Many species of fish, such as the brown trout pictured on the left, hatch in streams and then travel away from those streams in order to mature. However, when it is time to reproduce, they end up navigating back to the same stream in which they hatched so they can spawn there. How do they accomplish this? How do they know where they are and which way to swim in order to get back to that special stream? Based on behavioral studies, scientists have thought that these fish are able to sense the earth’s magnetic field and use it as an aid in their navigation. However, the specific source of this magnetic field sense has been elusive…until now.
A recent study has shed a lot of light on this magnetic sense, at least for trout (and presumably other similar fish, like salmon). The authors of the study set out to determine what gives the trout their magnetic sense, and they developed a rather ingenious method to aid them in their search. First, they took tissue samples from the trout’s nasal passages, because previous studies indicated that there was magnetite (a mineral that reacts strongly to magnetic fields) in those tissues.1 Then, they put cells from the tissues under a microscope and exposed the cells to a rotating magnetic field. In response, some of the cells rotated with the field.2 You can actually see a video of this happening here! Just click on the links for downloading the movies.
This is a very simple, very sensitive method for finding the cells responsible for the trout’s magnetic sense. As you can see from the video, the cells that are sensitive to the rotating magnetic field are smaller than the other cells in the tissue. Also, the authors found that only 1 in 10,000 cells in the nasal tissue have a magnetic sense. No wonder these cells haven’t been found until now! Of course, as the authors studied the cells more closely, they found evidence of thoughtful design.