Great white sharks make an annual journey from South Africa to Australia. The trip is over 12,000 miles. There is no GPS to guide these sharks, nor are there landmarks they can use to set their course. Currents and water temperatures vary. Yet, the sharks chart a straight line across the ocean every year.
Scientists have speculated that sharks use Earth’s magnetic field as a type of GPS for decades. However, the theory is hard to prove given the difficulty of studying sharks. However, a recent study in Current Biology found a way.
The Study
Bryan Keller, a researcher at Florida State, built an apparatus that can mimic specific magnetic fields. Keller constructed a 10-foot wooden cube with a large tank in the middle. Next, he coiled over a mile of copper wiring around the cube at specific intervals. Once connected to power, Keller can vary the intensity of the field, mimicking conditions the sharks may encounter in the ocean. Keller’s hypothesized that the sharks would orient themselves in a certain way based on the angle and intensity of the magnetic field. The result would be an indication that sharks use this information to understand their position on the plant.
While Keller’s method is similar to other studies on other marine life, this was the first instance where it is shown that sharks use electromagnetic fields to infer location.
However, there is a limitation. The cube’s magnetic field is too small to track the movements of great white sharks. Therefore, Keller chose to study 20 wild juvenile bonnetheads. Bonnetheads migrate thousands of miles annually, traveling from the Florida coast to the Gulf of Mexico.
In his study, Keller tests bonnethead sharks in three artificially generated magnetic fields. The first mimics the angle and strength of fields naturally encountered in Florida. The second mimics the field of a point 600km south along the sharks’ migratory route. The third field mimics a point in Tennessee, where the sharks have never been. The field from Florida did not generate a response. The bonnetheads did not react to the third field either. However, when exposed to the second field, the sharks consistently oriented themselves with their heads pointing north. Keller concludes that bonnethead sharks use this information to choose which direction to travel. The process is similar to how a hiker uses a compass.
Similarities
Many marine animals use a similar method to migrate. Salmon use magnetic data as well. They also use scents to detect spawning grounds. Keller believes sharks also use olfaction for fine-scale movement. However, he does not believe it is powerful enough to guide sharks hundreds of miles.
Exactly how sharks sense magnetic fields remains a mystery. One theory is they contain magnetite crystals that sense true north somewhere in their nervous systems. Another theory is that magnetic fields affect visual systems. Perhaps superimposing colors of light patterns could point in magnetic locations. For example, red might indicate north.
Sharks detect electrical currents in the water using pores called ampullae of Lorenzini. The pores are located in their snouts. Hypothetically, these receptors might also sense magnetic fields or pick up on them indirectly based on how they interact with electrical currents. Currently, there is not enough research to make definitive claims. There is also no reason to think that there is only one mechanism contributing to sensing magnetic fields.
Impact
Studies like Keller’s are crucial because they give us a better understanding of how our marine technologies affect sharks. The topic is significant to understand as offshore wind farms become more popular, disrupting these fields. The underwater cables in wind turbines create their own little magnetic fields in the ocean. Anomalies like these might confuse small animals in the ocean and send them off course.
Kyle Newton, a biologist at the Washington University of St. Louis, highlights the need for more research. Newton tells Wired that “it is not clear yet whether any disruptions are actually happening; these anomalies are small and might not have any effect at all.” More research is crucial to understand how our technology affects migrating marine life. Therefore, we can avoid derailing important migrations.