Sharks-Sensing the Body Electric

Posted by Haylie Romero on July 28, 2022 in Animals, Neuroscience | Leave a comment

One of the greatest fears people have about going into the ocean is the fear of being attacked by a shark. Although this fear is not truly warranted, as cows kill more people annually than sharks do [4]. But what is it about sharks that makes us so afraid of them? One major factor is their widespread portrayal as man-eating monsters in cinematic productions like Jaws and Deep Blue Sea. However, despite the fact that sharks pose very little threat to humans, there is no denying they have evolved tremendous hunting and survival capabilities. Sharks have been around for roughly 450 million years, having first evolved during the Paleozoic era. They’ve survived multiple mass extinction events, including the 6-mile wide asteroid that ended the age of dinosaurs [12]. Their large mouths full of sharp teeth, sleek bodies, and highly muscular physique all contribute to their great hunting success. In addition, sharks have also evolved two unique and highly advantageous abilities that allow them to better find and catch their prey: the ability to detect pressure changes in their environment and, the main one I will focus on in this article, the ability to perceive electrical activity, which is known as electrodetection.

You may be wondering why a shark would want to be able to detect electrical currents. To answer that, let’s talk about what electric currents are and where they exist. You may not think about this often, but every living organism on the planet utilizes electricity and, therefore, emits it as well. Even Earth itself has an electric field due to its magnetic poles. A more relevant and familiar example is the electric fields generated by animals such as fish, sea lions and even you. The cells that make up your nervous system utilizes electrical signals called action potentials to very quickly send signals along their length to communicate with other cells. That is why when you go to move your body, it happens very quickly, seemingly immediately, after you think about it. Your muscles also use electrical activity to rapidly contract and move. Therefore, your muscles and nervous system are constantly emitting small levels of electrical activity, which you may have been able to visualize if you’ve ever had an electrocardiogram (EKG) or electroencephalogram (EEG), where electrodes are placed either on the chest to monitor the electrical activity of the heart or on the head to monitor the electrical activity of the brain, respectively. Fish, rays, marine mammals, and other shark prey also emit constant electrical signals. Fortunately for sharks, these creatures live in very salty water, which is great at conducting electricity, meaning electrical signals easily spread through salt water. This allows sharks to be able to very effectively perceive and find their prey by sensing the electricity they emit.

Now we know why it would be very advantageous for sharks to be able to detect electric currents, but how do they do it? Sharks, as well as other members of their subclassification of animals, known as Elasmobranchii, which also includes rays and skates, are able to detect electrical activity due to highly specialized receptors called Ampulae of Lorenzini. These Ampulae of Lorenzini are made of tubes filled with highly conductive jelly which are open at the surface of the skin. These tubes allow the animal to detect electrical activity across the ampulla membranes, which results in a signal being sent back to their brain [9]. The Ampulae of Lorenzini are specialized extensions of another sensory system sharks and many fish species have, called the lateral line. This sensory system extends down the length of the body in a thin fluid-filled tube on either side and allows the shark to detect small pressure changes and vibrations in its environment [8]. For example, if an injured sea lion is thrashing around, it will create waves in the water, which then in turn will be detected as pressure changes by the shark’s lateral line.

A research team led by Dr. David Julius at the University of California San Francisco was interested in how sharks are able to navigate and respond to small changes in a sea of electrical noise. A shark’s ability to perceive the electrical signals from another animal can be compared to you being able to hear the tiniest, practically inaudible sound in the midst of a noisy cocktail party. Unlike with hearing, where the overall perception of sound is dampened during loud situations, sharks seem to be able to perceive really strong electric currents as well as nanoscopic ones simultaneously. To provide some context for just how sensitive sharks are to electric currents, great whites have been known to respond to one millionth of a Volt in water [9]. For comparison, a triple-A battery is 1.5 Volts. So it is quite surprising, given that all life in the ocean, many man-made objects, and Earth itself are producing electric currents, that sharks are able to perceive currents from individual organisms and respond.

Prior to their study, the responses caused by stimulation of Ampulae of Lorenzini had only been extensively studied in skates, where, as you’d probably expect, the size of electric current perceived was proportional to the invoked response in the skates’ nervous systems. This means a bigger electric current would cause a bigger response in cellular activity as well as the behavior of the animal. Surprisingly, this was not the case when they looked at sharks. In sharks, it didn’t matter how big the electric stimulus was within their experimental range, it induced a large response in their Ampulae of Lorenzini cells as well as their behavior every time. This result suggests the shark’s electrodetection system is unique compared to skates and that it’s used predominantly to find prey due to the fact that it is tuned to attack, unlike rays and skates which also utilize electrodetection for other activities like avoiding predators and finding mates.  To understand the underlying mechanisms behind these differences, the team looked at the specific channels in the shark and skate Ampulae of Lorenzini which are responsible for the electricity-induced responses. They found that while both sharks and skates use similar channels to initiate cellular activity, sharks have specialized downstream channels that modulate this activity differently. The electrosensory cells in sharks support large activity spikes which cause the cells to release maximal amounts of neurotransmitter filled vesicles, as opposed to the tunable activity responses that elicit stimulus-dependent vesicle release in skates. In other words a large electrical stimulus will drive a large response in both skates and sharks, whereas a small electrical stimulus will drive a small response in skates, but a large response in sharks. They then showed that a wide range of electrical stimuli will cause downstream physiological changes in sharks, such as an increase in breathing rate, suggestive of their preparation to attack prey. Dr. Julius explains that these results suggest a shark’s electrosensing organ is tuned to react to any changes in electric current in a sudden, all-or-nothing manner, as if to say, “attack now” [2].

Now you may be thinking this is all the more reason to fear sharks, but rest assured you have very little to worry about. In 2021, there were only 73 shark attacks on humans globally and of those, only 9 were fatal [14]. Conversely, humans kill about 100 million sharks per year. Although sharks’ electrodetection systems are tuned to finding and attacking prey, they also rely on many other senses to navigate their world; they have a keen sense of smell as well as the ability to detect pressure changes in water. With increasing access to drones and better cameras, we are realizing more and more how frequently humans are in very close proximity to sharks in the ocean, very rarely with any sort of interaction, let alone attack. Because they are so elusive and difficult to study, we still know very little about them, but it is becoming increasingly clearer that they are more complex and intelligent than we ever imagined.

Sharks and rays are incredibly important in the complex ecosystem on Earth, as they play a vital role in the intricate food web in the ocean. Here is one example of an important role sharks play in helping to maintain the health of the ocean as well as your health. As sharks are a natural predator of sea turtles, the very presence of sharks keeps turtles moving, out of fear. Otherwise the turtles will remain stationary where they will overgraze on the seagrass in their local environment. It has been shown that a reduction in shark populations, therefore, results in huge reductions in the health and overall size of our ocean’s seagrass meadows which are very important for taking CO₂ out of our atmosphere and releasing oxygen back into it [3]. Sadly, the number of sharks in our world’s oceans is plummeting at a staggering rate. Their numbers have declined by at least 71 percent since 1970, mostly due to overfishing [13]. Fortunately, there are people that are working on methods of protecting sharks, in part by utilizing their ability to detect electricity. For example, there are proposals to add devices which emit electric currents to fishing lines, surf boards, boats and other objects which potentially pose a threat to sharks that will deter sharks from approaching [6]. While this may take time, it is a promising goal and just one thing we as humans can do to try to protect these beautiful, majestic and very important creatures.

References

1. Ampullae of Lorenzini. (2005, October 15). Wikipedia, the free encyclopedia. Retrieved July 26, 2022, from https://en.wikipedia.org/wiki/Ampullae_of_Lorenzini
2. Bellono, N. W., Leitch, D. B., & Julius, D. (2018). Molecular tuning of electroreception in sharks and skates. Nature, 558(7708), 122-126. https://doi.org/10.1038/s41586-018-0160-9
3. Blogger, M. (2017, November 14). Why we need sharks for healthy oceans—and a healthy planet. Mystic Aquarium. https://www.mysticaquarium.org/2017/07/26/need-sharks-healthy-oceans-healthy-planet/#:~:text=Sharks%20keep%20ocean%20ecosystems%20in%20balance&text=Sharks%20limit%20the%20abundance%20of,structure%20in%20healthy%20ocean%20ecosystems
4. “Cows Kill More People Than Sharks | Nature and Wildlife | Discovery.” Discovery, http://www.discovery.com, 1 Nov. 2019, https://www.discovery.com/nature/cows-kill-more-people-than-sharks.
5. “EEG AND BRAINWAVES – BRIGHT BRAIN CENTRE – LONDON’S EEG, NEUROFEEDBACK AND BRAIN STIMULATION CENTRE.” BRIGHT BRAIN CENTRE – LONDON’S EEG, NEUROFEEDBACK AND BRAIN STIMULATION CENTRE, http://www.brightbraincentre.co.uk, 23 May 2021, https://www.brightbraincentre.co.uk/electroencephalogram-eeg-brainwaves/.
6. Electric fields could repel sharks. (2008, May 6). NBC News. https://www.nbcnews.com/id/wbna24484864
7. Half a century of global decline in oceanic sharks and rays. (2021, January 27). Nature. https://www.nature.com/articles/s41586-020-03173-9
8. Info, S. (2022, February 23). Sharks lateral line – How does it work? Sharks Info. https://sharksinfo.com/sharks-lateral-line-functions/
9. King, Benedict, and John Long. “The Shocking Facts Revealed: How Sharks and Other Animals Evolved Electroreception to Find Their Prey.” The Conversation, theconversation.com, 13 Feb. 2018, https://theconversation.com/the-shocking-facts-revealed-how-sharks-and-other-animals-evolved-electroreception-to-find-their-prey-91066.
10. Shark evolution: A 450 million year timeline. (n.d.). Natural History Museum. https://www.nhm.ac.uk/discover/shark-evolution-a-450-million-year-timeline.html
11. (n.d.). Steilacoom Historical School District No. 1 /. https://www.steilacoom.k12.wa.us/cms/lib/WA01001786/Centricity/Domain/666/20180430%20Week%20Marine%20Lectures.pdf
12. Wall, M. (2021, December 13). Asteroid that killed the dinosaurs: Likely origin and what we know about the famous space rock. Space.com. https://www.space.com/dinosaur-impactor-origin
13. Worm, B., Davis, B., Kettemer, L., Ward-Paige, C. A., Chapman, D., Heithaus, M. R., Kessel, S. T., & Gruber, S. H. (2013). Global catches, exploitation rates, and rebuilding options for sharks. Marine Policy, 40, 194-204. https://doi.org/10.1016/j.marpol.2012.12.034
14. Yearly worldwide shark attack summary. (2022, March 14). Florida Museum. https://www.floridamuseum.ufl.edu/shark-attacks/yearly-worldwide-summary/
15. (n.d.). YouTube. https://www.youtube.com/watch?v=0QVE1dFXFRo
16. (n.d.). YouTube https://www.youtube.com/watch?v=jboPBNkjNdQ