Ironman triathletes competing in the annual Cambridge event, and Eastern Shore residents enjoying the surf at Ocean City, probably can’t suppress the chilling Jaws movie soundtrack in their minds as they venture into the shark’s habitat.

Sadly, Erica Fox, a triathlete swimming off Lovers Point in Monterey Bay, California, was attacked and killed by a shark on December 21, 2025. It is a place I know well, having spent countless hours scuba diving there when I lived in California and conducted research as a marine biologist at nearby Moss Landing Marine Labs and Stanford University’s Hopkins Marine Lab.

When Erica’s body was recovered a week later, she was found wearing a magnetic shark-repellent bracelet on her ankle, which is claimed to overwhelm the shark’s special senses.

As a marine biologist who studied sharks in the laboratory and in the wild, I contributed to the discovery that sharks and their relatives can detect weak electric fields in seawater.

I would like to briefly explain the science of electroreception and explore the likely reasons why the device failed to protect the swimmer. Few people in the general public understand this amazing sense, called electroreception, which humans lack, and there is much misunderstanding and confusion about it.

You often hear that sharks can detect magnetic fields, and that they use their electro sensory sense to detect the heartbeat of prey, and that the magnetic ankle bands overwhelm the shark’s electro sensory system. None of these is true.

The Shark’s Sixth Sense

If you look closely at the head of a shark, you will see that it is speckled with small pores. Peeling back the rough skin reveals that these are openings of long, clear tubes filled with transparent jelly. The tubes can be as thick as a strand of spaghetti and just as long or longer, depending on the size of the shark or ray. These unique sensory organs, unlike anything in any other animal, are called ampullae of Lorenzini.

These peculiar structures were a mystery until the late 1970’s and 1980’s, when it was determined that they are a sensory system that detects electricity. They are so sensitive that, in theory, a shark could detect a 1.5-volt battery switched on across the distance of the Atlantic Ocean.

They detect DC battery-like electric fields, not higher frequency signals like the EKG of a heartbeat. Extremely weak DC voltages are generated by all types of chemical and biological processes in nature. The tubes serve as conductors of electricity, and the end of the tube swells into an eyedropper-like bulb, which is in effect the voltmeter, where nerves emerge to transmit neural signals to the shark’s brain.

These tubes radiate in all directions around the head, especially around the mouth. This enables sharks, rays, and chimaeras (bizarre deep-sea fish that I also studied) to measure the field strength and its shape in three dimensions.  Each tube works like an electrician measuring voltage differences between two points in contact with the probes of a voltmeter.

The Origin of Bioelectric Fields in Seawater

All animals in seawater have a weak bioelectric field around them. This is simply the result of differences in salts inside the body and in seawater. Salts dissolved in water are charged molecules (ions). If there is a difference in the number or type of ions across a barrier, like the skin, you have a battery.

The electric field around a normal fish radiates most intensely between its mouth and gills, because these membranes have the lowest electrical resistance. The two poles create a “dipole,” an electric field resembling the pattern of iron filings radiating from the poles of a magnet. The strength of the bioelectric field pulsates as the fish opens and closes its mouth, pumping water over its gills to extract oxygen. Sharks can detect this extremely weak, slowly fluctuating electric field to locate prey, even when hidden in murky water or buried under sand.

In experiments my wife and I did in the 1980’s in the open ocean off Woods Hole Oceanographic Institution, in Massachusetts, we attracted blue sharks at night to our specially designed 21-foot Boston Whaler. Laboratory experiments by us and others had shown that these organs detect electric fields. Still, we wanted to test the hypothesis that wild sharks in the open ocean used electroreception to detect prey.

To do this, I designed a T-shaped apparatus that we lowered through a square hole cut through the deck of our boat. At each end of the T, an electrode generated an electric field resembling that of a normal fish. We pumped chum (ground-up fish) through a central port between the two legs of the T-shaped apparatus to draw sharks into the experiment. My wife and I took turns randomly switching on one of the electrodes at either end of the T, while the other one observed the shark’s response, not knowing which electrode was active.

The results were so clear that no statistical analysis was really needed. The shark would scream in like a torpedo toward the apparatus, tracking the bloody odor source. At the last moment, the shark invariably pivoted its head and bit the electrode that was on. The strength of an electric field from a dipole decreases very rapidly with distance, so that it is only detectable within less than a meter from the source.

But the experiment showed that at the moment of attack, electroreception overrides the senses of sight, taste, and smell to orient the jaws for attack. In this way, electroreception gives sharks something like invisible whiskers.

How Magnetic Bracelets are Supposed to Repel Sharks

Why would a magnetic ankle bracelet repel a shark? Could it even pique the curiosity of a shark and draw it to the swimmer? Here is where rigorous scientific research is lacking.

Sharks detect DC or slowly changing electric fields, not magnetic fields. The idea behind the shark-repellent ankle band is that a magnet moving through a conductor, such as seawater, will induce a weak electric current. That is how electric generators work.

So as the swimmer kicks their feet, the magnet on their ankle generates a weak fluctuating voltage signal that changes strength and polarity with their kicking action. The shark would no doubt sense this electrical field, but the essential questions then become, what would the shark think it was, and would it be repelled by it?

Here is where experiments like the one I just described with blue sharks can lead to misunderstanding. Such prey-detection experiments give people a simplistic view of electroreception as a kind of beacon that draws the shark to food, like a moth to a lightbulb. The fact is that electroreception is a highly sophisticated sensory system, likely as vivid to a shark as vision is to us.

All manner of factors affect a battery’s strength, including temperature, the chemical properties of seawater, and more, which provide the shark with a rich sensory ability. A shark can interpret electrical fields as well as we can discern intricate details of objects from photons bounced off and transmitted through them. The shark can tell from the shape and changes in the electric field around a fish whether it is alive or dead, how big it is, and probably what kind of fish it is.

The manufacturers claim the magnetic bracelet overwhelms the shark’s electro sensory system. Still, any metal in seawater, for example, a rusty hook, generates an electric field from electrolysis millions of times stronger than the fields induced by a moving magnet or an animal in seawater. Iron in seawater creates about half a volt through electrolysis, but a shark’s electro sensory system can detect half a nanovolt. A nanovolt is one billionth of a volt.

Our Boston Whaler was designed to have no metal of any kind in contact with seawater for that reason.

The second issue is shark behavior. A “feeding frenzy” is how people refer to sharks partaking in their meal. I’ve seen it many times. The water absolutely explodes and boils when sharks attack their prey, and nothing seems to deter them.

In the experiments I described with the T-shaped apparatus, the sharks hit the electrode only on their first pass, biting it, spitting it out, and sending the apparatus spinning and swinging violently. During subsequent attacks by the same shark, the animal went into a frenzy, biting anything in sight, including our boat, and even thrusting its open jaws up through the deck cutout, snapping at our faces.

Shark attacks are rare, and they are nearly always instances of sharks mistaking a human for their preferred prey, like a seal. They will take the person in their jaws, mortally or grievously wounding the swimmer, and then spit them out like they did our experimental apparatus when they find it is not food. Shark attacks are horrible, and the loss of Erica Fox is a sad tragedy, but it is important to keep them in perspective. The most dangerous animal on the planet that kills more humans than any other is the mosquito. As a resident of Taylor’s Island, that is a threat I dodge every summer!

First published in The Cambridge Spy