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Male of the tropical fish species Danionella cerebrum: small but loud

Photo: Ralf Britz / dpa

The expectation of loud animals is often that they are also big.

The males of a certain species of tropical fish do not correspond to this at all.

They are just around one centimeter in size - and produce sound pressure that is similar to that of a jet taking off.

A research team has now examined in more detail how Danionella cerebrum achieves this using, among other things, high-speed cameras.

In the animals, a so-called drum cartilage shoots against the swim bladder with 2,000 times the acceleration due to gravity, reports the group led by Verity Cook from the Berlin Charité University Medicine in the specialist journal “PNAS”.

The approximately twelve millimeter small species was only described in autumn 2021 and lives in rivers at the foothills of the Bago Yoma Mountains in Myanmar.

The species name D. cerebrum refers to the fact that the transparent fish - one of the smallest vertebrates of all - has the tiniest vertebrate brain known.

Ralf Britz from the Senckenberg Natural History Collections, who first described the species in 2021 and was also involved in the current study, says that he himself heard the rattling sounds of such fish in the aquarium.

The sounds therefore serve for communication in the murky rivers.

“You can’t see five centimeters in the water,” says Britz.

Measurements in aquariums showed that the noise level generated by the animals at a distance of one body length - about one centimeter - is 147 decibels.

For comparison: a jet engine produces a sound pressure of 140 decibels at a distance of 100 meters, while elephants produce 125 decibels.

“Such an amplitude is highly unusual for an animal of this size,” notes the team.

However, with such a small sound source, the sound pressure decreases rapidly with distance.

At a distance of just one meter it is a good 100 decibels.

To clarify sound production, the team placed groups of three to four of the transparent fish in an aquarium, including at least one male.

They then filmed the animals using high-speed cameras that deliver up to 8,000 images per second.

The sounds are produced so quickly that the team only saw a contraction of the swim bladder in one image - this occurred within 125 microseconds.

Because this is much faster than any previously known muscle contraction, the researchers looked for a different mechanism.

They discovered that with each sound, one of the animal's ribs moves, stops and returns to its original position.

The team also found the tiny drum cartilage, just 250 micrometers long, which is also involved in the sounds.

Micro-CT images then showed that the males of this species each have a drum muscle on the left and right.

Its contraction pulls the fifth rib forward, which in turn puts tension on the drum cartilage.

If it is suddenly released, it shoots at the swim bladder at lightning speed.

What's curious is that in fish, the parts of the body that produce sound are also located directly on the hearing apparatus.

Britz compares this to a person who regularly hears the noise of a jet plane directly in their ears.

"It's a mystery how the fish manage not to become deaf."