American Scientist

A little more than 50 years ago, Adrianus “Ad” Kalmijn conclusively proved that sharks and rays can sense electromagnetic fields and use them to locate hidden prey. The finding transformed our understanding of the ways in which animals respond to their environments. I spoke about this discovery with Steve Kajiura of Florida Atlantic University, who is one of only a few experts studying sensory physiology in sharks and rays. “We hadn’t discovered a whole new sensory system in centuries,” he told me. “Imagine discovering that animals can see or hear, just a few decades ago.”

Kalmijn, who studied the intersection of biology and physics at Woods Hole Oceanographic Institution and Scripps Institution of Oceanography for more than 40 years, died this past December of leukemia. His work continues to influence recent discoveries about how sharks sense their world, and how the workings of their electrosense can be used to guide conservation and fisheries management.

It’s difficult to summarize how influential Kalmijn’s greatest discovery continues to be to this day. It has changed the way that biologists (including me) think about ocean life. The fact that sharks sense electromagnetic fields has become such common knowledge that it’s been mentioned in four of the last five children’s books about sharks that I’ve reviewed. Although I never knew Kalmijn, early in my career I developed a relationship with his work that deepened my respect for his contribution and the subsequent research it spawned. Even though decades passed before biologists began studying the implications in earnest, recent and emerging areas of research have shown that exploring the electrosensory worlds of sharks and rays continues to lead to surprising and exciting discoveries.

The Story of Kalmijn’s Discovery

Biologists have long been aware that sharks and their relatives have a peculiar system of mucus-filled pores, called ampullae of Lorenzini, on and around their snouts. I’ve been a shark nerd my whole life, and for me (and many other shark nerds I know) “ampullae of Lorenzini” was one of the first science-jargon terms I ever learned. This name comes from Italian ichthyologist Stefano Lorenzini, who formally described these anatomical features in the 17th century. What no one knew was what these pores were used for. There were many guesses, but no evidence.

“We hadn’t discovered a whole new sensory system in centuries.”

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A key breakthrough came in the 1930s, although no one realized it for decades. In 1935, comparative physiologist Sven Dijkgraaf at the State University of Utrecht in the Netherlands was studying sharks for an unrelated project when he observed that a captive shark reacted when it approached a rusty wire in its tank. The shark retreated from the wire after encountering it, recoiling in apparent disgust or pain even though the fish had never touched it. Moreover, this escape reaction occurred even when the shark was blindfolded and could not see the wire. Clearly, the wire was providing a nonvisual stimulus that the shark could pick up. Dijkgraaf seems to have filed away this observation as a curiosity outside of his focus of study at the time.

More than 30 years later, Kalmijn began working with Dijkgraaf and duplicated these results with a few other species of sharks and rays, showing that the ability to sense something near an electrical source is widespread in this group of fishes. Around the same time, chemist Royce Murray of the University of North Carolina at Chapel Hill documented that sensory cells in the ampullae of Lorenzini reacted to electromagnetic fields. This work was done in the lab with just the ampullae organs, not a living, swimming shark or ray.

By this point, scientists knew that sharks reacted to some stimulus given off by an unshielded wire, and that the ampullae of Lorenzini reacted in the presence of electromagnetic fields. Kalmijn wanted to test whether sharks and rays could sense the electric field given off by living marine animals, such as prey. Kalmijn’s breakthrough came from asking the following questions, as reported in his seminal 1971 paper in the Journal of Experimental Biology: “Are there electric fields in the natural habitat of the sharks and rays that can be detected by these animals?” and “If so, do the sharks and rays make a significant use of these fields?”

To answer these questions, Kalmijn performed a series of clever experiments, enclosing live prey animals in protective cases so that sharks couldn’t see, hear, or smell them but could still sense their bioelectric fields. The sharks reacted to their otherwise hidden prey, leading Kalmijn to conclude that “all criteria . . . have now been satisfied to accredit these animals with an electric sense, and to designate the ampullae of Lorenzini as electroreceptors [emphasis in the original]”! This new way of approaching the problem was revolutionary. At the time, physiologists tended to look at tissues and cells, as Murray had when studying the ampullae of Lorenzini, rather than the whole animal. “Kalmijn didn’t just look at tissues in a petri dish, he looked at the whole animals, alive and interacting with their environment,” Kajiura told me. “That’s what we finally needed to learn that they’re using this as a way to detect electric fields.”

Kalmijn’s 1971 paper documenting these experiments was the first scientific paper that I ever read in detail. That’s because my undergraduate honors thesis work at the Duke University Marine Lab was based on these experiments. Kalmijn’s research inspired the first scientific project I ever performed: I looked at whether smooth butterfly rays could tell the difference between the bioelectric fields of different prey animals. The 1971 paper is wonderfully written, with enthusiasm shining through in a way one rarely sees in scientific writing. Kajiura told me that when he first read it during his early career, he “marveled at such a well-written paper.”

Kyle Newton, a research associate at Oregon State University who works on how electromagnetic fields given off by offshore energy facilities affect sharks and rays, told me that humans are “so visually dominant that the idea that other animals can detect a whole other type of stimulus is just totally beyond our perception. What would it feel like? It’s beyond our comprehension.” Newton pointed out that some animals can see ultraviolet or infrared light (wavelengths humans cannot see), but that’s a different range of a sense we have, rather than an entire sense we don’t have.

Studying Electrosenses Today

After Kalmijn’s discovery that sharks and their relatives can sense electromagnetic fields, researchers began exploring what exactly they do with that capability. Only a handful of researchers in the world study topics related to electrosenses and are equipped with the expertise to speak on Kalmijn’s legacy. Among them is Timothy Tricas, who studies animal electrosenses at the University of Hawaiʻi. He told me that we now know that sharks can use their electrosense not just for finding prey buried under sand or mud, but also for hunting in the open ocean in the dark, as well as for locating mates and avoiding predators.

The dearth of published studies on how sharks and rays use the electrosense is especially striking compared with research on other senses.

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In a 2013 paper poetically titled “Survival of the Stillest,” a team led by Ryan Kempster, a graduate student at the University of Western Australia at the time, found that shark embryos in eggs stop moving when they detect an electrical field, a powerful predator defense behavior for an animal that can’t yet swim away. Just last year, a team of scientists led by Bryan Keller of the U.S. National Oceanic and Atmospheric Administration showed conclusive evidence that sharks can use the Earth’s magnetic field for precise long-distance open-ocean migration.

Even after five decades, the field of electroreception in sharks and their relatives remains wide open. Kajiura told me that the dearth of published studies on how sharks and rays use the electrosense is especially striking compared with research on other senses. And Newton said, “There aren’t a lot of behavioral studies happening with whole animals in wild environments. It’s been a while since people worked on this topic, and Dr. Kalmijn’s work was essentially a huge breakthrough that’s seen relatively little follow-up.” But that’s changing. There are several areas of active or emerging research on shark electrosenses.

Sensory Traps and Wires in Seawater

Knowing that sharks can sense electromagnetic fields, but most other fish cannot, gives us shark researchers the chance to create a sensory trap. “We can sucker [sharks] in or repel them, with their own biology,” Newton told me. This tactic has many uses, including one that could help turn the tide in the fight to conserve sharks, and one that may help protect people from sharks.

Sharks are some of the most threatened vertebrate animals in the world. Given their ecological importance in structuring coastal and marine ecosystems that humans depend on for food, that’s bad news for everyone. One of the biggest threats to sharks occurs when commercial fisheries target one type of fish but accidentally catch something that is swimming near it—referred to as bycatch. In addition to animals such as sea turtles, seabirds, and marine mammals, sharks are commonly caught as bycatch, killed in the process, and often simply discarded.

The basic principle of bycatch reduction is that we want to reduce the number of animals caught accidentally without reducing the amount of target catch—for example, we want to catch fewer sharks but still bring in the same amount of tuna. Fishers are unlikely to adopt a solution to save sharks that makes it harder for them to catch what they’re trying to catch. They’re also unlikely to adopt a solution that’s expensive or difficult to use.

Attaching magnets made from rare earth metals to fishing hooks may be effective at reducing the loss of sharks to bycatch. In the case of longline fishing for tuna, such magnets create a powerful electromagnetic field near the baited hook, which sharks can detect and tuna cannot. Ideally, this field would repel sharks without repelling tuna. Early trials have had mixed success, in some cases even increasing the number of sharks caught, but for some species of sharks, rare earth metal magnets significantly reduce bycatch. Given recent reports that open-ocean shark species commonly caught as bycatch in tuna fisheries have declined by 71 percent since Kalmijn’s paper was published in the 1970s, it’s certainly worth exploring the magnetic-repellent approach further.

Electromagnetic fields might also prove useful for keeping sharks away from swimmers. Although the odds of getting bitten by a shark are astronomically low, the risk has long captivated our imaginations and fears. There are several personal shark repellent devices on the market that aim to use electrosense of sharks to repel them. I am skeptical of the need for and utility of these devices—it’s worth noting that in 2016, a Florida teenager was wearing a Sharkbanz shark repellent when he was bitten—but clinical trials published by Charlie Huveneers’s lab at Flinders University in Australia indicate that some of these devices truly can repel some shark species under certain conditions.

Answering that question is the focus of Newton’s research at Oregon State University. “We really don’t know what, if anything, all these EMF’s [electromagnetic fields] in the water will do,” he told me. “Is this going to change their behavior or migrations, like beachfront resort light distracts sea turtle hatchlings? Make them forage where there’s no food, or avoid areas where there is food? Will the EMFs mask the signal of prey? We can’t even sense these fields, which means we can do all kinds of things to the environment without understanding the possible impacts.”

The initial breakthrough in the discovery of sharks’ electrosense came from the observation that sharks reacted to a wire in their tank. Over the next few years, humans are planning to put a lot more wires into seawater, in the form of high-voltage cables carrying energy generated by offshore wind farms. Sharks have a long history of disturbing submarine cables— the webcomic The Life of Sharks amusingly claimed that sharks were biting the underwater infrastructure of the internet because “those photos CAN’T get shared” (see comic on the right). But the types, quantity, and location of cables about to be constructed are unprecedented. How will all these new electromagnetic fields in the ocean affect the sharks?

Kalmijn was aware of and interested in these latest implications of his work and even attended a recent workshop on the environmental effects of offshore wind power. Kalmijn stayed involved with this research through his illness, and when he died he had two papers in preparation for peer review, to be submitted to journals posthumously by colleagues.

Kalmijn’s research revolutionized what we know about how animals perceive the world around them, with implications we’re only beginning to understand decades later. He lived to see his revolutionary ideas not only become widely accepted enough to be included in textbooks and children’s books, but also shape the cutting edge of emerging fields. Kalmijn opened a door to a new field of research, and his contributions will continue into the future.