Electric fish

Introduction:


An electric fish is any fish that can generate electricfields. A fish that can generate electric fields is said to be electrogenic  while a fish that has the ability to detect electric fields is said to be electroreceptive. Most electrogenic fish are also electroreceptive.Electric fish species can be found both in the ocean and in freshwater rivers of South America (gymnotymers) and Africa (mormiridae). Many fish such as sharks,rays and catfishes can detect electric fields and are thus electroreceptive, but they are not classified as electric fish because they cannot generate electricity. Most common bony fish (teleosts) including most fish kept in aquaria or caught for food, are neither electrogenic nor electroreceptive.


Strongly and weekly electric fish:

Electric fish produce their electrical fields from a specialized structure called an electric organ. This is made up of modified muscle or nerve cells, which became specialized for producing bioelectric fields stronger than those that normal nerves or muscles produce (Albert and Crampton, 2006). Typically this organ is located in the tail of the electric fish. The electrical output of the organ is called the electric organ discharge (EOD).

Fish with an EOD that is powerful enough to stun prey are called strongly electric fish. The amplitude of the signal can range from 10 to 600 volts with a current of up to 1 ampere. Typical examples are the electric eel (Electrophorus electricus; not a  true eel but a knifefish), the electric cat fishes (family Malapteruridae), and electric rays (order Torpediniformes). Strongly electric marine fish deliver low voltage, high current electric discharges while freshwater fish have high voltage, low current discharges. This is because of the different conductances of salt and fresh water. To maximize the power delivered to the surroundings, the impedances of the electric organ and the water must be matched. In salt water, a small voltage can drive a large current limited by the internal resistance of the electric organ. Hence, the electric organ consists of many electrocytes in parallel. In freshwater, the power is limited by the voltage needed to drive the current through the large resistance of the medium. Hence, these fish have numerous cells in series.

By contrast, weakly electric fish generate a discharge that is typically less than one volt in amplitude. These are too weak to stun prey and instead are used for navigation, object detection (electro location)and communication with other electric fish (electrocommunication). Two of the best-known and most-studied examples are peters elephantanose fish(Gnathonemus petersi) and the black ghost knife fish (Apteronotus albifron)