They are known as silent killers. Now, unprecedented recordings reveal first known shark sounds

(CNN) — Sharks have a reputation for being silent killers — the top ocean predators are stealthy hunters, but they also lack sound-producing organs found in many other fish. Now, a groundbreaking new study has revealed one shark species may be more vocal than previously thought, making noises comparable to the sound of a balloon popping.

For the first time, scientists have recorded evidence of the rig shark — Mustelus lenticulatus — actively producing sound by snapping its teeth, according to research published Tuesday in the journal Royal Society Open Science.

“How sharks maintain social groupings or communicate was thought to be largely body language and possibly through chemical signals; but for the most part has been a mystery,” said Neil Hammerschlag, president of Atlantic Shark Expeditions and executive director of the Shark Research Foundation, a not-for-profit group that aims to increase knowledge and promote conservation of sharks. Hammerschlag was not involved in the research. “This study opens up a whole new possibility of communication via sound.”

The rig shark, a small species that inhabits the coastal waters of New Zealand, typically lives near the seafloor and plays a key role in the region’s commercial fishing industry. Unlike most fish that rely on a swim bladder — a gas-filled organ that helps fish maintain their buoyancy and produce and detect sound — sharks lack this feature, making noise production seem unlikely.

Lead study author Dr. Carolin Nieder first became curious about sharks’ acoustic capabilities after hearing an unexpected clicking sound during behavioral training experiments while earning her doctoral degree at the University of Auckland in 2021. Nieder is now a postdoctoral investigator with the T. Aran Mooney Lab at the Woods Hole Oceanographic Institute in Massachusetts.

Although she was unable to investigate the mysterious clicking at the time, it held her interest. Now, Nieder and her team have reported fascinating findings from studying 10 juvenile rig sharks — five males and five females — caught off the coast of New Zealand.

Making some noise

The scientists housed the sharks in large marine laboratory tanks, maintained with proper seawater conditions and food, from May 2021 to April 2022.

To carefully record any sounds made, the team transferred the sharks to individual tanks equipped with underwater microphones or hydrophones. As the sharks were moved between tanks or gently held, they began making clicking noises, similar to the ones Nieder noticed years ago.

Each click was extremely short, lasting an average of 48 milliseconds, which is faster than a human eye blink.

Researchers also identified the clicks as broadband, meaning they occurred over a wide range of frequencies from 2.4 to 18.5 kilohertz — some of which can be detected by humans, according to the study.

In terms of volume, the clicks were intense, reaching about 156 decibels.

The study noted most clicks occurred within the first 10 seconds of handling, becoming less frequent over time. During the initial 10 seconds, the sharks emitted an average of seven clicks, compared with the final 10 seconds with an average of only two clicks.

“As the animals got used to the daily experimental protocol, they then stopped making the clicks altogether, as if they got used to being in captivity and the experimental routine,” Nieder said via email. “This led us to consider that maybe we are observing a sound making behavior rather than a strange artifact.”

The team also observed that around 70% of clicks occurred when the shark was swaying side to side slowly, while about 25% of the clicks happened when the shark was exhibiting explosive movements, waving its head or body around. Only 5% of the movements happened when the shark wasn’t obviously moving its body, according to the study.

What’s causing rig sharks to snap?

With no specialized sound-producing organs found in these sharks, the researchers are confident that the clicks come from their strong, interlocking teeth snapping together.

These platelike teeth, designed to crush prey such as crustaceans, may be responsible for producing the distinctive clicking sound, similar to the sounds some fish make by grinding their teeth.

The consistent pattern and frequency of the clicks also suggest the sounds are intentional, rather than accidental, Nieder said.

Researchers are still trying to investigate why exactly the rig shark emits these sounds. One possibility is that the clicks serve as a distress signal, which could be a reaction to being handled during the experiment.

Most of the clicks fell outside of the hearing range of rig sharks themselves, which extends to about 800 hertz, according to the study. If sharks aren’t using their vocal capabilities to communicate with one another, it’s possible the clicks could serve as a warning signal or form of aggression around prey or in dangerous situations, the researchers wrote.

Vocal abilities in natural habitats

With more than 500 species of sharks worldwide, it’s still unclear whether other sharks share this ability to produce sound.

“I think there is a chance other sharks are making similar noises,” Nieder said. “This documentation could help (us) start listening to sharks, and maybe we can learn more interesting things about their ecology and lifestyle in their various ecological niches.”

Additionally, because the sounds were recorded in a controlled laboratory setting, researchers are eager to discover whether rig sharks also produce these noises in the wild and under what conditions.

“This study opens up the possibility of these smaller sharks ‘sounding the alarm,’” Hammerschlag said via email, referencing how smaller sharks scatter in the presence of larger sharks, even if they are out of sight. “While we really don’t know if the sound produced by the rig sharks was simply a byproduct of being handled … it does open up some new questions, possibilities and avenues for future research.”

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