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Gerhard von der Emde
present in their natural aquatic habitats. When light conditions are poor, electric
fields can still be used for orientation, navigation, and communication.
Aquatic electrical fields are of either abiotic or biotic origin. Abiotic fields are
usually DC or low frequency fields with the exception of the high frequency fields
caused by lightning (Hopkins 1999). Low-frequency abiotic fields can originate
from geochemical sources, seismic activity, or the movement of water masses
through the earth's magnetic field. Certain geological formations can cause
electrical fields if they are connected through water, recalling DC batteries and
forming a stationary pattern of electrical field lines. An electrosensitive organism
swimming through this pattern will experience it as a low-frequency electrical
field, depending on swimming speed and direction of movement (Kalmijn 1974).
Organisms living in the water are sources of biotic electrical fields. Muscle
activity is accompanied by an electrical event at the muscle cell membrane, as
well as by an electrical nerve potential. However, because of the multiple
orientations of the many membranes over which electrical potentials are produced,
the net electrical field outside an entire muscle or even a whole animal is
extremely weak. Electrical fields produced by membrane-separated fluid
compartments containing different ion compositions are probably more important
than fields produced by muscles. An example are the gills of aquatic animals,
which exchange oxygen, C02, and various ions between blood and water.
Concentration differences of ions will lead to a net ionic flux across the thin gill
membranes. Movements of gill covers, such as the operculum or the mouth,
modulate these electrical fields in time at low frequencies. All other thin and
therefore leaky barriers between internal body fluids and external media form
additional sources of biotic electrical fields. When an animal is injured, the
resulting ion movements may increase greatly and thus intensifY the electrical
field (Kalmijn 1974).
Most of the biotic electrical fields are DC or low frequency fields (0 to 30 Hz)
similar to the abiotic fields. In many African and South American rivers, however,
biotic electrical fields containing much higher frequencies are found as well. They
originate from electric fishes, which have evolved specialized electric organs for
the production of high frequency electrical signals, called electric organ
discharges (EODs; Fig. 1). Electric organs have evolved, with one exception, from
electrically excitable muscle tissue. The modified muscle cells are called
electrocytes. They form regularly orientated stacks and are activated in synchrony.
Thus, summation of the weak electrical potentials generated across the membranes
of each single electrocyte results in a significant net electrical potential produced
by the whole organ. Using their large electric organs, strongly electric fish living
in freshwater habitats such as the South American electric eel can generate
electrical potentials larger than 600 V. Strongly electric marine fish such as the
electric ray Torpedo achieve voltages of only 60 V which are, however,
accompanied by large electrical currents. In both fresh and marine waters, strongly
electric fishes use their electric signals to stun or kill their prey and to defend
themselves against predators (Moller 1995).
Gerhard von der Emde
present in their natural aquatic habitats. When light conditions are poor, electric
fields can still be used for orientation, navigation, and communication.
Aquatic electrical fields are of either abiotic or biotic origin. Abiotic fields are
usually DC or low frequency fields with the exception of the high frequency fields
caused by lightning (Hopkins 1999). Low-frequency abiotic fields can originate
from geochemical sources, seismic activity, or the movement of water masses
through the earth's magnetic field. Certain geological formations can cause
electrical fields if they are connected through water, recalling DC batteries and
forming a stationary pattern of electrical field lines. An electrosensitive organism
swimming through this pattern will experience it as a low-frequency electrical
field, depending on swimming speed and direction of movement (Kalmijn 1974).
Organisms living in the water are sources of biotic electrical fields. Muscle
activity is accompanied by an electrical event at the muscle cell membrane, as
well as by an electrical nerve potential. However, because of the multiple
orientations of the many membranes over which electrical potentials are produced,
the net electrical field outside an entire muscle or even a whole animal is
extremely weak. Electrical fields produced by membrane-separated fluid
compartments containing different ion compositions are probably more important
than fields produced by muscles. An example are the gills of aquatic animals,
which exchange oxygen, C02, and various ions between blood and water.
Concentration differences of ions will lead to a net ionic flux across the thin gill
membranes. Movements of gill covers, such as the operculum or the mouth,
modulate these electrical fields in time at low frequencies. All other thin and
therefore leaky barriers between internal body fluids and external media form
additional sources of biotic electrical fields. When an animal is injured, the
resulting ion movements may increase greatly and thus intensifY the electrical
field (Kalmijn 1974).
Most of the biotic electrical fields are DC or low frequency fields (0 to 30 Hz)
similar to the abiotic fields. In many African and South American rivers, however,
biotic electrical fields containing much higher frequencies are found as well. They
originate from electric fishes, which have evolved specialized electric organs for
the production of high frequency electrical signals, called electric organ
discharges (EODs; Fig. 1). Electric organs have evolved, with one exception, from
electrically excitable muscle tissue. The modified muscle cells are called
electrocytes. They form regularly orientated stacks and are activated in synchrony.
Thus, summation of the weak electrical potentials generated across the membranes
of each single electrocyte results in a significant net electrical potential produced
by the whole organ. Using their large electric organs, strongly electric fish living
in freshwater habitats such as the South American electric eel can generate
electrical potentials larger than 600 V. Strongly electric marine fish such as the
electric ray Torpedo achieve voltages of only 60 V which are, however,
accompanied by large electrical currents. In both fresh and marine waters, strongly
electric fishes use their electric signals to stun or kill their prey and to defend
themselves against predators (Moller 1995).