There are two distinct branches of the electric fish family, the
South American Gymnotiforms and the African Mormyriforms. All weakly
electric fish emit either "hums" or "clicks" as a means of exploring their
environment. "Hummers" have a wave-type electric discharge organ (EOD)
and emit a wave-like hum, whereas "clickers" have a pulse-type EOD and
emit short pulses. The distinction in method of perception is essentially
a different means to the same end for the two types of fish. For humans,
understanding the sensory techniques has proven useful in learning about
the human condition. For example, by understanding the timing of the
hums or pulses released by the weakly electric fish, researchers have
developed a better understanding of how the timing mechanism in the
human ear functions.
Both hummers and clickers have a pacemaker in their brain sending
electric impulses via supermotorneurons to the electric organ of the fish.
The electric organ in hummers emits waves that vary in frequency depending
on the sex of the fish. For example, male hummers emit a lower frequency
wave than do females or their young. This is a direct result of estrogen
opposing the effects of androgens. Prior to the generating of the
electric organ, the stalk of the brain must be depolarized by
electromotorneurons from the brain. Essentially the electromotorneurons
flow from the posterior end of the stalk and depolarize the electrolytes,
but the electric organ is never fully depolarized. This in turn produces
a sinusoidal wave that we interpret as a hum, as they are able to maintain
constant voltage and constant electric field.
When hummers come into contact with other fish, they react by adjusting
their frequency, which is known as jamming avoidance response (see JAR lab). The fish with
a higher frequency will adjust to accomodate the higher end of the electric field,
whereas the fish with the lower frequency will adjust to the lower end of the
electric field. This is done to avoid confusion between the fish whose signals are
being received.
The clickers emit an arrythmic "clicking" or "pulsing" noise to sense their
environment and communicate. When a pulse is released from the electric organ,
typically the male pulse will last longer that the female pulse. Testosterone and
other androgens are responsible for lengthening the pulses. Clickers use the same
depolarization technique as hummers, the only difference being that the electric
organ is actually momentarily depolarized, causing the fish to produce a click, as
all the electrolytes fire simultaneously. Clickers cannot maintain a constant
voltage or electric field.
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