An electric fish is any fish that can generate electric fields. A fish that
can generate electric fields is said to be electrogenic while a fish that has
the ability to detect electric fields is said to be electroreceptive. Most
electrogenic fish are also electroreceptive. Electric fish species
can be found both in the ocean and in freshwater rivers of South America and
Africa. Many fish such as sharks, rays and catfishes can detect electric fields
and are thus electroreceptive, but they are not classified as electric fish
because they cannot generate electricity. Most common bony fish,
including most fish kept in aquaria or caught for food, are neither
electrogenic nor electroreceptive. Electric fish produce their electrical
fields from a specialized structure called an electric organ. This is made
up of modified muscle or nerve cells, which became specialized for producing
bioelectric fields stronger than those that normal nerves or muscles produce.
Typically this organ is located in the tail of the electric fish. The
electrical output of the organ is called the electric organ discharge.
Strongly electric fish Strongly electric fish are fish with an
EOD that is powerful enough to stun prey. Typical examples are the electric
eel, the electric catfishes, and electric rays. The amplitude of the
signal can range from 10 to 600 Volts with a current of up to 1 Ampere,
according to the surroundings, for example different conductances of salt
and fresh water. To maximize the power delivered to the surroundings, the
impedances of the electric organ and the water must be matched:
Strongly electric marine fish deliver low voltage, high current electric
discharges. In salt water, a small voltage can drive a large current
limited by the internal resistance of the electric organ. Hence, the electric
organ consists of many electrocytes in parallel. Freshwater fish have high
voltage, low current discharges. In freshwater, the power is limited by
the voltage needed to drive the current through the large resistance of the
medium. Hence, these fish have numerous cells in series.
Weakly electric fish Weakly electric fish generate a
discharge that is typically less than one volt in amplitude. These are too
weak to stun prey and instead are used for navigation, object detection and
communication with other electric fish. Two of the best-known and most-studied
examples are Peters' elephantnose fish and the black ghost knifefish. The males
of the nocturnal Brachyhypopomus pinnicaudatus, a toothless fish native
to the Amazon basin, give off big, long electric hums to attract a mate.
The EOD waveform takes two general forms depending on the species. In some
species the waveform is continuous and almost sinusoidal and these are said to
have a wave-type EOD. In other species, the EOD waveform consists of brief
pulses separated by longer gaps and these are said to have a pulse-type EOD.
Jamming avoidance response It had been theorized as early as the
1950s that electric fish near each other might experience some type of
interference or inability to segregate their own signal from those of
neighbors. This issue does not arise, however, because the electric fish
adjust to avoid frequency interference. In 1963, two scientists, Akira Watanabe
and Kimihisa Takeda, discovered the behavior of the jamming avoidance
response in the knifefish Eigenmannia sp. In collaboration with T.H. Bullock
and colleagues, the behavior was further developed.Bullock, Theodore Holmes;
Heiligenberg, Walter, eds.. Electroreception. Wiley. Finally, the
work of Walter Heiligenberg expanded it into a full neuroethology study by
examining the series of neural connections that led to the behavior.
Eigenmannia is a weakly electric fish that can self-generate electric
discharges through electrocytes in its tail. Furthermore, it has the ability to
electrolocate by analyzing the perturbations in its electric field.
However when the frequency of a neighboring fish’s current is very close
to that of its own, the fish will avoid having their signals interfere through a
behavior known as Jamming Avoidance Response. If the neighbor’s frequency is
higher than the fish’s discharge frequency, the fish will lower its
frequency, and vice versa. The sign of the frequency difference is determined
by analyzing the "beat" pattern of the incoming interference which consists of
the combination of the two fish’s discharge patterns.
Neuroethologists performed several experiments under Eigenmannia's natural
conditions to study how it determined the sign of the frequency difference.
They manipulated the fish’s discharge by injecting it with curare which prevented
its natural electric organ from discharging. Then, an electrode was
placed in its mouth and another was placed at the tip of its tail. Likewise,
the neighboring fish’s electric field was mimicked using another set of
electrodes. This experiment allowed neuroethologists to manipulate different
discharge frequencies and observe the fish’s behavior. From the results, they
were able to conclude that the electric field frequency, rather than an internal
frequency measure, was used as a reference. This experiment is
significant in that not only does it reveal a crucial neural mechanism
underlying the behavior but also demonstrates the value neuroethologists
place on studying animals in their natural habitats.
Species Following is a table of all known
electric fish species within fresh water. There are two groups of marine
fishes, the electric rays and the stargazers capable of generating strong
electric pulses. See also
Feature detection Passive electrolocation in fish
References