157
animals. These nanoparticles “permanently magnetized bar magnets that twist into
alignment with the earth’s magnetic fi eld if allowed to rotate freely. Single-domain
magnetite crystals might transduce geomagnetic fi eld information to the nervous
system in several different ways. One possibility is that such crystals exert pressure
or torque on secondary receptors (such as stretch receptors, hair cells or mechanoreceptors) as the particles attempt to align with the geomagnetic fi eld. Alternatively,
the movement of intracellular magnetite crystals might open ion channels directly
if, for example, cytoskeletal fi laments connect the crystals to the channels,”
(Lohmann and Johnsen 2000 ; see also Davila 2005 ).
According hypothesis (Walker et al. 1997 ), in animals, the functional activity of
magnetite nanocrystals as magnetoreceptors supposes immediate contact with the
nervous system. However, the strong evidence of such kind of contacts on anatomical level is still absent.
Some organisms possess so called superparamagnetic nanostructures, which differ from typical single-domain crystals in size (they are smaller) and have different
magnetic properties (Hanzlik et al. 2000 ). As described by Lohmann and Johnsen
( 2000 ):
“One characteristic is that the magnetic axis of a stationary superparamagnetic
crystal can move about to track the direction of an ambient, earth-strength fi eld.
By contrast, the magnetic axis of a single-domain crystal is fi xed and stable under
the same conditions, and the crystal itself must rotate physically to track the fi eld.
Superparamagnetic crystals generate fi elds strong enough to attract or repel adjacent
crystals,” (Lohmann and Johnsen 2000 ).
The magnetic sense of animals has generally been related to orientation and
navigation purposes. The two most common orientation mechanisms observed in
animals, based on magnetic fi eld parameters, have been described: The inclination
compass for directional information, and the navigational map for positional information (Davila 2005 ). With few exceptions, the biological mechanism underlying
magnetic fi eld perception in animals is still unknown. However, it has been suggested
that inclusions of biogenic magnetite particles connected to nerve structures can
provide a suitable transducer mechanism of the geomagnetic fi eld (see for review
Kirschvink et al. 2001 ).
Previously, Kramer ( 1961 ) suggested that animals transported to an unfamiliar
site need both a map sense to determine the direction of the displacement from
home, and a compass to fi nd that direction. The Map-and-Compass model involves
a two-step process for orientation (Davila 2005 ):
1. the animal establishes its position relative to the loft with the help of the map;
2. a compass system is used to locate the direction that will lead it home.
Over the past 60 years, it has been shown that several groups of animals have
developed biological mechanism to extract map and compass information using
geomagnetic fi eld (Wiltschko and Wiltschko 1995 ; Fischer et al. 2001 ; Freake et al.
2006 ).
The geomagnetic fi eld is relatively stable over biological time scales and is axial,
with the magnetic fi eld lines roughly directed north-south and symmetric in both
3.5 Biomagnetite in Marine Vertebrates
animals. These nanoparticles “permanently magnetized bar magnets that twist into
alignment with the earth’s magnetic fi eld if allowed to rotate freely. Single-domain
magnetite crystals might transduce geomagnetic fi eld information to the nervous
system in several different ways. One possibility is that such crystals exert pressure
or torque on secondary receptors (such as stretch receptors, hair cells or mechanoreceptors) as the particles attempt to align with the geomagnetic fi eld. Alternatively,
the movement of intracellular magnetite crystals might open ion channels directly
if, for example, cytoskeletal fi laments connect the crystals to the channels,”
(Lohmann and Johnsen 2000 ; see also Davila 2005 ).
According hypothesis (Walker et al. 1997 ), in animals, the functional activity of
magnetite nanocrystals as magnetoreceptors supposes immediate contact with the
nervous system. However, the strong evidence of such kind of contacts on anatomical level is still absent.
Some organisms possess so called superparamagnetic nanostructures, which differ from typical single-domain crystals in size (they are smaller) and have different
magnetic properties (Hanzlik et al. 2000 ). As described by Lohmann and Johnsen
( 2000 ):
“One characteristic is that the magnetic axis of a stationary superparamagnetic
crystal can move about to track the direction of an ambient, earth-strength fi eld.
By contrast, the magnetic axis of a single-domain crystal is fi xed and stable under
the same conditions, and the crystal itself must rotate physically to track the fi eld.
Superparamagnetic crystals generate fi elds strong enough to attract or repel adjacent
crystals,” (Lohmann and Johnsen 2000 ).
The magnetic sense of animals has generally been related to orientation and
navigation purposes. The two most common orientation mechanisms observed in
animals, based on magnetic fi eld parameters, have been described: The inclination
compass for directional information, and the navigational map for positional information (Davila 2005 ). With few exceptions, the biological mechanism underlying
magnetic fi eld perception in animals is still unknown. However, it has been suggested
that inclusions of biogenic magnetite particles connected to nerve structures can
provide a suitable transducer mechanism of the geomagnetic fi eld (see for review
Kirschvink et al. 2001 ).
Previously, Kramer ( 1961 ) suggested that animals transported to an unfamiliar
site need both a map sense to determine the direction of the displacement from
home, and a compass to fi nd that direction. The Map-and-Compass model involves
a two-step process for orientation (Davila 2005 ):
1. the animal establishes its position relative to the loft with the help of the map;
2. a compass system is used to locate the direction that will lead it home.
Over the past 60 years, it has been shown that several groups of animals have
developed biological mechanism to extract map and compass information using
geomagnetic fi eld (Wiltschko and Wiltschko 1995 ; Fischer et al. 2001 ; Freake et al.
2006 ).
The geomagnetic fi eld is relatively stable over biological time scales and is axial,
with the magnetic fi eld lines roughly directed north-south and symmetric in both
3.5 Biomagnetite in Marine Vertebrates