The tiny thing measures just one centimeter, but the sound pressure it produces is similar to that of a jet plane taking off. A German research team has now investigated how the males of the tropical fish species Danionella cerebrum, which was discovered just a few years ago, achieve this using, among other things, high-speed cameras. In animals, a so-called drum cartilage shoots against the swim bladder with 2000 times the acceleration of gravity, reports the group led by Verity Cook from the Charité Universitätsmedizin Berlin in the scientific journal PNAS.
The approximately 12 millimeter small species was only described in autumn 2021 and lives in rivers in the foothills of the Bago Yoma Mountains in Myanmar. The species name D. cerebrum refers to the fact that the transparent fish, as one of the smallest vertebrates, has the tiniest known vertebrate brain.
He himself heard the rattling sounds of such fish in the aquarium, says Ralf Britz from the Senckenberg Natural History Collections, who first described the species in 2021 and was also involved in the current study. 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 – i.e. about one centimeter – was 147 decibels. A jet engine produces a sound pressure of 140 decibels at a distance of 100 meters, and elephants produce a sound pressure of 125 decibels. “Such an amplitude is highly unusual for an animal of this size,” the team now notes in their paper. However, with such a small sound source, the sound pressure decreases rapidly with distance. At a distance of just one meter it is only 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 with 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.
Since this is much faster than any previously known muscle contraction, the team 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 is 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 go deaf.”