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I. Physical Features
Electric eels are not really eels, they are actually ostariophysians, but have a strong physical resemblance to true eels. . An electric eel is mostly tail. The internal organs are compressed into the anterior (front) 1/5th of the body, and the rest of the fish consists of the long, electricity-producing tail. The tail contains the electric organs: Sachs’ organ, Hunter’s organ, and main organ. Extending down the entire tail region is an elongated anal fin. There are no dorsal or pelvic fins. An adult eel can reach any size from six to nine feet in length an weigh up to sixty pounds. It is cylindrical in shape with a slightly flattened head and large mouth. They do have gills, though it is not their primary source of oxygen intake. A thick, slimy skin covers the entire body. The skin is used as a protective layer, often from their own electrical current that is produced. Electric eels range from gray to brownish/black in color with some yellowish coloration on the underside of the body and have tiny scales. Electric eels do not have teeth, enabling them to swallow their prey easier. The eyes are tiny, and as this fish ages, its vision diminishes.
II. Physiological Features
The Main and Hunters’ organs are the high voltage producers, used for protection, fright reflexes and stunning prey. The Sachs’ organ is capable only of producing low voltage pulses – its purpose is mainly electro communication and navigation. . Electric organs are made up of cells called Electrolytes. Some scientists believe these cells are derivative of a muscle-cell since nerve cells synapse onto them and they behave much like a muscle-cell post-synaptically. However, they are unlike muscle cells in that they don’t contract. Flat and disk-like, the electrocytes are stacked in a sequence with the head as the positive pole and the tail as the negative pole. Each electrocyte generates .15 volts, which is a very small amount. However, when 4,000 electrocytes are lined up generating electricity at the exact same time at .15 volts each, the shock equals at least 600 volts, which can paralyze or kill a human, especially after repeated shocks.
At rest, the Na+K+ pump, concentration and electrical gradient keeps the inside of the electrocyte at a resting potential of .08 volt. When the electric eel electrolocates its prey, the brain sends a signal through the nervous system to the electric organs. Acetylcholine is dropped onto the electrocyte which binds to the corresponding receptor on the ion. This opens the ion channels of the cell, allowing Na+ to rush in. The cell then depolarizes, momentarily reversing the charge, and fires. Since the electrocytes are lined up, current flows through like a battery, emitting a charge. However, they must all discharge at the same time. The electric eel’s design resolves this at the neuronal connections by delaying the signals and action potentials. The closer connections have longer and thinner pathways, which decelerates the signal and allows all electrocytes to synchronize their discharge.
The electric eel may be found in the basins of both the Amazon River and Orinoco River, as well as the surrounding areas. They live in Murky, low-oxygenated freshwater streams and pools. The Amazon river habitat has a low concentration of dissolved oxygen in the water. For this reason the eel surfaces frequently to gain sufficient oxygen for respiration. Up to 80% of the total oxygen intake is from the atmosphere, as opposed to the 20% extracted by the gills. Emission of carbon dioxide is done primarily through the skin (80%), but a small amount of carbon dioxide is emitted through the mouth (20%).
The electric eel seems to have adapted and evolved to its preferred environment. It is able to swim forward and backward with its undulating anal fin. This allows it a better sense of its surroundings. An electric eel will die if left in water for more than 20 minutes. The cloudy water does not create visual obstacles for the fish either. Not only is the electric eel nocturnal, a fully grown fish can hardly use their beady eyes. Although the youth can see, the adults become increasingly blind due to the constant exposure to the generated electrical field. Electric fields from other fish are generated regardless of whether or not they are electrogenic. The use of any muscle creates an electric field, so the contractions of a heart could be detected in another fish. The eels can locate objects accurately by measuring frequencies and amplitudes on different parts of the body as well as static electricity, caused by flow of water against the bottom of the river.
By using its own electric organ discharge (EOD) and sensing differences in the current around itself, the eel is able to deduce the conductivity of any surrounding objects, another way of navigation and obstacle detection. In addition, the EOD from other electrogenic fish can be used for communication.
Electric eels have been reclassified several times. Originally a species in Gymnotus, it was later given its own family Electrophoridae, and only demoted to a genus of Gymnotidae alongside Gymnotus. The electric eel requires scientific permits to obtain. It would be extremely harmful for humans and animals if released into waterways. Electric eels are one of the preferred subjects for research of the enzyme, Acetylcholinesterase (AchE). The electric eel is used for cancer research as well. Occasionally, they are eaten by locals of the Amazon area. They are, however, commonly avoided due to the electrical shocks that can be given out up to eight hours after death. Although there is no commercial value, the electric eel has been a constant source of study for many years. The scientific community is very interested in studying the electrical capabilities of electric eels. Electric eels can fatally electrocute a horse. A human can withstand one discharge, but would not survive several.