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Gerhard von der Emde
signals are produced continuously. Interruptions are rare and serve certain
purposes during electrocommunication. In contrast, each pulse-type EODs is
followed by a pause which lasts longer than the signal itself.
The variety of EODs in different species of electric fish reflects the variety in
habitat and life-style of these animals. Presumably, different types of signals are
the result of an adaptation to different physical habitats and social structures.
However, even within a certain habitat, demands on signal design for different
purposes can be conflicting. For example, for electrocommunication it might be
advantageous to produce high-amplitude and low-frequency electric signals. For
active electrolocation of small objects (see below), signals containing higher
frequencies are better suited. However, in order not to attract predators such as
large catfishes or the electric eel (which use EODs for detecting their electrogenic
prey), an electric fish should produce a low amplitude and high-frequency signal
(Westby 1988; Stoddard 1999). Because of such conflicting constraints, signal
evolution in weakly electric fish has been the result of a compromise satisfying the
demands of electrolocation, electrocommunication, and predator avoidance.
Interestingly, both wave-type and pulse-type EODs developed independently in
Africa and South America. Presumably, each type of signal has certain advantages
in a given ecological situation.
2 Sensing of Aquatic Electrical Fields
The ability to sense electrical fields appears to be a primitive vertebrate capacity
(Bullock et a!. 1983). Besides fishes, very few vertebrates, and no invertebrates
are known to possess an electrical sense, which is always associated with an
aquatic habitat. Among nonteleost fishes all taxa except two (the Myxiniformes
and the Holostei) possess electroreceptor organs and related brain structures. The
electroreceptor organs used by these species are called ampullary organs (Szabo
and Fessard 1974) and they can be found in both freshwater and marine species. In
marine elasmobranchs, the ampullary organs are called Ampullae of Lorenzini.
The common ancestor of all teleosts was not electroreceptive. For unknown
reasons (New 1997), it lost its ability to detect weak electric fields. However,
during teleost evolution, electroreception was "rediscovered" independently at
least twice (maybe four times). In Africa, the mormyriforms, which produce highfrequency electric signals, and the nonelectrogenic African knifefish
(Notopteriforms) developed an electric sense. Whereas the African notopteriforms
possess only ampullary electroreceptor organs, the mormyriforms have, in
addition, electroreceptors located in so-called tuberous organs. These receptors are
sensitive to high-stimulus frequencies and thus can be used to detect the fish's own
EODs as well as those of other fishes. In South America, gymnotiform weakly
electric fish have developed electroreception independently of their African
counterparts. Like mormyrids they possess both ampullary and tuberous
electroreceptor organs. Catfishes (siluriforms), which are closely related to the
Gerhard von der Emde
signals are produced continuously. Interruptions are rare and serve certain
purposes during electrocommunication. In contrast, each pulse-type EODs is
followed by a pause which lasts longer than the signal itself.
The variety of EODs in different species of electric fish reflects the variety in
habitat and life-style of these animals. Presumably, different types of signals are
the result of an adaptation to different physical habitats and social structures.
However, even within a certain habitat, demands on signal design for different
purposes can be conflicting. For example, for electrocommunication it might be
advantageous to produce high-amplitude and low-frequency electric signals. For
active electrolocation of small objects (see below), signals containing higher
frequencies are better suited. However, in order not to attract predators such as
large catfishes or the electric eel (which use EODs for detecting their electrogenic
prey), an electric fish should produce a low amplitude and high-frequency signal
(Westby 1988; Stoddard 1999). Because of such conflicting constraints, signal
evolution in weakly electric fish has been the result of a compromise satisfying the
demands of electrolocation, electrocommunication, and predator avoidance.
Interestingly, both wave-type and pulse-type EODs developed independently in
Africa and South America. Presumably, each type of signal has certain advantages
in a given ecological situation.
2 Sensing of Aquatic Electrical Fields
The ability to sense electrical fields appears to be a primitive vertebrate capacity
(Bullock et a!. 1983). Besides fishes, very few vertebrates, and no invertebrates
are known to possess an electrical sense, which is always associated with an
aquatic habitat. Among nonteleost fishes all taxa except two (the Myxiniformes
and the Holostei) possess electroreceptor organs and related brain structures. The
electroreceptor organs used by these species are called ampullary organs (Szabo
and Fessard 1974) and they can be found in both freshwater and marine species. In
marine elasmobranchs, the ampullary organs are called Ampullae of Lorenzini.
The common ancestor of all teleosts was not electroreceptive. For unknown
reasons (New 1997), it lost its ability to detect weak electric fields. However,
during teleost evolution, electroreception was "rediscovered" independently at
least twice (maybe four times). In Africa, the mormyriforms, which produce highfrequency electric signals, and the nonelectrogenic African knifefish
(Notopteriforms) developed an electric sense. Whereas the African notopteriforms
possess only ampullary electroreceptor organs, the mormyriforms have, in
addition, electroreceptors located in so-called tuberous organs. These receptors are
sensitive to high-stimulus frequencies and thus can be used to detect the fish's own
EODs as well as those of other fishes. In South America, gymnotiform weakly
electric fish have developed electroreception independently of their African
counterparts. Like mormyrids they possess both ampullary and tuberous
electroreceptor organs. Catfishes (siluriforms), which are closely related to the