Magnetic fields felt by sharks
By Jan TenBruggencate
Advertiser Science Writer
Scientists at the Hawai'i Institute for Marine Biology at Coconut Island, in groundbreaking research conducted earlier this year, have proven that sharks detect changes in magnetic fields, which may be a key to how they navigate in the deep ocean.
The work was done with juvenile sandbar sharks and a scalloped hammerhead shark — species common in Kane'ohe Bay — but it probably applies to others as well, said the University of Hawai'i's Carl G. Meyer, who worked with fellow marine biologists Kim N. Holland and Yannis P. Papastamatiou on the research. The research was published this week in the Journal of the Royal Society, a leading British scientific journal.
"We suspect that all sharks have the ability to sense magnetic fields but that sensitivity to magnetic fields may vary among species," Meyer said.
Shark researchers have long suspected that the animals have the ability to respond to changes in the planet's geomagnetic field, and that they use it to find their way when swimming across vast stretches of the sea.
"The open ocean is an apparently featureless environment. The fact that sharks were able to maintain a constant heading across open ocean strongly suggests that they use a reliable, environmental cue to navigate. The Earth's magnetic field is a relatively constant, reliable cue," he said.
However, there has been little hard evidence until now that sharks actually can detect magnetic changes.
The Coconut Island research team constructed a circular tank a little more than 20 feet across, and on the outside of the tank built a coil of 100 strands of copper wire. When low power is applied to the coil, it creates a magnetic field that is detectable with a compass. At its strongest point, the field is less than three times the strength of the earth's own magnetic field.
Next, they put seven young sharks in the saltwater tank. When the magnetic field was turned on, the sharks would be fed at a particular location in the pool. Their diet was chopped fish and squid.
Later, the researchers turned on the magnetic field without feeding the fish, being careful to remain out of view and to control sounds and other potential signals to the animals.
The sharks reacted quickly to the changed magnetic field, making quick turns, swimming faster and cruising repeatedly over the location where they were normally fed. The change in behavior was significant, and clearly indicated that the sharks could sense the magnetic field, Meyer said.
While scientists have long suspected that other species navigate by magnetic fields, it hasn't been possible with most animals and birds to clearly show that this happens. With sharks, though, there is now no question, the research team said.
What is still not clear is how they do it — what kind of sensory system the sharks possess that lets them pick up magnetic fields.
That's one of several questions that the team will attack next, Meyer said.
Reach Jan TenBruggencate at jant@honoluluadvertiser.com or (808) 245-3074.
