Electrocommunication
Electric fish can use electricity as a communicative device,
much as humans use auditory signals. Using its electric
organ, the
fish
produces
an electric organ discharge (EOD), which
is broadcast through
the surrounding water and received by other fish in the environment. These
other fish detect the signals and process various aspects of the signal
to determine
its significance. Fish constantly emit EOD's, and thus are continuously
providing informational
cues to their surrounding environment. Electrocommunicative signals can express
a fish's species, gender, reproductive intent, social status, and even level
of aggression. While some progress has
been made to identify and characterize different signals and their meanings,
decoding electrocomminicative "fish speak" is a difficult process,
and much remains to be discovered. Each species of electric fish varies its
EOD differently to communicate different cues.
| The Elephant Nose fish (Gnathonemus petersii )is
a weakly electric fish that uses its pulse-type EOD to communicate
with other fish. Image used with permission of Simon
Quellen Field. |
EOD Variation
Fish EODs vary according to many factors, including signal
type, frequency, duration, and structure. Different combinations of these
factors produce signals with different meanings.
Pulse and Wave: Some fish produce short bursts
of communicative electric signal called "pulses," and
others produce longer signals called "waves" also called
"tones" (Hopkins
1972). Pulse discharges are "brief with respect to the
period between discharges" and have variable frequency (Hopkins
1972).
Wave or tone signals have discharges nearly equal in length to the period
between discharges,
and relatively stable frequency (Hopkins
1972). Individuals of a single species
produce
either wave signals or pulse signals, but not both. The
Black Ghost Knife fish (Apteronotus albifrons) is a wave producing
fish, while the Elephant Nose fish (Gnathonemus petersii)
produces pulses (von der Emde 1999).
| Depiction of two different types of EOD. Pulse-type
are single bursts of electric signal, while wave-types are repeated.
Image used with permission of Masashi
Kawasaki |
EOD Shape: Fish also produce EODs of different structures
or shapes. A signal may begin with a strong peak and gradually fade, or
it may rise slowly to
peak
at the
end of the signal. EOD shapes vary from species to species, between males
and females,
or even with age. They also vary from individual to individual.
Other Factors: Even if a fish keeps the structure of its
EOD discharge consistent, it can vary the frequency, amplitude, or duration
of
a signal.
There are
many ways
in which
fish electrocommunicative signals differ.
Examples of Electrocommunication
Instead of trying to give a comprehensive account of EOD variation
in makeup and meaning, I have listed a number of specific examples below to
give an indication of how fish use electrocommunication.
Species: One indication a fish may give of its
species is the type of EOD it emits. As note above, some species produce wave
signals, and
others
produce
pulses. However, as many wave or pulse generating fish may live in a single
area, other indicators are necessary for further distinction.
Within wave-type fish, different frequencies can indicate different species.
For example, some species in the Moco-moco creek in the Rupununi District
of South America are distinguishable by EOD frequency. Sternopygus macrurus EOD
frequencies
range between 50 and 150 hz, Eigenmannia virescens frequencies average
240 to 580 hz, and Apteronotus albifrons frequencies are even higher-
750-1250 hz (Hopkins
1972).
Gender: Gender information can be conveyed
by the frequency of a fish's EOD, though the way in which frequency is
related to gender
may vary across species. Brown Ghost Knifefish (Apteronotus leptorynchus),
males typically have higher EOD frequencies than females: 800-1000 hz
for males, 600-800 hz for females(Bastian et al. 2001). However, in Sternopygus
macrurus individuals, the females exhibited higher frequency signals
(120.1 for females, 66.8 for males) (Hopkins
1972).
Gender information can be indicated by aspects of the EOD other than frequency. Brienomyrus
brachyistius triphasic females and males differ in the structure of their
EOD pulse. Males emit longer pulses characterized by an initial peak, a gradual
decrease, and a shallow dip. Females and juveniles have shorter pulses, no appreciable
decrease, and a deeper dip (Hopkins and Bass 1981). |
| Depiction of a single EOD pulse of Brienomyrus
brachyistius triphasic fish. Note the shape difference
between female and male pulses. Image adapted from Hopkins and
Bass 1981. |
|
Courtship: Males of the Sternopygus macrurus species exhibit
specific patterns of variation in their usual wave-type EOD to indicate courtship
intent.
This
pattern is generated by enhancing the normal wave pattern through signal cessation
and frequency variation. S. macrurus males interrupt their normal
wave pattern or briefly increase the EOD frequency to create a unique pattern,
and they
exhibit this pattern only in courtship situations (Hopkins
1972).
The males of a species of West African pulse-type electric fish called Brienomyrus
brachyistius triphasic also vary EOD frequency in courtship. They drastically
increase their normal EOD pulse briefly to produce high frequency bursts of
electric signal to attract potential mates (Hopkins and Bass 1981).
Aggression: Studies with the Brown Ghost
Knifefish Apteronotus
leptorynchus have
shown that their aggressive behavior is expressed by varying the duration of
their
EODs.
Short, low frequency “chirps” (~15-30 ms) indicate aggression,
while longer “rises” or higher frequency chirps seem to show submission
(Trifenbach and Zakon 2002).
Size: Fish size seems to be correlated with
the magnitude of the EOD: the larger the fish, the more electrical signal it
can produce, and
thus
the stronger
its signal. In male Brown Ghost Knifefish, size
and EOD frequency are also positively correlated. The results for females of
the same species are less conclusive. (Trifenbach
and Zakon 2002).
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