The acoustic world of sharks: how do they hear?
Show notes
Before diving into the acoustic world of sharks, we start by getting to know Lucille a little better [06.43] – and it might surprise you to know that her most memorable ocean experience has nothing to do with sharks, or ears. Lucille was searching for sharks while snorkelling the Exmouth Gulf, at the infamous Ningaloo reef in Western Australia, when another ocean giant showed up to play: a humpback whale! The gigantic animal passed right underneath her, coming so close that they were almost eye-to-eye. For around five minutes Lucille and the whale swam with one another, until she realised that they were not quite alone. Another dark shape, even more massive than the last, appeared out of the distance and it quickly transpired that the whale Lucille had been swimming with was just a calf, and now its mother was also coming to say hello!
Even before this magical encounter, Lucille had had a lifelong fascination with the ocean. This was in part due to her upbringing in landlocked Switzerland, where the ocean was a distant paradise only visited on ‘exotic’ holidays [12:18]. Because of this association, Lucille now feels that her job as a marine scientist “feels like working in a world of holidays” – a very nice position to be in! It was on these childhood vacations that sharks first came into Lucille’s life. As apex predators, they are to the ocean what lions are to the Savannah; iconic, charismatic species that capture the imagination. As Lucille says, sharks have existed for over 400 million years, and across that time have evolved to become highly efficient and incredibly well adapted to their environment. This is particularly evident in their sensory systems. Lucille describes her research into these senses as akin to “working on a very expensive, fancy car” – systems that have been perfected over time, to become highly advanced, efficient and unique [13:40]. This is what drew her interest, and eventually Lucille travelled to the University of Perth to start her PhD on shark behaviour and sensory cues.
Of all shark senses, hearing is probably one of the most understudied and least understood. In fact, a common misconception is that sharks don’t even have ears [23:21]. They do, but they look very different from our own. Sharks lack the middle and external ear – in other words, the two ‘flappy things’ that we have on either side of our head. But they do have an inner ear. If you look very carefully at the top of a shark or ray’s head, just behind the eyes, you will see a tiny pore or hole. These are called the ‘endolymphatic pores’ and they lead to the inner ear. The inner ear is made up of many of the same components that are found in all vertebrates: three, semi-secular canals (which are known to help with balance), and the ‘end organs’. In the end organs are beds of hair cells, which detect sound and send signals to the brain, telling the brain that the shark is hearing something. These beds of hair cells are known as the maculae, and sharks typically have four: the saccule, the utricle, the lagena and the macula neglecta.
Sharks also differ from us in the way that they experience sound [26.44]. To understand this we need to go into some physics. Sound travels as a wave, which causes particles in the surrounding medium (air or water, for example) to vibrate. In bony fishes, this movement is felt in the swim bladder – a gas-filled sac that they use to control their buoyancy – and it causes the swim bladder to inflate or deflate. But sharks, who lack a swim bladder, feel these vibrations with their whole body. As their body is mostly made up of water, their body will move with the vibrations. It’s very subtle (so the shark isn’t juddering backwards and forwards like a tuning fork!) but it does trigger a behavioural response. Also in the inner ear are tiny crystals, called the otoconia, which lie on top of the maculae and are denser than the rest of the shark’s body. As the shark’s body moves with the soundwave, these crystals lag behind ever so slightly, causing the hair cells to bend and send signals to the brain.
We also experience sound differently from sharks because we use our outer ear, those flappy parts of cartilage on our heads, to capture sound [30:28]. We also have an extra part of our ears – the middle ear – which amplifies the sound and then transfers it to our inner ear. This is largely because we are typically hearing sound that travels through air. Sound travels much faster underwater, and so fishes have no need for such complicated ears. Instead, they take sound directly through their bodies and are incredibly sensitive to the surrounding vibrations. It’s almost like when you’re standing at a concert, right next to the speakers – you feel the bass vibrate through your whole body. That’s a good comparison for how a shark experiences sound!
But what are sharks using this sense for [34:58]? It might sound like a simple question, but it’s very difficult to know for sure. Scientists hypothesis that sharks and rays use sound to hunt, and to navigate their environment. All different habitats have a distinct ‘soundscape’. Coral reefs and mangroves, for example, both of which are important to sharks, are noisy places and could produce sound cues that sharks use to orientate themselves, find prey, and even locate important sites like nursery grounds. More recently, scientists have made a new and potentially very exciting discovery that sharks and rays may also be capable of producing sound [36:20]. Stingrays have been recorded making curious ‘knocks’ and ‘clicks’ when divers got too close, with the sounds getting louder as the divers drew nearer. This suggests that sharks and rays could be using sound as a way to communicate. In the case of the divers, the stingray is probably using sound to indicate it’s annoyance – if it were speaking, it would be saying “back off”!
Lucille’s research looks at how sharks use sound to navigate their environment, and is particularly interested in how they interact with the sounds we, humans, produce. Her PhD looked at the behavioural responses of white sharks to different anthropogenic sounds, to examine the effectiveness of acoustic sound deterrents to prevent shark bite incidents [45:31]. Her research was quite unique! Lucille set up a floating sound station, which she used to play different sounds to the sharks. The first was a recording of an orca or killer whale – the ocean’s top predator – who are one of the only species known to hunt white sharks. The other was a curious sound Lucille herself had produced, a mixture of beeps and whistles that sound a little like R2D2 from Star Wars, except at a much lower frequency (sharks can only hear low frequency sounds). This sound was deliberately chaotic and unpredictable, as many sounds found in nature are actually quite rhythmic.
The results were interesting [52.04]. Lucille found that other species of shark, like reef sharks, avoided both sounds. But the responses of white sharks were less conclusive. Interestingly, Lucille noticed behavioural differences between individuals. Where some didn’t return to the area, others were very curious. One reason as to why this may be is that orca calls are very specialised, and different depending on the type of prey they hunt and their location, almost like an accent. It could be possible that the white sharks in the sites Lucille had tested had just never heard an orca call like that before, and didn’t know to associate it with predation. Lucille would like to re-do the experiment in False Bay, where white sharks have been heavily predated by orca in recent years. But, either way, she doesn’t recommend playing music to white sharks while you’re out surfing!
Another very important aspect to Lucille’s work is testing the responses of sharks to noise pollution [59.04]. This is the focus of her project funded by Save Our Seas Foundation, The Ghosts of Oceans Future , which looks at the potential effects of noise pollution on the Australian ghost shark. A deep water species, the Australian ghost shark relies heavily on its sense of hearing. They have one well-known nursery site in Western Port Bay, a shallow, tidal bay near Melbourne. To get there, the ghost shark has to migrate across a deep channel, which has become increasingly busy with boat traffic, particularly large cargo vessels that generate huge amounts of underwater noise. Although it’s early days in Lucille’s research, she has detected that the soundscape of this area is dominated by loud, anthropogenic noise. It also appears that the ghost sharks are altering their swimming activity and showing signs of stress when exposed to this noise in the lab. Lucille has only tested a few individuals so far, and trials have all occurred in captivity, which means we have to be careful about what conclusions we draw. But, it is concerning. Especially has we learn more about the hearing capabilities of sharks, and as our oceans get steadily busier.
About our guest
DR LUCILLE CHAPUIS
Originally from Switzerland, Lucille has always been fascinated by the ocean and inspired by its wonders. She has a soft spot for tiny creatures like cleaner shrimps, and for bigger things like sharks. While her MSc thesis focused on mutualism in the former, her PhD investigated the acoustic ecology of sharks. Since then, she has been interested in underwater sound and concerned about the effect of the ever increasing human-made noise in aquatic environments.
Lucille is now a Postdoctoral Fellow funded by a Marie Sklodowska-Curie Individual fellowship and a Save Our Seas Foundation project leader. As a sensory ecologist, she aims to understand how aquatic organisms sense and adapt to their ever-changing environment. She especially focuses on the hearing systems of marine fauna and explore this sensory modality using a multidisciplinary toolset, including morphological, bioimaging, electrophysiological and behavioural techniques. For her project, Lucille is studying the potential effects of anthropogenic noise pollution on the Australian ghost shark.
Instagram: @lucillechapuis, twitter: @sharkslikejazz
