From sea turtles to the humble pigeon, many species have the ability to sense the Earth’s magnetic field and use it as a compass to navigate. But despite decades of research, scientists are still not quite sure how they manage it.
Now, a new paper published in Biology Letters seems to shed some light on how rainbow trout are able to return to the streams from which they were born in order to spawn. The researchers identified a series of genes that seem to be involved in the fish’s magnetoreception, allowing them to pick up on the weak magnetic field of the planet.
While trout are born in freshwater, they migrate to the sea and spend the next three years maturing in the ocean before cues kick in that they need to return to spawn. Despite having traveled hundreds of kilometers over this period of time, the fish still somehow manage to make it back to the exact river they were born from all those years previous, and the cycle starts again.
It is thought that they use the Earth’s magnetic field to do this, so a team of researchers set out to see if they could identify which genes may be responsible. They took live trout and put them in a special tank that was then placed inside a coil of wire. The researchers ran an electric current through the coil to generate a magnetic field to disrupt the fish’s magnetoreception.
They then looked at the gene activity in these fish, and compared them to others used as controls. Out of 40,000 genes, the researchers found around 181 that changed activity during the experiment. Out of these, they found an increase in activity in the brain's expression of genes involved in making ferritin, a protein that usually stores and transports iron. There were also alterations in the genes linked to the development of the optic nerve.
"The results suggest that the detection system is based on iron that may be connected with or inside the eyes," says researcher Sönke Johnsen in a statement. The discovery fits nicely with previous studies that linked magnetoreception with a compound known as magnetite, which also contains iron. The team think that the gene activity observed in the trout may be a result of the fish trying to repair their magnetite that was disrupted by the magnetic blasts from the coil.
To follow-up on this, the scientists plan to run the same tests but will instead focus on other organs (such as the eye) to see if similar gene activity occurs. They also plan to run experiments on other species of migratory fish and compare the results.