Category Archives: geomagnetic field

How eels navigate to the Sargasso Sea

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The migration of European eels (Anguilla anguilla) between freshwater habitats in Europe and North Africa and their spawning ground in the Sargasso Sea is one of the unsolved mysteries in animal navigation. Scientists have speculated that the eels may use the Earth’s magnetic field as a guide during their 6000 kilometer trip, and an article published today by Durif et al. strongly supports this idea.

To test the ability of eels to orient using a magnetic field, the authors used a carefully controlled laboratory setting, in which they eliminated other potential orientation cues, including odors, vibrations, light, and sounds.  They also created an artificial magnetic field, the same strength as the Earth’s magnetic field, which could be oriented in different directions.  Turning the artificial magnetic field between test runs helped rule out the possibility of the eels using other orientation cues.

The authors found that the eels consistently oriented in a particular direction with respect to magnetic north and that the eel’s particular orientation varied with water temperature.  Below 12 oC, which is the temperature range associated with eel migration, the eels oriented in the direction they had been transported from the holding tank to the testing tank.  This also corresponded with the direction of increasing water temperature.  Above 12 oC, the eels oriented at right angles to the direction they had been transported, which the authors speculate might reflect foraging behavior during times of the year the eels are not migrating.

Thus, eels seem capable of using the Earth’s magnetic field as a navigational guide.  They also seem to integrate this information with other cues, such as water temperature, to determine their direction of movement.

While these results provide some insight into how eels navigate to their spawning ground, other mysteries about eel migration remain, including where the spawning ground in the Sargasso Sea is precisely located.  For further reading about eel migration, see another paper published in PLOS ONE in October, in which Béguer-Pon et al. tagged adult American eels (Anguilla rostrata) to map their migration to the Sargasso Sea but instead learned something about porbeagle shark predation.

Image credit: Steffen Zienert (http://nas.er.usgs.gov/queries/factsheet.aspx?SpeciesID=308)

Citations: Durif CMF, Browman HI, Phillips JB, Skiftesvik AB, Vøllestad LA, et al. (2013) Magnetic Compass Orientation in the European Eel. PLoS ONE 8(3): e59212. doi:10.1371/journal.pone.0059212

Béguer-Pon M, Benchetrit J, Castonguay M, Aarestrup K, Campana SE, et al. (2012) Shark Predation on Migrating Adult American Eels (Anguilla rostrata) in the Gulf of St. Lawrence. PLoS ONE 7(10): e46830. doi:10.1371/journal.pone.0046830

No sense of direction? Consider consulting some carp

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The ability to navigate using the earth’s magnetic field is a skill that is not unique to humans.  Over the last few decades, scientists have discovered that numerous organisms have an ability to tell which way is north.  And the list is growing.

In one study, “Magnetic Alignment in Carps: Evidence from the Czech Christmas Fish Market,” Hart et al. reported that carp tend to align themselves along a north-south axis.  The authors photographed over 14,000 carp swimming in plastic tubs at pre-Christmas fish markets and found that, on average, the fish positioned themselves facing either the North Pole or the South Pole.

While the authors have not yet proven that carp can sense the geomagnetic field, they did rule out other possible orientation cues, including light, wind, temperature, and water flow. What benefit a common orientation may provide the fish remains unknown.  One possible explanation the authors present is that it may help the fish coordinate their movement when they swim in a school.

Some other organisms also have an ability to detect localized magnetic fields.  In a paper titled “Desert Ants Learn Vibration and Magnetic Landmarks,” Buehlmann et al. demonstrated that ants can sense a strong magnetic field created by two small magnets and use this as a landmark to find their nest.

In the absence of any other landmark (such as a vibrational, visual, or olfactory cue), ants who had been trained to associate the magnetic field with the nest entrance spent a lot more time near the magnetic field than ants who were naive to this landmark.  It is unclear how relevant this experiment is to ants in their natural environment, but the study nevertheless highlights the ants’ ability to sense a magnetic field.

While little is known about how carp align with the earth’s magnetic field or how ants sense a localized magnetic field, more is known about how some tiny organisms, aptly named magnetotactic bacteria, orient with a magnetic field.  These bacteria form straight chains of nano-size magnetic particles within the cells. The magnetic chains are attached to intracellular structures, thus allowing the bacteria to align passively with the earth’s magnetic field, like compass needles.

In a paper published earlier this month, Kalirai et al. showed that some magnetotactic bacteria form anomalous magnetic chains, with some sections of the chain oriented north and others south.  This finding contradicts scientists’ previous understanding that all the magnetic particles in a single chain would have the same alignment.  The study raises many questions: Would bacteria with anomalous magnetic chains have a competitive disadvantage in their natural environment?  Is there a single genetic mutation that leads to the anomalous magnetic chains?

All three of these studies raise intriguing questions, and we look forward to future discoveries from these scientists.

Image: Arrows indicate the orientation of carp swimming in a plastic tub (Hart et al. PLOS ONE 2012)

References:

Hart V, Kušta T, N?mec P, Bláhová V, Ježek M, et al. (2012) Magnetic Alignment in Carps: Evidence from the Czech Christmas Fish Market. PLoS ONE 7(12): e51100. doi:10.1371/journal.pone.0051100

Buehlmann C, Hansson BS, Knaden M (2012) Desert Ants Learn Vibration and Magnetic Landmarks. PLoS ONE 7(3): e33117. doi:10.1371/journal.pone.0033117

Kalirai SS, Bazylinski DA, Hitchcock AP (2013) Anomalous Magnetic Orientations of Magnetosome Chains in a Magnetotactic Bacterium: Magnetovibrio blakemorei Strain MV-1. PLoS ONE 8(1): e53368. doi:10.1371/journal.pone.0053368