Gills Aloud? Tiny fish found making very big noise

They were prompted to investigate after hearing mysterious clicking noises coming from the fish tanks in their lab.

They found that the fish, Danionella cerebrum, drums out a powerful rhythm on an organ called a swim bladder.

In the waters close to the fish, it taps out 140 decibels, which is as loud as a gunshot.

Researchers believe the 12mm long species is the loudest fish for its size yet found.

They believe the drumming may be a form of social communication.

Under the water it's a different story - where the minuscule species is now one of the noisiest yet discovered.

Scientists have known that other creatures, such as the aptly named pistol shrimp can create very loud noises as they hunt other species, up to around 200 decibels.

Danionella is valued by science as its transparency means you can see its brains in action and this allows researchers to closely study its behaviour,

But while working with these fish in their lab in Germany, the scientists noticed something odd.

"People were just walking past the fish tanks, and they could hear these sounds, and were wondering where they were coming from," said Verity Cook, the lead author of the study, and a PhD Student at Charité University, Berlin.

"It turned out that they're coming from the fish themselves. And it's extraordinary, because they're so tiny and so loud."

Using an array of microphones and video cameras, the research team were soon able to work out just how loud.

"At a distance of one body length away, around 140 decibels is the amplitude of the sound, that's how loud we believe the sound to be perceived by other fish," she told BBC News.

"Sound attenuates with distance, so at one metre away, the amplitude is around 108 decibels."

That's still roughly equivalent to the noise made by a bulldozer.

Much of this sound is reflected back into the water so when humans stand beside the fish tanks, they hear these pulses as a continuous buzzing sound.

While fish including the Plainfin Midshipman and the Black Drum and others are louder, they are all much larger than danionella.

"In terms of communication signals, I couldn't find another animal of this size that makes sounds this loud," Ms Cook added.

The researchers argue that the drumming mechanism that the fish use is a very sophisticated instrument.

All bony fish have a swim bladder, a gas-filled organ which helps them remain under the water.

Many species use their muscles to drum on this bladder to produce sounds but danionella goes several steps further.

When it contracts its muscles, these pull on a rib, which creates tension with a piece of cartilage that sits inside the muscle.

When the cartilage is released it hits the swim bladder.

Only the males of the species make this sound, and they only do it in company. Some are louder than others.

"We know that when you have maybe like eight males together in a large tank, then three of them will dominate the sound production and the other ones will be quiet. So we think there is some sort of hierarchy," Ms Cook said.

The researchers believe that evolving in murky waters in Myanmar played a role in the development of this ability to make a big noise to help them communicate.

"Evolution has come up with lots of interesting ways to solve lots of interesting problems," said Ms Cook.

"And we shouldn't assume that we know how things work, just because of how things work in other species."

The study has been published in the journal, Proceedings of the National Academy of Sciences.

-bbc