Stingrays are no longer the silent residents of the sea scientists once thought them to be. Caught on camera, two different stingray species have been discovered making weird clicking sounds in a lucky discovery that has delighted marine ecologists – but also left them stumped.

"That we only just realized that these commonly encountered stingrays are making sounds demonstrates, once again, how little we know about the oceans," says marine ecologist Lachlan Fetterplace of the Swedish University of Agricultural Sciences, who led the study.

Nearly 990 species of bony fish are known to make sounds. But elasmobranchs – a group of cartilaginous fish that includes rays, sharks, and sawfish – were always thought to be the stealthy, silent type.

Not so, it seems. Says Fetterplace: "We now have multiple recordings and observations of two species of stingrays making sounds in the wild", thanks to a few avid naturalists.

Two mangrove whiprays (Urogymnus granulatus) and one cowtail stingray (Pastinachus ater) were observed by divers who captured them making weird sounds, which likely serve as a warning or defense signal, Fetterplace says.

Unlike haunting whale songs that travel far across the oceans, or bewitching bird songs that filter through forests, the wild stingrays in these videos make short, sharp, hollow-sounding clicks that sound more like a percussion instrument than a marine animal.

As you can see below, the two species produce clicks of slightly different pitch as they glide through the water, an environment that often sounds muted to human ears but is actually full of noisy creatures making a hubbub.

Based on what little is known about the range of sounds elasmobranchs can hear, Fetterplace and colleagues surmised that the stingrays and their predators can indeed hear these clicking sounds, which fall into the expected hearing range (40–1,500 Hz).

That led them to propose that stingrays might produce the sounds to ward off or startle approaching predators, so the rays can make a speedy getaway.

In all recorded observations – captured using hand-held digital cameras, in 2017 and 2018 near Indonesia's Gili Islands and on the Great Barrier Reef – the rays started making the 'clicking' sounds as the observer approached, and then stopped when the diver or ray moved away.

But it could also be that stingrays' sounds are a call to arms, to recruit other stingrays when one feels threatened, as one snorkeler observed.

The question is: How does an animal so flat, whose body parts are streamlined for an aquatic world, make a sound other than swoosh?

Researchers think the rays might make these sounds in ways similar to how humans snap our fingers or click our tongues.

"We can't be certain of how the rays are producing the sounds," says co-author and marine scientist Joni Pini-Fitzsimmons of Macquarie University, "but it appears to involve rapid movement of the head or jaw and spiracles, an opening behind the eyes used for respiration."

A mangrove whipray resting on a sandy seafloor between mangrove roots. A mangrove whipray at Magnetic Island, Australia. (J. Javier Delgado Esteban)

Stingrays noises have been observed previously, but only rarely. Decades ago a few captive rays were observed making 'crunching' and 'rumbling' sounds when feeding (let's be honest, who doesn't); there were also some anecdotal reports of cowtail stingrays making loud clicking sounds when fleeing from divers in murky waters.

This, however, seems to be the first documented evidence of the animals actively producing sounds without being provoked.

More research is needed though to appreciate the hearing range of elasmobranchs, "especially considering the limited number of examinations in this group to date," the researchers write.

"Elasmobranchs are most sensitive to low frequency sounds between 40–1,500 Hz, with peak sensitivities between 200–400 Hz, but audiograms have only been produced for 10 species."

Considering how animals perceive the worlds they inhabit in ways we cannot really ever truly fathom, we shouldn't be surprised yet continually marvel at new discoveries that upend what we thought we knew about life on Earth.

The videos capture only a few instances of two stingray species making noise, which might be because they only do so very occasionally, when threatened.

Some species of stingrays are solitary creatures.

"They can also be quite difficult to study because they are often highly mobile and elusive," Audrey Looby, a marine community ecologist at the University of Florida who was not involved in the work, told National Geographic.

Despite the challenges, with over 1,200 known elasmobranch species, researchers think many more observations could soon surface, now that people know what to listen out for.

The research was published in Ecology.