The Earth’s Magnetic field is a “GPS” navigator for sharks

Sampriti Roy, University of Calcutta

Sharks (Subclass: Elasmobranchii) are among those species that exhibit site fidelity, which means that no matter how far they migrate, they always find their way “home”(just like turtles). It has been speculated by scientists for some time now that migratory species like sharks can do this by using electrosensory organs to obtain information about navigation routes from the geomagnetic field (GMF).

The GMF provides both Compass and map information to those who can perceive it and many Elasmobranchs seem to be able to differentiate between the two components. However, if sharks are among those Elasmobranchs that can obtain magnetic cues and derive spatial information for maintaining their orientation during navigation was something inconclusive for about 50 years. 

A recent study published in the journal Current Biology now presents solid evidence for the first time that proves that sharks are indeed dependent on magnetic fields for their long journeys across the sea.

The study

One of the main reasons for the theory of sharks using GMF for navigation remaining inconclusive for about 50years is that sharks have proven to be difficult to study. This is why project leader Keller decided that it would be easier to perform the study in smaller sharks.

Selection of species: A species was to be selected that was known to return each year to specific locations or its home. The species ultimately settled upon was the Bonnethead shark (Sphyrna tiburo), since it has been found to return to the same estuaries every year from any location it migrated to.

Now, to find out if the bonnetheads planned those return trips by depending upon GMF maps, the researchers first selected 20 juvenile, wild-caught bonnetheads. These were captured off the coast of Florida and taken to Florida State University Coastal and Marine Lab. After acclimatization and other necessary procedures, the following steps were then taken:

• The sharks were exposed to magnetic conditions that represented locations that were hundreds of kilometers away from where they were originally captured. This was done to find out if the sharks were relying upon magnetic cues and subsequently orienting themselves.

• The researchers predicted that if sharks indeed were dependent upon geomagnetic field cues, they would show the following observations to compensate for the displacement perceived by them concerning their capture area:

i) Orientation predicted in a southern magnetic field: Northward 

ii) Orientation predicted  in a northern magnetic field: Southward 

iii) Orientation predicted if the magnetic field matched their capture site: No preference in orientation.

It was found that when the sharks were exposed to fields that were in their natural range (fields they are more familiar with, either from evolutionary history or individual experience) they acted just as predicted by the researchers. 

 In the case of Southern magnetic field exposure, the sharks perceived it as something different than the field in which they were captured (off the coast of Florida). The response was with homeward orientation (northward). However, the Northern field did not cause a difference in orientation than the field of capture. The speculation behind this was the sharks not having much of an experience with such strong magnetic fields. 

Significance of the study

• Explanations of other species’ impressive navigation feats:

According to Keller, it was very unlikely that it was only the bonnethead species that developed this ability and not its cousin species. It was found that a great white shark migrated between South Africa and Australia and returned to the same location the next year. The reason behind being able to achieve this is the map that it followed, which has now been proven to be the Earth’s geomagnetic field.

• Importance in understanding current biogeographic patterns and migrations:

The information about bonnetheads deriving spatial information from geomagnetic cues is very important in this regard. One instance in which this is useful is that the genetic differences between different shark populations may be predicted by observing the spatial variation in GMF.

• An important addition to literature: The work has been said to be important in the growing body of literature that builds on the map-like use of GMF and how it can be considered an “evolutionary underpinning for how animals across a variety of taxa successfully derive spatial information from diverse habitats”.

As for Keller, his further plans include studying the effects of magnetic fields that are generated from anthropogenic sources (like submarine cables) on sharks. Along with this, he also plans to study the dependence of sharks on magnetic cues not just for migration over long distances, but in everyday life.

Also read: COVID-19 Vaccines Around The World

Source:-

• “Map-like use of Earth’s magnetic field in sharks” by Bryan A. Keller, Nathan F. Putman, R. Dean Grubbs, David S. Portnoy and Timothy P. Murphy, 6 May 2021, Current Biology.
  DOI: 10.1016/j.cub.2021.03.103
• https://scitechdaily.com/sharks-use-earths-magnetic-fields-to-guide-them-like-a-map-it-really-is-mind-blowing/

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