If you are lost in the woods and your phone is dead, you probably need a map and a compass. The map gives you a general sense of where you are, and the compass pinpoints you in the right direction. Animals, on the other hand, use environmental cues, instincts, and internal cues to help them navigate. Migratory birds for example use the sun, the stars and the earth’s magnetic field as their compass. Using the earth’s magnetic field for the purpose of navigation (magnetoreception) is a relatively unexplored sensory modality although it is present in a large number of both aquatic and terrestrial animals.
Among these “magneto-sensitive” animals are fish. Different species of fish are now known to use the earth’s magnetic field as their GPS. For example adult salmon are known to swim back (more than 4000 km) to their original hatching stream to deposit eggs using their magnetic sense. This is particularly useful because marine habitats like open oceans and seas provide very few cues required to successfully navigate.
The developmental stage at which most fish start using their magnetic sense for navigation and orientation is, however, not well known. The vast majority of fish have a life cycle that consists of two parts—offshore and demersal (close to the sea floor). They typically spend their larval stages in offshore waters, and then move to deeper waters where they settle down and complete the rest of their life cycle.
Coral-reef fish larvae were observed to swim to settlement habitats as far as 1 km away in the absence of any visual, olfactory or auditory cues. Thus, it is thought that they use other large-scale directional cues such as magnetoreception.
To test this idea, O’Connor and Muheim collected pre-settlement larvae of the coral-reef damselfish Chromis atripectoralis. Each larva then underwent an orientation test in a plastic circular arena situated under a magnetic coil. O’Connor and Muheim were able to manipulate the horizontal component of the magnetic field in the test arena by shifting magnetic north in the coil by +90 degrees. The arena was also covered with opaque plastic sheets to block direct sunlight and other visual cues from the environment. An HD video camera was used to record the movements of the larvae.
The researchers found that the larvae shifted their orientation by roughly +90 degrees when magnetic north was shifted by +90 degrees, indicating that they use information from the earth’s magnetic field for the purpose of orientation. This finding confirmed their hunch—“offshore stage” fish are also capable of using their magnetic sense for orientation and navigation. This is a remarkable finding, although very little is known about the physiology and mechanisms that underlie magnetorecetion.
Overall, this makes us more aware of the potential effects of human induced electromagnetic fields on aquatic organisms.
Reference: O’Conner J. & Muheim R. (2017). Pre-settlement Coral-reef Fish Larvae Respond to Magnetic Field Changes during the day. Journal of Experimental Biology 220: 2874-2877
