A twilight chorus that can be heard 18,000 feet under the sea

Off the Japanese island of Minamitorishima at twilight, after the sun dips below the horizon, the voices of many animals come together in chorus. It cannot be heard on land, because the participants in this chorus are fish. They don't sing using vocal cords—they don't have any—but rather they sing by contracting their gas-filled swim bladders to make a sound like a kazoo or by rubbing their flippers or teeth to make a sound like a cricket. The overlapping calls of all these grunts, buzzes, chirps and squawks combine into a chorus that soars across the vast ocean at distances where the sun's rays might disappear.

“Light does not travel well in seawater, so it is very dark at great depths,” Christine Erby, director of the Center for Marine Science and Technology at Curtin University in Perth, wrote in an email. “But sound transmits very well.”

The chants of the fish chorus also travel, not only across the waves, but also beneath them. They form part of the ocean's soundscape, which includes all the sounds made by animals, natural forces such as wind and earthquakes, and man-made technology. Different ocean ecosystems have different soundscapes, said Tzu Hao Lin, a researcher and self-described “ocean listener” at Academia Sinica in Taiwan. He devoted himself To study the acoustic atmosphere of the deep sea. He has it Recorded audio clips From hydrothermal vents, the twilight zone and abyssal plains more than 18,000 feet deep in the water next to the remote island of Minamitorishima. In these exceptionally deep waters, Lin thought he discovered a chorus of fish singing shortly after sunset and calming down at midnight. But the recording was so short, representing less than a day at depth, that it may have been just a fleeting blip, a fleeting school of chattering fish.

“It's in the deep sea, so we don't really have a lot of scientific questions to begin with,” Lin said. “We just want to explore what we will be able to discover.”

Lin came up with a plan to record the sounds of the deep sea abyss for an entire year. In March 2020, Shinsuke Kawaguchi, a researcher at the Japan Agency for Marine-Earth Science and Technology, dropped an underwater recording device called a hydrophone over the side of a ship south of Minamitorishima, which is off-limits to and isolated from civilians. Shipping movement. The hydrophone sank until it settled on the sea floor at a depth of 18,000 feet. He stayed there for a year, in an area too deep and remote to be photographed, recording for two minutes every four hours. When researchers recovered the device the following April—remotely triggering it to loosen its anchor and rise more than three miles to the surface—they recovered a year-long record of the abyss's soundscape, which Lin and Kawaguchi published in the journal. Limonology and oceanography Last fall.

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Dropping the hydrophone from the ship. | Shinsuke Kawaguchi (JAMSTEC)

Back in the lab, Lin studied 4,260 minutes of recordings. Surrounding audio clips often don't sound like much to the human ear. “It's like the white noise coming from a broken TV,” Lin said. Ironically, the best way to understand these sounds is to see them. Lin ran a computer program that produces spectrograms that visualize audio data, revealing changes in frequency and intensity over time (here Good example From soundscape and associated spectrogram).

The data revealed that by day, the Minamitorishima abyss was a quiet place, occasionally punctuated by whistles and clicks from passing marine mammals or distant ship engines. But the abyss came alive every night, resounding with a chorus of fish at sunset and calming down at midnight. He added that even Lin, who is just a human, can hear a “huge difference” between daytime and nighttime sounds. He was surprised. His previous statements were not just a passing picture. this First chorus They can be heard every day from 8pm until midnight between May and September. From December to June, a Second chorus It seemed, also around 8 pm

At first glance, this finding may seem mundane. After all, the ocean is full of fish. But the timing of these fish choruses—both of which materialize at twilight—means that different times of day in the abyss feature different sounds. The researchers consulted previous studies and found that similar evening choruses can be heard across swaths of the Pacific and Indian Oceans at depths of approximately 1,000 feet to more than 18,000 feet. The researchers wondered whether these sounds were related to fish migration, the daily phenomenon in which billions and billions of plankton, fish and other animals rise toward the ocean surface at night and sink again into the deep seas at dawn. If songfish migrate along this vertical path, they may descend to depths where their sounds become felt in the abyss at sunset. In other words, while deep-sea creatures may not see the sunset, they may be able to hear it.

A diagram explaining how deep-sea sound travels from the surface to the depths
A diagram explaining how sound travels in the deep sea. | Tzu Hao Lin

As creatures of the sun, we keep time with it Circadian rhythms– Our internal clocks that oscillate in 24-hour cycles in response to light and darkness. Scientists have long assumed that the deep sea, a world far removed from the cycles of sunlight and darkness that dictate much of life on Earth, has no such rhythms. But as deep-sea exploration technology improved, allowing scientists to record abyssal environments for longer periods of time, they discovered circadian rhythms in the deep sea that could be explained by biological clocks. For example, in the sparkling waters of hydrothermal vents more than 5,500 feet deep, Bathymodeulus thermophilus Mussels obey 24-hour rhythms.

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Hans van Haaren, an experimental physicist and physical oceanographer at the Royal Netherlands Institute for Marine Research who was not involved in the research, observed deep-sea plankton engaging in a daily vertical migration at depths where shifts in sunlight are impossible to detect. Plankton aggregate during the day and disperse near the surface at night, and these movements are actually initiated by plankton groups living deep underground. So, if sunlight isn't an indicator, the trigger is probably present somewhere within the plankton. “Our main hypothesis was control by stable biological clocks, although we took into account the possibility of acoustic noise,” van Haren said, adding that the soundscapes revealed in the new paper still do not explain how these deep plankton populations drive Migrations.

But our internal clocks are not entirely biological. They are also synchronized through environmental cues called, charmingly, zeitgebers – derived from the German word for Time givers. The new study suggests that the sound of migratory species from above could serve as a sound that allows many deep-sea species, with or without biological clocks, to tell time. However, the identity of these migratory species remains a mystery, as Lin and Kawaguchi were unable to identify any individual species during the evening choruses. But the precise effects of these sounds on other species remains an open question. “It seems strange that a species would betray itself to predators by making noise,” Van Haren said.

Although there is a paucity of research that actually tests the hearing and noise-making abilities of deep-sea fish, the case remains anatomy Many species appear to be attuned to hearing, with oversized ears or inner ear structures with special adaptations. “We know very little (very little) about creatures that live in the deep sea, but given the lack of light, it is plausible that many of these species use sound to sense their environment, navigate, and communicate,” said Irby, who was not involved in the study. search. Given the daily patterns of sounds emitted by fish, marine mammals and other animals closer to the surface, “it would not be surprising if deep-sea benthic organisms responded to this, or evolved to take advantage of these sounds from higher up in the water column,” she said.

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A big blue sperm whale swims in Dominica
Sperm whales are deep-diving marine mammals whose sounds reach great depths. | Arturo de Frias via Getty Images

Lin and Kawaguchi's recording also picked up occasional whistles and echolocation clicks from passing marine mammals, which they suggested could mean the area may be a research ground for sperm whales and beaked whales. Certain species of fish and mammals issue feeding calls as they follow prey, which may migrate vertically, so a whale song or dolphin whistle would show these daily patterns, Erby noted. Given the high speed of sound in the ocean, these calls would quickly travel to the depths, “where they may induce rhythmic patterns within the same 24-hour period,” Irby said.

Lin admits that the new data is somewhat limited, and is sampled with a single hydrophone at intervals of a day. He would like to get back to making higher quality recordings. Irby suggested using technology that could indicate the source of the sound, as the presence of sound at great depths does not necessarily indicate that the sounds also come from great depths. “Once we can identify the source, we may have a much better idea of ​​the animal or species making the sounds we hear at depth, and ultimately the function of those sounds,” she said.

The abyssal plains near Minamitorishima are filled with manganese nodules, which contain rare minerals that plans are looming to exploit. Deep sea mining. Like any other type of mining, deep-sea mining will be a noisy business, conjuring up the sounds of operations as well as surrounding ships. This pollution would overwhelm the local soundscape, drowning out any distant aurora choruses, dolphin clicks, and whale songs. For deep-sea creatures that can sense these sounds, this could be like blocking out the sun. But more research is needed to understand the role of sound in the deep sea, and how important it is to mute it.

“When we talk about deep-sea mining, we really need to be careful about how this kind of noise affects deep-sea ecosystems,” Lin said. “We still know very little.”

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