Paper: Mickle, Megan F., and Dennis M. Higgs. “Towards a new understanding of elasmobranch hearing.” Marine Biology 169.1 (2022): 1-13. doi: https://doi.org/10.1007/s00227-021-03996-8
Not so hard of hearing
When you think about sharks, maybe you imagine their swimming abilities, rows of sharp teeth, or dorsal fins peeking out of the water. But have you ever thought about how these top predators are able to hear? While sharks may not have ears exactly like us humans, they do have their own way of detecting sounds in their ocean environments.
The shark ear is only internal, though many species have noticeable holes on their skin to indicate where these inner ears are located. The entire shark inner ear structure consists of three semicircular canals, which are big looping bones that connect to a center structure that holds four important organs. Three of these organs: the uticulus, sacculus, and lagena, contain sensory hair cells (which look like very tiny, short hairs sticking up) with a dense structure called the otoconia on the top of them. The fourth organ, the macula neglecta, connects directly to the external environment and contains sensory hair cells covered in a gelatinous structure. Humans also have semicircular canals in our inner ears, but instead of so many inner ear organs, we have three inner ear bones: the malleus, incus, and stapes.
When sound occurs near a shark, it will cause vibrations that move both the shark’s body and the water around it. The otoconia and gelatinous structures are slightly denser than the rest of the shark’s body, and so will move slightly after the body does. This delayed movement causes the hair cells in the hearing organs to bend, and the direction and amount of this bending will tell a shark where a sound is coming from and how close it is. While all sharks have their hearing system set up in this way, the size of the different inner ear pieces can vary, leaving different species with different hearing capabilities.
What’s within earshot?
Most sharks can hear sounds from over 1 km (about 0.6 miles) away – a distance of about 10 and a half football fields! The sharks that can hear the widest ranges of sound are those that swim in the middle of the water column – think bull sharks or lemon sharks. Sharks that tend to rest on the ocean floor, such as bamboo sharks, cannot hear a wide range of sound frequencies, particularly really high frequencies. Scientists think this is likely because high frequency sounds mimic splashing sounds, which only occur at the surface of the water, and are not very relevant for sharks on the ocean floor.
Many shark species seem to be attracted to sounds that are steady, fast, pulsing beats. These sounds likely mimic those of stressed prey, and could therefore mean an easy meal for the sharks. On the other hand, sharks also tend to avoid certain sounds, namely very loud sounds or sounds that mimic dangerous predators such as killer whales.
So why should you care about how and what sharks are able to hear? Well, it is no secret that shark populations across the globe are on a decline, in part because of overfishing and bycatch incidents (where sharks are accidentally caught by fishermen). If we know what sounds sharks will avoid, it is possible that fishing vessels could play those types of sounds via underwater speakers to deter sharks from coming close to their nets. Knowing the specific types of sounds sharks can hear, and how far away they can hear those sounds, is also helpful in determining how underwater noise pollution affects these top predators. If noise from shipping traffic is too disruptive, it may interfere with sharks’ ability to hear prey, which could lead to them having less successful prey capture events, starving, and potentially dying. Even though these powerful animals may seem intimidating and like they do not need our help to survive, that is simply not true. With a few changes to our fishing industry, we can have a big impact on the populations of these extremely important animals.
I am a PhD candidate at Wake Forest University, and I received a B.S. in Biology from Cornell University. My research focuses on the terrestrial locomotion of fishes. I am particularly interested in how different fishes move differently on land, and how one fish may move differently in different environments. While I tend to study small amphibious fishes, I’ve had a lifelong fascination with all ocean animals, and sharks in particular. When not doing science, I enjoy running, attempting to bake and cook, and reading.