If you've ever heard a thump in the night, you've probably been glad that you could tell where it was coming from. The ability to tell where a sound comes from plays a crucial role for many vertebrates. For vertebrates that live on land, like humans, direction of sound can be determined by measuring the time delay and sound intensity between each ear. For vertebrates that live underwater, it's not so easy. Because sound travels five times faster in water, the difference in arrival time at the two ears becomes very small. This means that fish need to have a different mechanism for localizing sound. Scientists now think they have an answer. An international team of researchers from Germany, France, and the USA led by Johannes Veith and Thomas Chaigne from Charité - Universitätsmedizin Berlin, Germany, have shown how fish might be localizing sound. The researchers turned to a tiny, transparent fish, Danionella cerebrum, from Myanmar, which communicates via loud clicks, making it an ideal candidate to find out how fish hear. What they were trying to understand is what part of a sound fish are using to determine direction.
To determine how the tiny fish figure out which direction a sound is coming from, the scientists put the animals into a square tank with underwater speakers placed on various edges of the tank. In a series of trials involving playing sound from one, two and three sides, the scientists played a short beep. When a standard beep was played from the left, the fish startled to the right. For each trial, they recorded the direction the fish fled when startled. Underwater, sound can be thought of as having two parts: one is how loud it is, the pressure change as the sound moves through the water – which can be compared between ears – and the second is how much the sound moves the water molecules by oscillating them, the particle motion, which can be sensed using special organs. The researchers created loud sounds by adjusting the water pressure or transmitted only the movement component of sound. By varying which component the sound had, and playing it from different directions, the scientists were able to see whether fish startled away from the speaker. The researchers set up the speakers to do this so that different parts of each sound would interact, sometimes giving contrasting information causing the fish to startle towards the active speaker instead of away from it, ‘tricking’ the fish. They arranged a speaker that played a standard beep on the left side and added speakers on the top and bottom walls producing sounds that carried only loudness information, which conflicted with the speaker on the left side and caused the fish to move towards the speaker on the left. This told the scientists that the fish were not just sensing both parts of the sound but comparing the pressure and particle motion components to one another to determine which direction it was coming from.
But how do D. cerebrum sense the particle motions and pressure differences that are an essential component of their sound direction sensing equipment? Veithe and Chaigne X-rayed the fish to get a closer look at their internal organs and also detected sound vibrations moving through the sound-sensing organs using a laser coupled with a powerful microscope. These observations confirmed that the organs are able to measure the difference in pressure and movement of sound. This is quite the feat for the little fish given the speeds that sound travels in water and allows the fish to figure out which direction to turn when they need to flee.
