zinc from the seabed off Okinawa. "We need research cruises to incorporate soundscapes as part of their surveying," Chen said, adding that the process should also be included in baseline environmental studies of potential mining sites.
Many deep-sea mining interests overlap with biodiversity hot spots, such as sulfide-rich hydrothermal vents. Chen suspects that vent soundscapes may offer cues to deep-sea larvae looking to settle on the seafloor. "Chemical cues get diluted by seawater, but sound propagates very far, so it potentially has a very important role," Chen said. If deep-sea mining were to interrupt larval settlement, communities could take years to recover.
Lin continues to scan his soundscapes for new patterns. The recordings are still cryptic, a bramble of ambience. But for now, some reflect the racket of a deep sea that's still noisy in the ways it's supposed to be.
Written by: Sabrina Imbler
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