Electroreception: The Shark’s Sixth Sense
SHOW NOTES
Stephen has over 30 years of experience studying sharks and rays, which began with an undergraduate research project looking at the movements of wrasse [4.21]. The research involved scuba diving to observe wrasse underwater, and noting if a predator swam past to see if their behaviour changed. This was Stephen’s first dive with the research team, and he remembers being extremely happy to be fulfilling his dream of conducting fieldwork underwater! After some time, Stephen saw a black-tip reef shark pass behind his buddy’s head. “I just noted it down, and thought ‘oh, this probably happens all the time’!” He laughs. But, as it turns out, this was a rare occurrence. “[My buddies] couldn’t believe that I had seen a shark, on my very first dive, and they had never seen one, after weeks and weeks of working in the field!” This was also a special experience for Stephen, as it was his first encounter with a shark in the wild.
Growing up in a small, rural town in southern Ontario, Canada, marine biology was considered an odd career choice [8.08]. Most of Stephen’s community were farmers, or part of the agricultural industry. But for reasons he can’t explain, Stephen was obsessed with the ocean, and sharks, from a very young age. He remembers being hooked on documentaries with scientists and explorers, such as Jaques Cousteau, and taking out all the books he could find at the local library that had anything to do with the underwater world. “Now here I am, as a big kid, and I’m doing exactly what I wanted to do,” says Stephen. “I couldn’t ask for a happier resolution to my childhood desires, it’s all come together. And how many kids get to do, what they’ve wanted to do for their whole life? I really consider myself fortunate.”
Stephen’s lab is now primarily interested in the integration of sensory biology with functional morphology – essentially, how sharks experience the world, and the physiological mechanisms that allow them to do that. Like us, sharks are able to navigate and explore their environment using touch, smell, sight, hearing and taste. But, they have an additional ‘sixth sense’: electroreception, or the ability to detect electrical signals.
But where are these electrical signals coming from [10.21]? “Anything that is alive maintains a concentration of ions in their body, that is necessary for survival.” Explains Stephen. An ion is an atom, or group of atoms – the basis of all things in the universe – that have positive or negative electrical charges. “The concentration in our body is different to the concentration of ions in the seawater. In seawater, you have sodium chloride, NaCl, and the concentrations of those are not quite the same as what you have in your body.” Our skin acts as a barrier, holding in most of those ions and keeping the concentration in our tissues at the right level. But, some still escape into the surrounding environment, through soft membranes such as the mouth, or gills in the case of fishes. As a result of these leakages, a charge is created around the body of the animal. “The very fact that these animals are alive, means that they are little living batteries, that are leaking. And they’re creating electric charges,” says Stephen. These electrical signals are minute, but sharks, and their relatives, the rays, are able to pick up on them.
How ‘leaky’ you are (or, how many ions are discharged from the body) can depend on what you are doing [15.27]. “You can see, as a fish is opening its mouth or opening its gills, the voltage go up,” Stephen says. The opposite is true when the mouth or gills are closed. “So you can be electrically quiet, if you were to close your mouth, close your gills, and just sort of sit there, and not move…but as soon as you start opening your mouth and exposing your mouth and gills, the voltage goes up again.” This naturally happens with respiration, meaning that, to avoid detection, fishes will literally hold their breath! But what about if an animal is moving, or even thrashing around in distress – does this send the shark’s electroreceptive senses tingling? “The contraction of muscles does generate an electrical charge,” Stephen explains. “But, what I will point out is that those signals are of a very high frequency, compared to the modulation of your mouth and gills…those sorts of stimuli, the twitches of your muscles or a heartbeat, are very high frequencies, that’s usually outside the range of what shark electroreceptors can detect. So sharks are probably not picking up on the muscles, or the heart, they’re picking up on the leakiness of the gills.” So, the idea that sharks can pick up on a heartbeat from some distance away is likely a myth – rather, they are tuned in to the frequency of prey breathing.
So how are sharks able to detect these tiny signals? [20.21] If you take a look at the head of a shark, you’ll see lots of tiny dots, or pores. Every one of these pores is connected to a thin tube, that is filled with a highly conductive jelly-like substance. “You can think of that tube as an insulator, on a wire, and the jelly-like substance is the copper wire inside it,” says Stephen. It acts as a conductor, transferring an electrical field from the outside environment, near the pore, into the shark’s head. At the end of the tube is a small bulb – an ‘ampulla’ – which is lined with special sensory cells that can detect voltage differences between the external electrical field, and the shark’s own body. These cells send signals to the brain, which is then able to process this information. “Sharks can have hundreds, in some cases thousands, of these electroreceptive pores, distributed across the head and pointing in all different directions. It’s like having an array of antennas all around.” It gives the shark the ability to distinguish where the stimulus is coming from, and act accordingly.
But just how sensitive is this sixth sense? [24.00] The problem is, with any sort of electric field – especially in a conductive environment, like seawater – the field grounds out quite rapidly, meaning that electrical signals decay very quickly. Because of this, sharks can likely only detect electrical signals from about a metre away. “It’s a very close-range sense, it’s not something that they can use to find prey at a distance. They have to already be right there,” says Stephen. Beyond short distances, sharks are likely employing their other senses – then, once they are very close, tuning into those electrical signals to hone in on their target.
The general anatomy, or basic structure, of the electrosensory system is the same across all sharks and rays. But, there are over 1, 200 species of sharks and rays, which inevitably means there are variations on that theme [30.32]. The main difference between species is in how the electrosensory pores are distributed around the body. This was a particular research interest of Stephen’s, and he has published articles on this very subject. “How do these differences cause the animal to sense the world differently? Does a hammerhead sense electrically a different sort of habitat than a pointy nose shark?” On a hammerhead shark, for example, the electroreceptors are shaped across a much larger surface area, thanks to their unique, t-shaped noggin. This might give them an advantage in not only detecting prey, but also in locating it. “You don’t get weirdness without a good reason,” Stephen adds.
But sharks don’t just use this sense to detect prey. They can also use it to navigate [36.15]. “If you look at our earth, it has a north pole and a south pole.” Stephen explains. “So you’ve got this pervasive magnetic field around the whole planet. Magnetic fields and electrical fields can interact, through a process called induction. And so what happens – we think, this is all theoretical! – is that as you’re moving through a magnetic field, you induce an electrical field with your body.” Based on this theory, a shark swimming from east to west across the earth’s magnetic field would induce an electric field around its body as a result, which sharks should be able to detect via their electrosensory system. As they change direction, the electrical field will change. For example, a shark swimming from north to south will detect very little change, because it is running in parallel to the earth’s own magnetic field. But say they change direction, they will produce a different electric field around their head and know which way they’re moving based on the magnitude of that field. Imagine it almost like an inbuilt sat nav, alerting the shark whenever it’s heading down the wrong route. Scientists believe this is how sharks are able to navigate huge distances across the oceans, often without any visual cues or landmarks. “That’s what I think is so cool about this sense – it’s double purpose!” smiles Stephen.
We might also be able to use electroreception as a way to manage human-shark interactions [44.00]. Although shark bite incidents are rare, they can happen. Stephen’s lab is currently looking into the efficacy of magnetic shark deterrents as a way to repel sharks from fishing gear: “One of the neat things is because the sharks have this electrosense that the bony fishes don’t have…it gives us a channel to communicate to the sharks with the bony fishes being blind to it, not knowing. And so the idea has always been, can we come up with a type of shark repellent that repels the sharks away from the bait, but still allows the fishermen to catch the target species, the swordfish or the tunas or whatever.” An effective deterrent would significantly reduce the amount of sharks that are caught as incidental bycatch, while still allowing fishermen to make a living. “It could potentially save millions of sharks a year,” Stephen adds.
The deterrents work by using a strong magnet to induce an electric field. Like with the earth’s magnetic field, as the shark swims the magnet will induce an electrical field around the shark’s head, which it will pick up with its electroreceptors. “Because that magnet is so much stronger than anything these animals have ever encountered in the wild, as they’re swimming toward this, it’s going to produce this unnatural stimulus, this unnaturally strong electric field, and it’s going to startle them more than anything. They’re going to jump and say: ‘What is this? I don’t know what that is’, and they’re going to swim away.”
It’s exciting research, which Stephen is eager to share with the world. “I just had a student who finished her thesis in December and we are currently working on the manuscript…I’m excited to get that out the door!”
You can keep up to date with this research here, and by following @sharkmigration on Instagram!
ABOUT OUR GUEST
PROFESSOR STEPHEN KAJIURA
Dr. Stephen Kajiura is a Professor in the Department of Biological Sciences at Florida Atlantic University. He received his PhD in Zoology from the University of Hawaii, a MS in Marine Biology from the Florida Institute of Technology, and a BSc (Hons) in Marine Biology from the University of Guelph (Canada). Dr. Kajiura has conducted research for various agencies including the National Science Foundation, the Department of Defense, and the National Marine Fisheries Service. He has published over 70 papers in peer-reviewed journals, authored 5 book chapters, and has presented numerous talks at scientific conferences. He has supervised more than 30 graduate students and post-doctoral researchers, and has served on numerous thesis committees for students from around the world. Dr. Kajiura maintains a strong public outreach service, primarily through television documentary appearances, and has served as an elected member of the American Elasmobranch Society Board of Directors. He has over 30 years of experience studying the biology of sharks and rays.
