Electric Fields and Electroreception
317
gymnotiforms, are electroreceptive, but possess only low-frequency ampullary
electroreceptor organs.
2.1 Passive Electrolocation:
Sensing of Low-Frequency Electrical Fields
Because of their high sensitivity to low-frequency electric fields, ampullary
electroreceptor organs are well suited for the detection of low-frequency
environmental fields of abiotic or biotic origin. This passive electrolocation
involves the detection of electrical fields which are not self-produced. Even
though these natural fields are very low in amplitude, the ampullary
electroreceptor organs of many fishes are sensitive enough to detect them. Skates,
for example, respond reliably to voltage gradients ofless than 0.01 J.!Vcm· 1 , which
corresponds to a voltage of 1 V over 1000 km. The high sensitivity to low voltage
gradients generally found in elasmobranchs enables these animals to detect
electrical fields emanating from the gill epithelia of prey, such as flatfishes buried
in the sand. Sharks have been found to be attracted by electrical dipole sources
emitting prey like signals. Thresholds for feeding responses in the absence of any
food odors are in the order of just 5 nVcm· 1 (Kalmijn 1974).
Elasmobranchs can also use electrical fields for orientation and navigation
(Pals et al. 1982), either by detecting electrical fields of geochemical origin or by
detecting magnetic fields indirectly through electrical induction. A moving water
mass or a shark swimming through the earth's magnetic field induces an electrical
current large enough to excite an elasmobranch's electroreceptors (Kalmijn 1974).
Even though the exact mechanisms are debated (Paulin 1995), elasmobranchs use
the information thus obtained to determine their magnetic compass heading.
Likewise, freshwater fish use their ampullary electroreceptors for orientation.
They detect voltage gradients in their habitat, which serve as electrical landmarks
(Peters and Meek 1973).In addition to using tuberous receptors and active
electrolocation (see below), weakly electric fish also employ their ampullary
receptors for locating low frequency electric fields that originate, for example,
from prey items. Gnathonemus petersii can find insect larvae (Chironomidae), its
natural prey, by passive electrolocation, i.e., by detecting and evaluating electric
gradients produced by the prey animal. Interestingly, passive electrolocation is just
one of many senses that are used simultaneously by G. petersii in search for prey
(von der Emde and Bleckmann 1998).
A hunting method based on electroreception is used by several South
American predators of weakly electric fish. Catfishes and electric eels (Westby
1988; Moller 1995) prey on gymnotiforms by eavesdropping on their electrical
discharges. Predator pressure by hunting catfishes, electric eels, and others might
have caused several species of gymnotiforms, over evolutionary times, to shift the
spectra of their EODs to higher frequencies in order to avoid detection (Stoddard
1999). Interestingly, throughout Peru (this is not true of other areas), wave fish
317
gymnotiforms, are electroreceptive, but possess only low-frequency ampullary
electroreceptor organs.
2.1 Passive Electrolocation:
Sensing of Low-Frequency Electrical Fields
Because of their high sensitivity to low-frequency electric fields, ampullary
electroreceptor organs are well suited for the detection of low-frequency
environmental fields of abiotic or biotic origin. This passive electrolocation
involves the detection of electrical fields which are not self-produced. Even
though these natural fields are very low in amplitude, the ampullary
electroreceptor organs of many fishes are sensitive enough to detect them. Skates,
for example, respond reliably to voltage gradients ofless than 0.01 J.!Vcm· 1 , which
corresponds to a voltage of 1 V over 1000 km. The high sensitivity to low voltage
gradients generally found in elasmobranchs enables these animals to detect
electrical fields emanating from the gill epithelia of prey, such as flatfishes buried
in the sand. Sharks have been found to be attracted by electrical dipole sources
emitting prey like signals. Thresholds for feeding responses in the absence of any
food odors are in the order of just 5 nVcm· 1 (Kalmijn 1974).
Elasmobranchs can also use electrical fields for orientation and navigation
(Pals et al. 1982), either by detecting electrical fields of geochemical origin or by
detecting magnetic fields indirectly through electrical induction. A moving water
mass or a shark swimming through the earth's magnetic field induces an electrical
current large enough to excite an elasmobranch's electroreceptors (Kalmijn 1974).
Even though the exact mechanisms are debated (Paulin 1995), elasmobranchs use
the information thus obtained to determine their magnetic compass heading.
Likewise, freshwater fish use their ampullary electroreceptors for orientation.
They detect voltage gradients in their habitat, which serve as electrical landmarks
(Peters and Meek 1973).In addition to using tuberous receptors and active
electrolocation (see below), weakly electric fish also employ their ampullary
receptors for locating low frequency electric fields that originate, for example,
from prey items. Gnathonemus petersii can find insect larvae (Chironomidae), its
natural prey, by passive electrolocation, i.e., by detecting and evaluating electric
gradients produced by the prey animal. Interestingly, passive electrolocation is just
one of many senses that are used simultaneously by G. petersii in search for prey
(von der Emde and Bleckmann 1998).
A hunting method based on electroreception is used by several South
American predators of weakly electric fish. Catfishes and electric eels (Westby
1988; Moller 1995) prey on gymnotiforms by eavesdropping on their electrical
discharges. Predator pressure by hunting catfishes, electric eels, and others might
have caused several species of gymnotiforms, over evolutionary times, to shift the
spectra of their EODs to higher frequencies in order to avoid detection (Stoddard
1999). Interestingly, throughout Peru (this is not true of other areas), wave fish