Electric eel

The electric eel (Electrophorus electricus) is a South American electric fish, and the only species in its genus. Despite the name, it is not an eel, but rather a knifefish. The electric eel has an elongated, cylindrical body, typically growing to about 2 m (6 ft 7 in) in length, and 20 kg (44 lb) in weight, making them the largest species of the Gymnotiformes. Their coloration is dark gray-brown on the back and yellow or orange on the belly. Mature females have a darker color on the abdomen. They have no scales. The mouth is square, and positioned at the end of the snout. The anal fin extends the length of the body to the tip of the tail. As in other ostariophysan fishes, the swim bladder has two chambers. The anterior chamber is connected to the inner ear by a series of small bones derived from neck vertebrae called the Weberian apparatus, which greatly enhances its hearing capability. The posterior chamber extends along the whole length of the body and maintains the fish's buoyancy. E. electricus has a well-developed sense of hearing. This fish has a vascularized respiratory system with gas exchange occurring through epithelial tissue in its buccal cavity. As obligate air-breathers, electric eels must rise to the surface every ten minutes or so to inhale before returning to the bottom. Nearly eighty percent of the oxygen used by the fish is obtained in this way. Despite its name, the electric eel is not closely related to the true eels (Anguilliformes) but is a member of the neotropical knifefish order (Gymnotiformes), which is more closely related to the catfish. 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 finds 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. Electric eels are also capable of controlling their prey's nervous systems with their electrical abilities; by controlling their victim's nervous system and muscles via electrical pulses, they can keep prey from escaping or force it to move so they can locate its position. In the electric eel, some 5,000 to 6,000 stacked electroplaques can make a shock up to 600 volts and up to 1 ampere of current. 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. Electric eels use electricity in multiple ways. Low voltages are used to sense the surrounding environment. High voltages are used to detect prey and, separately, stun them. Pairs of high voltage pulses separated by 2 milliseconds are used to detect and locate prey by causing them to twitch involuntarily; the electric eel senses this movement. A string of high voltage pulses at up to 400 per second are then used to attack and stun or paralyze the target, at which point the electric eel applies a suction-feeding bite.