This trout looks guilty for a reason. It's hiding one of nature's most intriguing mysteries: the secret of magnetoception. For decades, behavioral experiments have suggested that a variety of animals possess this ability to sense — and navigate by — the Earth's magnetic fields, but identifying what, exactly, endows these animals with their sense of direction has proven difficult.
Now, a team of German researchers has not only identified the cells thought to be responsible for trout's uncanny navigational skills, it's also developed a brand new technique for finding these cells under a microscope. This technique could be applied to studying magnetic sense in animals ranging from sharks to bees, representing a major advancement in the study of magnetoreception.
Consider this scenario: someone drops you off in the middle of the desert and tells you to quickly make your way East. "Easy," you think. "I'll just head toward the rising Sun, or navigate by way of the stars." You're about to be on your way when, suddenly, thick clouds set in. Your view of the sky is quickly and totally obscured. All seems lost.
But it's not. You reach in your pocket to find a compass, and suddenly the task at hand is entirely feasible. Screw the Sun and stars — now you're navigating with magnets.
Or, to be more precise, now you're navigating with magnetite. The iron-containing compound is the most strongly magnetic mineral on Earth, which is exactly why you'll find it used in compasses. By sensing the Earth's magnetic field, the magnetite in a compass needle keeps it pointing toward magnetic north, and you oriented in whatever direction you need to be heading.
It turns out the same is likely true for trout. The only difference is that where we humans use a tool to navigate, these fish use their biology. Like pigeons, turtles, and a variety of other animals, trout are thought to be capable of sensing the Earth's magnetic field. Unfortunately, identifying the physiological mechanisms of this so-called "magnetoception" has proven to be rather tricky.
Now, a team of researches led by Stephan Eder has come up with a refreshingly simple technique for identifying magnetite-containing cells. Previous research had suggested that the cells responsible for magnetoception in trout were likely located in the animals' noses. With that in mind, the researchers scraped tissue from the noses of trout, placed the samples beneath a microscope, and introduced a rotating magnetic field. Then they watched and waited. What they observed was nothing short of astounding.
This video featured here reveals what they saw: cells that would actually spin in unison with the rotating magnetic fields. The spinning cells are pretty rare — about 1 in 10,000 — but the researchers say these numbers make sense. These magnets, while small, are powerful; the researchers' experiments revealed the cells to respond to the rotating magnetic field 100 times more forcefully than they predicted. Gather too many of these magnetosensing cells in one place and they start to interfere with one another. In order to be of any navigational use, these cells need their space.
Here's a confocal microscopy image of a spinning cell. The blue is the cell's nucleus, and the dotted white line the cell membrane — but you might as well think of it as the outline of a compass needle. See that clump of white material highlighted by the yellow box? Researchers think those are magnetite crystals. By attaching firmly to the cell membrane, Eder and his colleagues believe these crystals are able to coax the cell along in its spinning motion. These results, write the researchers, show that "the magnetically identified cells clearly meet the physical requirements for a magnetoreceptor capable of rapidly detecting small changes in the external magnetic field."
The challenge now lies in clarifying how these spinning cells convey directional information to the trout, and whether magnetic cells can be identified in other species using the same technique. If they can, it could lead to rapid advances in our understanding of everything from magnetoception, to evoluton, to animal migration.
The researchers' findings are published in the latest issue of PNAS