After spending 3 years at sea and traveling up to 300 kilometers away from home, a rainbow trout can swim straight back to its original hatching ground, following freshwater streams inland and rarely heading in the wrong direction. This remarkable feat of navigation likely relies on many senses; the fish have superb eyesight and smell. But the trout also seem to rely on Earth's magnetic fields, which point them in the right direction. Now, for the first time in any animal, scientists have isolated magnetic cells in the fish that respond to these fields. The advance may help researchers get to the root of magnetic sensing in a variety of creatures, including birds.
"We think this will really be a game changer," says Michael Winklhofer, an earth scientist at Ludwig Maximilians University Munich in Germany who led the new study. "To study magnetic sensory cells, you have to be able to get hold of them first, and that's what we've finally developed a way to do."
Previous research has shown that many species of fish, as well as migratory birds, have the ability to detect differences in magnetic field strengths, which vary around the globe. Scientists think that the key to this ability is magnetite, the most magnetic of all minerals, which they've found embedded in bird and fish tissues. They've even narrowed down which tissues in these animals could contain magnetite by using dyes that bind to the mineral. But they've never been able to isolate individual cells that contain magnetite, and some of the staining methods have led to false positives and controversy in the field.
The challenge in isolating magnetic cells is that they are few and far between—if they were clustered together they would interfere with each other's magnetism. "If you have a tissue containing these cells, it's likely that only one out of ten thousand cells is magnetic," says Winklhofer. "That makes it very hard to do any research."
To isolate magnetic cells from their non-magnetic neighbors, Winklhofer and his collaborators placed a suspension of rainbow trout (Oncorhynchus mykiss) cells under a microscope that had a magnet rotating around the stage that the sample sat on. Any cells containing magnetic particles should slowly rotate along with the magnet, they thought. They tested the method in tissue isolated from the fish's noses, which contain magnetite. In each trout's olfactory tissue, they found between one and four cells that rotated in turn with the rotating magnetic field. The team transferred the rotating cells to individual glass slides to study them further under the microscope.
In each isolated cell, the magnetite particles were next to the cell membranes, the scientists report online today in the Proceedings of the National Academy of Sciences. And surprisingly, the magnetism in each cell was tens to hundreds of times stronger than researchers had hypothesized, says Winklhofer. This suggests that the fish may be able to detect not only the direction of North based on magnetism, but small differences in magnetic field strength that can give them more detailed information about their precise latitude and longitude.
"This result is really a step beyond anything we've done before," says ecologist Michael Walker of the University of Auckland in New Zealand, who led many of the initial experiments that homed in on trout's noses as the tissue-containing magnetic particles. "The idea that they came up with here is just great and it worked like a charm."
"What I think needs to be done now is that we need to demonstrate that these cells are actually sensory cells," he adds. Although the cells contain magnetic particles, Walker notes, that doesn't necessarily mean that they pass magnetic information along to the fish's brain.
Winklhofer hopes to next apply the technique to various tissues from pigeons to determine where birds' magnetic sensing cells are located.