Non-human hearing

Professor John Montgomery of the University of Auckland discusses how fish hear. He details the organs that fish use to detect sound, how well they can hear sound under water and how this might be useful as a survival mechanism.

Transcript

PROF JOHN MONTGOMERY
In the water, we are not conscious of the sound that surrounds us. There are two reasons. One is that our ear is set up for listening for pressure waves in air, which is sound in air, and sound is kind of different under water and we are not equipped to detect it. The other reason is that is a lot of the sound, particularly the sorts of sounds that we have been interested in, the background sound, is what we would call white noise. It just doesn’t have a lot of structure to it, and we are very good at tuning that out.

So once you have actually cottoned on to the idea that there is sound there, despite the fact our ears aren’t very good at it, you can detect the sound of the reef, you can hear the snapping shrimps, you can hear the sea urchins. The range of sound frequencies that marine animals hear goes from infrasound – would be something less than 20 cycles a second, less than 20 hertz – and then when you get into the ultrasound, which is above the range of our hearing. There are few fish that detect ultrasound, and we think that’s as an adaptation to being preyed on by marine mammals that use active echolocation where they are using very high frequencies, so fish have become sensitive to that because it’s a good way of knowing where the predators are coming from.

Hearing under water occurs through two main mechanisms. One is where there are dense ear bones in the inner ear. So you’ll obviously have seen that fish don’t have external ears. If you look at a fish, it doesn’t have ears, but these dense bones called otoliths, which are very heavy. So the fish moves in the sound field, and then there is a lag between the movement of the fish and these ear bones, and it’s that relative movement which provides the basic sense of what we call particle motion.

The second main mechanism depends on having some gas bubble, like a swim bladder in a fish, and the pressure wave will vibrate, you need some way of linking that vibration to a sense, and there is half a dozen or so different systems that have evolved. One that people would be interested in is the goldfish has a set of specialised ear bones that take the vibrating swim bladder and transmit that vibration into the inner ear. There are also some fish in tropical reefs that have connected the swim bladder to their lateral lines, so movement of the swim bladder are detected by the lateral line

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