What is an electric fish?

You may know that an electric eel can produce very strong electricity to shock large animals. But did you know that some tropical freshwater fishes use electricity for navigation and communication? They can use electricity to 'feel' its environment, and they can 'talk' each other using electrical signals.

All of these electric fishes produce electricity from an organ in the tail called an 'electric organ'.

The electric organ contains electrically excitable cells called 'electrocytes', which receive simultaneous command signals from the brain to 'fire'. At the moment of 'firing', the electrocytes are asymmetrically polarized acting as serially connected batteries.

The simultaneous firing of electrocytes results in the electric organ discharges (EODs) which are emitted in the surrounding water. In strongly electric fishes, such as the electric eel, electric catfish, and electric rays, the electric organ is huge containing numerous electrocytes. Therefore, their discharge voltage can reach as high as 600 volts. In weakly electric fishes, which use electricity for navigation and communication, the discharge voltage is small -- often less than a volt.

There are two types of EODs, pulse type and wave type. All strongly electric fishes and some weakly electric fishes are pulse-type electric fishes. They discharge short electrical pulses intermittently. Some weakly electric fishes are wave type. They produce wave-like continuous A.C. electricity.

All electric fishes mentioned so far not only produce electricity but sense it with a very sensitive sensory organ called 'electroreceptors' which are embedded in the skin.

Electroreceptors are used to detect a slight change of electric field cause by nearby objects. Electric fishes can thus electrically 'see' objects in an environment where vision is useless (at night, or in murky water). This process is called 'active electrolocation' because the source of electricity that they use for electrolocation is their own electric organ.

By the combination of electrogeneric and electroceptive capabilities, some electric fishes are known to communicate each other by electric signals. Dr. Carl Hopkins at Cornel University studies on electrocommunication. Visit Dr. Hopkins' page to learn more about electrocommunication.

Some fish can only sense electricity but cannot produce electricity. These fishes are also categorized as electric fishes. They are sharks, rays, skates, catfish, and paddle fish. These fish can sense very weak electricity generated by prey animals. So, sharks can find a small fish buried in sand by weak electricity given off by the prey. This type electrolocation is called 'passive electrolocation'.

Electric fishes are divided into the three main categories.

  • Strongly electric fish
    • electric eel
    • electric catfish
    • electric rays
  • Weakly electric fish
    • knife fishes
    • elephant nose
  • Fishes that can only sense electricity
    • sharks
    • rays
    • skate
    • catfish
    • paddle fish
    • Platypus (though not a fish, they are electroreceptive.)

 

The jamming avoidance response

The wave-type electric fish normally discharge at a fixed frequency. Each individual has its own frequency of discharge much as each radio station has its own broadcasting frequency. When two individuals with similar discharge frequencies meet, however, their EODs interfere each other causing problems in electrolocation. To avoid jamming, they shift their frequencies each other until their frequencies are separated enough for normal operation of electrolocation.

Since its discovery by Watanabe and Takeda in 1963, this electrical behavior, the jamming avoidance response, has provided neuroscientists with important scientific questions of broad implication, such as temporal and spatial pattern recognition, feature detection, and distributed computation of sensory information. My laboratory focuses on brain mechanisms for the jamming avoidance responses in central-south American and African electric fishes.