The Gymnotiformes are a group of teleost bony fishes
commonly known as the Neotropical or South American knifefish. They have long
bodies and swim using undulations of their elongated anal fin. Found
exclusively in fresh water, these mostly nocturnal fish are capable of
producing electric fields for navigation, communication, and, in the case of
the electric eel (Electrophorus electricus), attack and defense. A few species
are familiar to the aquarium trade, such as the black ghost knifefish
(Apteronotus albifrons), the glass knifefish (Eigenmannia virescens), and the
banded knifefish (Gymnotus carapo).
Aside from the electric eel (Electrophorus electricus),
Gymnotiformes are slender fish with narrow bodies and tapering tails, hence the
common name of "knifefishes". They have neither pelvic fins nor
dorsal fins, but do possess greatly elongated anal fins that stretch along
almost the entire underside of their bodies. The fish swim by rippling this
fin, keeping their bodies rigid. This means of propulsion allows them to move
backwards as easily as they move forward.
The caudal fin is absent, or in the apteronotids, greatly
reduced. The gill opening is restricted. The anal opening is under the head or
the pectoral fins.
These fish possess electric organs that allow them to
produce electricity. In most gymnotiforms, the electric organs are derived from
muscle cells. However, adult apteronotids are one exception, as theirs are
derived from nerve cells (spinal electromotor neurons). In gymnotiforms, the
electric organ discharge may be continuous or pulsed. If continuous, it is
generated day and night throughout the entire life of the individual. Certain
aspects of the electric signal are unique to each species, especially a
combination of the pulse waveform, duration, amplitude, phase and frequency.
The electric organs of most Gymnotiformes produce tiny
discharges of just a few millivolts, far too weak to cause any harm to other
fish. Instead, they are used to help navigate the environment, including
locating the bottom-dwelling invertebrates that compose their diets. They may
also be used to send signals between fish of the same species. In addition to
this low-level field, the electric eel also has the capability to produce much
more powerful discharges to stun prey.
Black ghost knifefish
The black ghost knifefish (Apteronotus albifrons) is a
tropical fish belong to the ghost knifefish family (Apteronotidae). They
originate in freshwater habitats in South America where ranging from Venezuela to the Paraguay–Paraná
River, including the Amazon Basin.
They are popular in aquaria. The fish is all black except for two white rings
on its tail, and a white blaze on its nose, which can occasionally extend into
a stripe down its back. It moves mainly by undulating a long fin on its
underside. It will grow to a maximum length of 50 cm (20 in). It does not have
scales.
Black ghost knife fish are nocturnal. They are a weakly
electric fish which use an electric organ and receptors distributed over the
length of their body in order to locate insect larvae.
Black ghost knifefish with its underside pointing towards
the camera
The black ghost knifefish natively lives in fast moving,
sandy bottom creeks in a tropical climate. South American natives believe that
the ghosts of the departed take up residence in these fish, hence the name.
The black ghost knifefish is a weakly electric fish as a
result of the electromotor and electrosensory systems it possesses. While some
fish can only receive electric signals, the black ghost knifefish can both
produce and sense the electrical impulses. Electrogenesis occurs when a
specialized electric organ found in the tail of the fish generates electrical
signals, which are thus called electric organ discharges (EODs). Then, for
these EODs to be sensed by the fish, electroreception occurs when groups of
sensory cells embedded in the skin, known as electroreceptor organs, detect the
electrical change. The EODs are used for two major purposes: electrolocation
and communication.
The kind of EOD produced can be used to distinguish between
two types of weakly electric fish: the pulse-type and the wave-type. The black
ghost knifefish are considered to be the latter type, because they can
continuously generate EODs in small intervals. Wave-type EODs have a narrow
power spectra, and can be heard as a tonal sound, where the discharge rate
establishes the fundamental frequency. By emitting its own continuous
sinusoidal train of EODs, the fish can determine the presence of nearby objects
by sensing perturbations in timing and amplitude of electric fields, an ability
known as active electrolocation. The particular organs used to sense the
self-generated high-frequency EODs are tuberous electroreceptor organs. On the
other hand, when low-frequency electric fields are generated by external
sources instead of the fish itself, a different class of electroreceptor organs
is used for this passive electrolocation, called ampullary organs. Therefore,
the black ghost knifefish uses an active and a passive electrosystem, each with
its own corresponding receptor organs. The fish can also use a mechanosensory
lateral line system, which detects water disturbances created by the motion of
the fish's body. As nocturnal hunters, the fish can rely on all three systems
to navigate through dark environments and detect their prey.
Each species has a characteristic EOD baseline frequency
range, which varies with sex and age within the species, as well. The baseline
frequency is maintained to be almost constant at stable temperature, but will
usually be changed due to the presence of others of the same species. Such
changes in frequency relevant to social interaction are called frequency
modulations (FMs). The role these FMs have in communication is significant, as
black ghost knifefish have developed jamming avoidance responses, which are
behavioral responses that avoid the overlapping of EOD frequencies between
conspecific individuals to prevent sensory confusion. Moreover, a study was
conducted that focused on sexual dimorphism in electrocommunication signals.
Female black ghost knifefish generate EODs at a higher frequency than the
males, an FM which can be used for gender recognition. A study found the
subdominant black ghost knifefish exhibited noticeable gradual frequency rises (GFRs)
in their EODs whereas the dominant fish did not, supporting the researchers'
hypothesis that GFRs during communication are indicative of submissive signals.
Elephant nose
Peters' elephant-nose fish (Gnathonemus petersii; syn.
Gnathonemus brevicaudatus Pellegrin, 1919, Mormyrus petersii Günther, 1862) is
an African freshwater elephantfish in the genus Gnathonemus. Other names in
English include elephantnose fish, long-nosed elephant fish, and Ubangi
mormyrid, after the Ubangi
River. As the Latin name
petersii confirms it is named after someone called "Peters" (probably
Wilhelm Peters), although the apostrophe is often misplaced and the common name
given as "Peter's elephantnose fish". It uses electrolocation to find
prey, and has the largest brain-to-body oxygen use ratio of all known
vertebrates (around 0.6).
Peters' elephantnose fish are native to the rivers of West
and Central Africa, in particular the lower Niger
River basin, the Ogun
River basin and in the upper Chari River.
It prefers muddy, slowly moving rivers and pools with cover such as submerged
branches. It is a dark brown to black in colour, laterally compressed
(averaging 23–25 cm), with a rear dorsal fin and anal fin of the same length.
Its caudal or tail fin is forked. It has two stripes on its lower pendicular.
Its most striking feature, as its names suggest, is a trunk-like protrusion on
the head. This is not actually a nose, but a sensitive extension of the mouth,
that it uses for self-defense, communication, navigation, and finding worms and
insects to eat. This organ is covered in electroreceptors, as is much of the
rest of its body. The elephantnose fish has poor eyesight and uses a weak
electric field, which it generates with specialized cells called electrocytes,
which evolved from muscle cells, to find food, to navigate in dark or turbid
waters, and to find a mate. Peters' elephantnose fish live to about 6–10 years,
but there are reports of them living even longer.
Electric eel
The electric eel (Electrophorus electricus) is an electric
fish, and the only species in that genus. Despite the name, it is not an eel,
but rather a knifefish.
The electric eel has three pairs of abdominal organs that
produce electricity: the main organ, the Hunter's organ, and the Sach's organ.
These organs make up four-fifths of its body, and give the electric eel the
ability to generate two types of electric organ discharges: low voltage and
high voltage. These organs are made of electrocytes, lined up so a current of
ions can flow through them and stacked so each one adds to a potential
difference.
When the eel locates its prey, the brain sends a signal
through the nervous system to the electrocytes. This opens the ion channels,
allowing sodium to flow through, reversing the polarity momentarily. By causing
a sudden difference in electric potential, it generates an electric current in
a manner similar to a battery, in which stacked plates each produce an electric
potential difference.
In the electric eel, some 5,000 to 6,000 stacked
electroplaques are capable of producing a shock up to 860 volts and 1 ampere of
current (860 watts) for a duration of two milliseconds. Such a shock is
extremely unlikely to be deadly for an adult human, due to the very short
duration of the discharge. Atrial fibrillation requires that roughly 700 mA be
delivered across the heart muscle for 30 ms or more, far longer than the eel is
able to produce. Still, this level of current is reportedly enough to produce a
brief and painful numbing shock likened to a stun gun discharge, which due to
the voltage can be felt for some distance from the fish; this is a common risk
for aquarium caretakers and biologists attempting to handle or examine electric
eels.
The Sach's organ is associated with electrolocation. Inside
the organ are many muscle-like cells, called electrocytes. Each cell can only
produce 0.15 V, though the organ can transmit a signal of nearly 10 V overall
in amplitude at around 25 Hz in frequency. These signals are emitted by the
main organ; the Hunter's organ can emit signals at rates of several hundred
hertz.
The electric eel is unique among the Gymnotiformes in having
large electric organs capable of producing potentially lethal discharges that
allow them to stun prey. Larger voltages have been reported, but the typical
output is sufficient to stun or deter virtually any animal. Juveniles produce
smaller voltages (about 100 V). They are capable of varying the intensity of
the electric discharge, using lower discharges for hunting and higher
intensities for stunning prey or defending themselves. They can also
concentrate the discharge by curling up and making contact at two points along
its body. When agitated, they are capable of producing these intermittent
electric shocks over a period of at least an hour without tiring.
The electric eel also possesses high frequency-sensitive
tuberous receptors, which are distributed in patches over its body. This
feature is apparently useful for hunting other Gymnotiformes.
Electric eels have been used as a model in the study of
bioelectrogenesis. The species is of some interest to researchers, who make use
of its acetylcholinesterase and adenosine triphosphate.
Glass knifefish
The Glass Knifefish (Eigenmannia virescens) is a weakly
electric freshwater fish found across South America.
It is marketed as an aquarium fish.
The appendix on the body has black lines running through the
bottom of the sides, with the most intense line running on the anal fin.
