An Electric Sense
In the murky, sediment-filled waters of the Amazon basin, vision is a limited sense. Here, a group of fish called the Gymnotiformes, or knifefish, have evolved an alternative: an electric sense. These fish, such as the glass knifefish (Eigenmannia virescens), generate a weak, continuous electric field from a specialized electric organ in their tail. This Electric Organ Discharge (EOD) creates a field around their body. By detecting distortions in this field with hundreds of electroreceptor organs in their skin, they can navigate, find food, and identify other fish in complete darkness.
The EOD is remarkably stable, like a personal radio frequency. For Eigenmannia, these frequencies typically range from 240 to 600 Hertz. This system allows them to perceive the world in a way humans can barely imagine, constructing a detailed electrical picture of their surroundings from the subtle changes in current flowing across their skin. This entire process of generating and sensing the electric field is called electrolocation.
The Jamming Avoidance Response
A problem arises when two weakly electric fish with similar EOD frequencies swim near each other. Their electric fields interfere, creating a lower-frequency "beat" pattern that scrambles their electrolocation, much like two radio stations causing static. This sensory confusion effectively blinds the fish. To solve this, they perform a behavior called the Jamming Avoidance Response (JAR). Discovered in 1963 by Akira Watanabe and Kimihisa Takeda, the JAR is the solution to signal interference.
When two fish sense this jamming beat, they reflexively shift their EOD frequencies apart. The fish with the slightly higher frequency increases its frequency, while the fish with the lower frequency decreases its own. This increases the difference between their two signals, resolving the interference. The frequency shift is small and fast, typically a change of about 3 to 6 Hz. This computation is a model in neuroethology, and the neural circuitry behind it has been mapped, largely through the work of neurobiologist Walter Heiligenberg in the 1970s. The fish's brain analyzes tiny differences in the timing and amplitude of the signal across its body to determine whether its neighbor's frequency is higher or lower than its own, and then adjusts its pacemaker neurons accordingly.