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Fish appear to steer with magnets

Scientists have zeroed in on the likely source of some animals' sense of direction. Rainbow trout seem to be guided by an 'internal compass' of sorts.

By Natalie WolchoverLiveScience / July 9, 2012

Toua Yang lands a rainbow trout while fishing among the rocks at Lost Lake Park on the San Joaquin River in Friant, Calif. Scientists found that rainbow trout find their way using internal magnetism.

AP Photo/The Bee, Craig Kohlruss

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Researchers have isolated what are essentially tiny compass needles in the noses of rainbow trout that may explain these and many other animals' incredible ability to navigate across vast distances.

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When cells scraped from the trout's nasal passages were placed in a rotating magnetic field, a clump of tiny iron-rich crystals inside the cells called magnetite — the same mineral used in compass needles — spun in synchrony with the field, turning the cells around with them.

The strength of the crystals' magnetic response, and their firm attachment to the surrounding cell membranes, lent strong support for what scientists have long suspected: That these crystals lean back and forth like a sail in response to Earth's weak magnetic field, and that the cells they are embedded in somehow convey their swaying movements to the brain. This is believed to confer trout and other migratory animals with a "magnetic sense" by which to judge direction.

As detailed in a new paper published online July 9 in the journal Proceedings of the National Academy of Sciences, the researchers found that the magnetic cells in the trout's noses swayed in response to a magnetic field 100 times more forcefully than had previously been predicted. "More importantly, we show for the first time that the internal compass needle has a strong connection to the plasma membrane [or outer membrane] of the cell, which is important to realize an immediate sensing process," said lead researcher Michael Winklhofer of the University of Munich in Germany.

The results show that the magnetic cells "clearly meet the physical requirements for a magnetoreceptor" capable of rapidly detecting small changes in Earth's magnetic field, the researchers said.

The strength of Earth's field varies in a predictable way across the planet's surface, allowing migratory animals to use it for position-finding. By learning the strength of a field that exists at a particular destination, the animals can home in on it. That much is pretty well established in the science field; what has remained mysterious is how these animals use magnetic-field changes to navigate. [What If Earth's Magnetic Poles Flip?]

Scientists think Earth's magnetic field might urge migratory animals in the right direction like a guiding hand pressing on them. "I think it is similar to touch or pressure. The magnetite-based magnetic sense is innervated by the trigeminal nerve, which mediates touch (heat, cold and pain). If the inner compass needle of a cell points in a certain direction in space, and the fish makes a 90-degree turn, the cell will fire and tell the brain: 'I am 90 degrees out of my preferred direction,'" Winklhofer told LiveScience.

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