It was 2005 and scientists in Cape Town had made a shocking discovery. Their tracking data showed a great white shark moving from South Africa to Australia and back again in a near straight line. It was the fastest transoceanic return migration ever recorded and it was carried out with near pinpoint accuracy. Today, it’s well known that sharks, skates and rays make yearly returns such as this to specific locations, but how exactly they do it has escaped consensus.
A group of scientists from Florida State University has taken on the question and concluded that sharks have an internal, GPS-like navigation system that allows them to read the Earth’s geomagnetic field. To conduct the research, the team first captured 20 juvenile bonnethead sharks in St George Sound off the Florida Panhandle, before placing them in a small pool surrounded by copper wire. The wire allowed the researchers to create a custom magnetic field in the centre of the pool. Exposed to the magnetic field from the capture location, the sharks swam in random directions at leisure; but when exposed to the geomagnetic field that would be found 600 kilometres south of that spot, they swam north in a ‘homeward orientation’.
Researchers have suspected that sharks, rays, skates and sawfish detect magnetic fields since the 1970s, but the exact mechanism by which they do so, and the prevalence of this skill in nature have proven elusive, partly because it’s so difficult to study. ‘We’ve known for some time that sharks have the ability to detect the magnetic field, but this is the first time it has been tested successfully,’ says Bryan Keller, a scientist at the US National Oceanic and Atmospheric Administration. ‘We expect these abilities are also observed in other species, like the great white, which migrate 20,000 kilometres out and back to the same spot.’ The results mean that some sharks can be added to the growing list of animals that navigate by magnetic sensation, which includes sea turtles, lobsters and birds.
With the shark navigation system now demonstrated, scientists want to understand the mechanism behind it. Two theories have emerged: some researchers believe that it depends on an iron mineral called magnetite; others believe it’s based on a magnetic-field-sensing molecule in the retina of the eye called cryptochrome. Both theories, or a combination of the two, are plausible. Magnetite has been isolated from many animal tissues, while evidence from studies in birds suggests that they sense the inclination of the magnetic field using cryptochrome molecules in their retinas; the direction of the field is transmitted by the optic nerve to the brain, which allows them to ‘visualise’ northand south. But scientists don’t yet know the precise location of the cryptochrome receptors, or the brain centres that process the information on the magnetic field. There's more work to do to truly understand these masterful navigators.