Sharks have long been thought to be mute, relying on stealth to hunt their prey and avoid predators. But now, sharks have been recorded making sounds for the very first time.
The new recordings reveal that rig sharks (Mustelus lenticulatus) — small, bottom-dwelling sharks native to New Zealand — emit distinct clicks when handled by researchers underwater. These sounds were consistent and repeated across multiple individuals, and were potentially tied to distress or defensive responses, according to a study published Wednesday (Mar. 26) in the journal Royal Society Open Science.
The recordings represent the first known case of a shark actively producing sound. "Sharks have sensory systems that are more refined than their hearing, like their electroreceptors, their smell and the way they propel themselves through the water," study lead author Carolin Nieder, a researcher at the Woods Hole Oceanographic Institution, told Live Science. "But I think the original notion that we had that sound isn't important at all is also likely not true."
Ten juvenile rig sharks were observed making sounds by accident during routine behavioral experiments at the University of Auckland's Leigh Marine Laboratory. When briefly handled by researchers underwater, all 10 sharks produced audible clicks.
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These clicking sounds were very frequent during the first few handlings but then stopped as the experiments progressed, Nieder said. "Maybe they weren't afraid for their lives anymore," she said, adding that in the wild, loud clicks may serve as a split-second distraction for juvenile sharks to make their escape when they are seized by predators.
Most sharks are thought to be silent because they lack swim bladders — air-filled sacs commonly used by fish to make sounds. MicroCT scans and 3D reconstructions of rig sharks also revealed no obvious sound-producing organs or structures.
The team proposed that the sounds could be coming from the sharks snapping their teeth.
Rig sharks have broad, blunt teeth arranged in tightly packed, plate-like formations, which are ideal for crushing hard-shelled prey like crabs. This "pavement dentition," may also serve a second function of producing noise when the jaws snap shut, the researchers said.
However, Nieder noted that without direct observation of the sharks' jaws during click production, the mechanism remains speculative.
Each click lasted around 48 milliseconds, with volumes sometimes exceeding 155 decibels, which is comparable to shotgun blast.
Roughly three-quarters of the clicks were single bursts, while the rest were short double-clicks. The scientists observed that about 70% of these clicks were accompanied by calm, swaying body movements, but a few occurred without any visible motion at all.
Whether the clicking is an accidental byproduct of handling or a purposeful behavior remains unknown.
The sharks' own hearing range is largely below 1 kilohertz, far lower than the frequencies of its clicks, meaning it's unlikely the clicks are meant for communicating with other sharks.
However, some known predators of rig sharks, such as New Zealand fur seals (Arctocephalus forsteri), are sensitive to higher frequencies and might be startled or confused by the sharp clicks.
Several species of rays and skates — close relatives of sharks — are also known to produce clicks when disturbed by divers. These sounds are thought to serve as warning signals or distress calls.
Nieder said future studies could target closely related shark species to investigate whether they are also capable of making noises, and whether they make noise in response to stress.
Adrian Gutteridge, a shark biologist with the IUCN Shark Specialist Group who was not involved in the study, said more research will be needed to establish what the shark clicks are for, and what they could mean.
"It's too early to tell whether it's a response, kind of saying, 'go away,' or if [it's] just their nervous system is firing off which just happens to make their teeth and jaws click," he told Live Science.
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