Bonnethead shark

Turns out some sharks always know where on Earth they are

Barkha Kumari

Maps, planes, road signs, vision, GPS… Humans have a lot of help to reach their destination. But things are bit murkier underwater. So how do sea creatures find their way through the vast darkness without any signpost?

In the case of some sharks, that would be a compass.

It was quite clear that sharks do have a good sense of direction. They return to the same location to breed and feed, several miles and months apart, a new study published in the journal Current Biology has confirmed.

Scientists have believed that elasmobranchs (sharks, skates and rays) are sensitive to magnetic fields. Then in 2005, a great white shark reportedly traveled from South Africa to Australia and back to the same site in nine months – in almost a straight line.

Unlike on land, odors spread and meander in currents – and visual cues are hard to find mid-ocean. The Earth’s magnetic field is perhaps the only cue they may have – and other researchers had suggested as much. But the latest research put that theory to test.

An overhead shot of the bonnethead sharks in the test tank
An overhead shot of the baby bonnethead sharks in the test tank. Credit Bryan Keller

Shark tank

The researchers from the Florida State University Coastal and Marine Laboratory put 20 baby bonnethead sharks (Sphyrna tiburo) in a small pool enclosed in a cage surrounded by 10,000 feet of copper wire. The sharks were exposed to magnetic conditions akin to the site where they were caught, hundreds of miles away, off an estuary in Florida. They were also exposed to cues different from their habitat.

When the sharks were convinced they were “home,” they swam in random directions. When they thought they were south of it, they headed “north.” They became disoriented when the magnetic condition resembled those found in northern waters. That is because sharks rarely migrate north and would have reason to wonder what got them there.
Soon it was clear that the sharks were responding to magnetic displacement, intensity and inclination.

“The main findings is that sharks use map-like information from the geomagnetic field as a navigational aid,” researcher Bryan Keller said in an email. “This ability is useful for navigation, and possibly maintaining population structure.”

One model of how sharks use the Earth's magnetic field
Possible mechanism for a magnetic compass based on electromagnetic induction As a shark swims through Earth’s magnetic field, it induces weak electric currents to flow through the surrounding seawater. The induced current depends partly on the heading of the shark relative to the magnetic field. In effect, the shark uses its electric sense to infer its magnetic heading. (After Kalmijn 1978.) Pic courtesy The Lohmann Lab at the University of North Carolina at Chapel Hill.

Uncooperative subjects

If simulating the sea environment and a magnetic field was a problem, what took scientists so long to design this test? After all, earlier experiments had recorded responses to magnetic fields in Pacific salmon, sea turtles and lobsters.

“Firstly, for the methods we used, we created a cube that was 3 meters in each direction,” Keller said. “The uniform magnetic field within this field was small, slightly over one meter in each direction. For this reason, it is not realistic to use our methods to study larger sharks.

“We needed a shark that was not only small but also exhibited site fidelity. This means that bonnetheads return to the same estuaries on a seasonal basis. This means the sharks have the capability to remember a specific location and to navigate back to it. There are not many species that are both small and have described site fidelity.”

There are more than 500 shark species. So does the bonnetheads’ ability to use Earth’s magnetic field apply to others? While the jury is still out, Keller felt it did make sense from an evolutionary viewpoint.

Keller pointed out that strong correlations have been found between the sharks’ swimming routes and local magnetic peaks and lows, extending from seamounts to feeding grounds. Many of these species are migratory and their movements often deliberately lead to target locations.

“It is unlikely that the bonnethead evolved this ability independently, given similarities in its ecology to other species,” Keller said. While some species are born with this magnetic “sixth sense,” others may just learn it over time.

Bryan Keller holds a bonnethed shark
Bryan Keller holds a bonnethead shark. Pic courtesy Colby Griffiths

Different strokes

Every species of shark may respond to different components of the magnetic field. This could be how each species keeps its groups close in an environment free of barriers, and maintain genetic diversity. As the apex predators of the oceans, sharks control prey populations and influence their distribution. For instance, turtles, kept away by the fear of tiger sharks, do not overgraze on seagrass. Sharks also act as carbon sinks and cycle nutrients across locations and depths.

Back to the navigation. Can sharks migrate without their natural GPS?

“Navigation is made possible by the use of multiple aids. These can include olfaction [smell], water currents, topography, etc.,” Keller said.

The question remains: Where is this GPS located in the shark? On the fins, in the tail, behind the eyes? That remains unresolved, Keller said. But he shared a revealing detail from a 2017 study by James Anderson of the University of Hawaii. It said that the ability is not limited to the system involved in sensing electric fields. Instead, it suggested, the shark’s head and snout may house cells and structures involved in gauging magnetic fields.

Next up

Asked about future plans about species and topics to study, Keller was circumspect.

“While studying a specific species is attractive, we are more focused on species research questions,” he said. “The first thing we would like to explore is the effect of EMFs [electromagnetic fields] on the spatial ecology of sharks. We would also like to study how sharks may use Earth’s magnetic field for fine-scale navigation.” Keller pointed to research by Jayne Gardiner and Andy Nosal showing that sharks do use odor cues when close to their destinations.

“But we would like to explore how these animals may use the magnetic field for fine-scale navigation,” he said.

Barkha Kumari is a freelance journalist based in Bengaluru, India. You can follow her on Barkha2803

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One thought on “The fish that used GPS”
  1. Neat article. I wanted to share the link via email to a friend who is working in a similar area but I’m not able to send email using your link. Wonder why

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