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Electric Fish
All ocean animals are surrounded by very low-frequency electric fields. Many
animals have magnetic or electric senses. Electric fish interest neuroscientists
today because of the extraordinarily sensitive and precise system of generating,
receiving, processing, and responding behaviorally to electric signals. Seven
families of fish deliver appreciable voltage outside of their bodies. A few
predators kill prey electrically. The majority, weakly electric fish, use
a specialized sensory guidance system for environmental navigation via
echolocation. Similar to the echolocation in bats and dolphins, the
weakly electric fish use electrolocation to analyse the reception and
transmition of electric current for communication. This is primarily for species and
sex recognition in their low visibility environments.
Electric Organ Discharge
The organ is located in the posterior of the fish and the
synchronous firing of the organ in controlled by the midline medullart nucleus, called the
pacemaker nucleus, located in the brain. The multinucleated cells that make up the organ
are either myogenically (muscle) or neuronally (nerve) derived. The cells
that make up the organ are called electrocytes. The columns of the
electrocytes are innervated by
supramoterneurons which carry the signal from the brain stimulating the EOD by
depolarizing the elecrocytes. In hummers the electrocyte is never fully depolarized
because the stimulation is constant, which results in the sinusiodal wave.
Strongly Electric Fish
Strongly electric fish (freshwater eels and catfish) kill prey and ward off
predators by delivering electric shocks of several hundred volts. With many strong
dischargers, the whole fish has enormous electric potential. Eels have the unique
ability to discharge both weak and strong electric current. The weak current is
used primarily to locate and and stun prey. The strong current is used almost
exclusively as a weapon to attack prey.
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| Image by Skyler R. Chapman |
Weakly Electric Fish
Weakly electric fish use their electric organs primarily for the detection of the
rough shape, conductivity, and location of nearby objects, recognition of members
of their own species, calling their mates, finding their position in a school, and
enacting other behaviors critical to their survival. Weakly electric fish live in a
variety of freshwater habitats in Central and South America
and in Africa. The South American Gymnotiforms and African Mormyforms
are so phylogenetically distant that they are thought to have evolved through
convergent evolution (independently). In both orders, the fish live in shallow
streams of dark murky waters so the EOD essentially replaces their vision; they sense
their surroundings by emitting an EOD which creates an electric field around the fish. The
fish can sense purturbations caused by objects in their electric field-called
electrolocation. Weakly electric fish have one of two patterns of electric discharge,
both of which are generated from modified muscle tissue usually near the tail or from
tissue near the eyes. Wave fish (hummers) produce continuous sinusiodal wave signals at
frequencies of 50-1000 Hz. Pulse fish (clickers) emit electrical pulses
lasting approximately one
millisecond which are spaced about 23ms apart. These pulses create time
gaps
in their
field. (see hummers and clickers) Hormones are now thought to
play an integral roll in the amplitude of the discharge and probably effects the anatomy
of the EOD control system in many other ways. The EOD varies with sex.
EOD?s: Social Communicaion
Electric fish emit sexually dimorphic EOD?s (waveform can be altered by a steroid
treatment), generated by a medullary pacemaker nucleus (PMN) composed of 2 cell
types. The output neurons of the PMN synapse located on electromotor neurons on the
spinal cord innervate the electric organ. The PMN only receives input from 2
sources, which are responsible for brief modulations that occur during social
interactions. This capacity to emit and sense weak EODs evolved only in South
American gymnotisforms and African mormyriforms. For these fish, EODs give
information on the species, sex, and possibly the individual identity of
another fish. Also,transient modulations in EOD frequency happen in
social situations and convey information on aggressiveness, readiness to
mate, etc. (See also http://www.hhmi.org/grants/lectures/97lect/behave/
(outside site) for an applet on the social and non-social
behavior of Gnathonemus petersii)
Jamming Aviodance Response (JAR)
The
Jamming Avoidance Response (JAR) is a very important mechanism for electric fish.
When two fish with nearly the same frequency meet each other, one alters his
frequency to be slightly higher and the other alters to be slightly lower.
The shifts are simultaneous and reflexive. This process prevents two
frequencies from interfering and jamming each other's electrical
signals-allowing the fish to operate in the same area. (See JAR
lab)
Electroreception
Nonconductive objects reflect the electric field-while
highly conductive objects weaken the field by attracting electrons and ions from the
field. The electroreceptors located in the epidermis of the head and sides of the fish
percieve these changes. Voltage sensitive channels open to detect a change in the electric
field-this stimulation of the cells cause the release of synaptic vescicles?release a
neurotransmitter, which binds to a lateral line nerve fiber which eventually sends a
signal to the brain where the information is processed. Ampularry receptors detect
electric fields given off by other fish. Tuberous receptors respond to the range of the
fish?s own EOD.
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