It's been known for a very long time that many organisms are sensitive to the Earth's magnetic field. Birds, fish, bats, and even bacteria use the field as a reference to find their way around. But how the magnetic field is detected has remained a mystery. But now, thanks to a fridge magnet, Jonathan Pierce-Shimomura has found that worms also do this and he's uncovered a pair of unusually shaped nerve cells inside the animals that appear to work like a neurological compass.
Jonathan - There are all these animals that migrate around the world using the Earth's magnetic field. But it's been a real mystery about how they sense the Earth's magnetic field. We don't have this sense. So, we don't have any intuition how they do it. No one has ever discovered neurons that are actually required for these migrations or actually sense the Earth's magnetic field.
Chris - And you've been looking in worms at this. So, they presumably do this, too?
Jonathan - Yeah. It wasn't obvious that tiny microscopic worms would do this. But my lab makes a business out of studying how they migrate to different stimuli and perform other behaviours. We thought, "Let's see if they move towards magnets?" So one day, we put a refrigerator magnet on one side of the dish and we saw that worms would migrate towards this. That was our first hint that they might sense magnetic fields. To determine if they could sense the Earth's magnetic field, we next injected them into a tube filled with Jell-O. When we did that, we found that, starved worms had a preference for burrowing downward in the tube. When they were well-fed however, they liked to burrow upward. This suggested that they might use gravity or the Earth's magnetic field to orient. To figure out if it's gravity or the magnetic field, we put the same tube inside of a coil where we could invert their Earth's magnetic field, flipping it now upward instead of downward. When we did this, worms moved in the opposite direction.
Chris - If you, therefore, take worms from a part of the Earth's surface where actually the magnetic field is going in the opposite direction. So, I'm thinking if you're in the northern hemisphere and you're working on worms from the northern hemisphere, and you compare them with worms from the southern hemisphere. Do they do the opposite?
Jonathan - Yes. That's indeed what we found. We took worms from Australia and we tested them in the same experiment, and they moved in the opposite direction. When they were starved, they liked to burrow upward. When they were well-fed, they liked to burrow downward. So, this is consistent, not with gravity, but with them responding them to the natural direction of the magnetic field where they come from on Earth.
Chris - Is it that the worms are detecting the magnetic field or are they sensitive to something that the magnetic field does to the tube they're living in?
Jonathan - To ask that question, we try to identify the neurons that might be required for performing this behaviour. So we systematically broke a bunch of the different sensory neurons with different mutations. And we quickly deduced that one pair of sensory neurons called AFD were required for this behaviour.
Chris - Ah right. So, there's a specific magnetic sensing pair of nerve cells in these worms. And if you deactivate them, then the worms can't do this.
Jonathan - That's right. They couldn't move towards the refrigerator magnet, nor could they show any preference for burrowing up or down like the normal worms could.
Chris - But are those nerve cells detecting the Earth's magnetic field, or are they detecting something that the Earth's magnetic field does to the environment?
Jonathan - Looks like these neurons detect the magnetic field directly. Because when we performed a physiological experiment to see if the neurons could be activated by magnetic fields, we saw that the neurons were turned on when we introduced a magnetic field that was Earth's strength. So, this neuron responds directly to the Earth's magnetic field.
Chris - Do you have any insight into how that nerve cell is registering the orientation of the magnetic field relative to it, so the worm effectively has this built-in compass. How is this working?
Jonathan - This neuron has a very intriguing shape. The tip of it has these microscopic antenna-shaped structures with the antenna pointed towards the front and towards the back of the animal. This shape would be really nice to serve as a miniature compass that can wiggle back and forth, perhaps if associated with iron, that could activate the neuron and tell them which direction they're going.
Chris - Do you think that this is an insight, what this worm's doing into how bigger, more complicated creatures like birds or butterflies, or turtles, salmon, for example, all of which migrate, we think, following the Earth's magnetic field actually achieve that feat?
Jonathan - It remains to be seen if the same sort of mechanism will be used in these larger animals. But this is a nice new clue about how animals can sense the Earth's magnetic field. What we're hoping is that the molecules that we find that are required for transducing the Earth's magnetic signal into an electrical-chemical signal in the neurons - once we identify those molecules, we can see if those molecules are in the sensory organs of birds, turtles, and butterflies.