A team of German researchers unveiled how fish determine the source of sounds underwater, according to a study published in Nature.
Humans cannot determine where a sound comes from underwater. Our brain determines the origin of a sound by analyzing the difference between when it arrives in one ear and then the other. This is not possible underwater, as sound travels five times faster than on land. In theory, fish should not be capable of directional hearing, as there is no difference in volume and arrival time between their ears. And yet, this has been observed in various aquatic species. Fish can locate sounds coming from prey or predators, but until now, researchers haven’t fully understood how.
Finally, a team of Neuroscientists from Charité – Universitätsmedizin Berlin, Germany, have solved the mystery. Prof. Benjamin Judkewitz used a tiny fish called Danionella cerebrum to unveil how fish can hear underwater. “To find out whether, and above all how, a fish can tell the direction of sound, we built special underwater speakers and played short, loud sounds,” explained Johannes Veith, one of the two first authors of the current study. “Then we analysed how often Danionella avoids the speaker, meaning that it recognises the direction the sound is coming from.”
What land animals perceive through the eardrum is sound pressure. Fish have a different hearing mechanism: They can also sense vibration. The team found this mechanism by taking images with a purpose-built laser scanning microscope and scanning the structures inside the fish ear in a strobe pattern while a sound is played.
Close to a speaker, vibration moves back and forth along an axis, going toward and away from the speaker. Fish can detect the vibration along this axis but cannot establish the direction from which the sound originally came. This is because sound vibrations move in a continuous back-and-forth movement. The fish solve this problem by analysing a combination of vibration and sound pressure. “Sound pressure sets the compressible swim bladder in motion, which in turn is recognised by hair cells in the inner ear. Through this second indirect hearing channel, sound pressure gives fish the reference they need for directional hearing. That’s exactly what one model of spatial hearing from the 1970s predicted – and now we’ve confirmed it experimentally,” said Judkewitz.
The hearing apparatus in Danionella is similar to about two-thirds of living freshwater fish, suggesting that the directional hearing strategy that the team has uncovered could be widespread. The researchers plan to continue to determine which nerve cells are activated when sounds are played underwater.
Veith, J., Chaigne, T., Svanidze, A. et al. The mechanism for directional hearing in fish.Nature 631, 118–124 (2024). https://doi.org/10.1038/s41586-024-07507-9