Electric fish can use electricity as a communicative device, much as humans use auditory signals. Using its electric organ, the fish produces an electric organ discharge (EOD), which is broadcast through the surrounding water and received by other fish in the environment. These other fish detect the signals and process various aspects of the signal to determine its significance. Fish constantly emit EOD's, and thus are continuously providing informational cues to their surrounding environment. Electrocommunicative signals can express a fish's species, gender, reproductive intent, social status, and even level of aggression. While some progress has been made to identify and characterize different signals and their meanings, decoding electrocomminicative "fish speak" is a difficult process, and much remains to be discovered. Each species of electric fish varies its EOD differently to communicate different cues.

The Elephant Nose fish (Gnathonemus petersii )is a weakly electric fish that uses its pulse-type EOD to communicate with other fish. Image used with permission of Simon Quellen Field.


EOD Variation

Fish EODs vary according to many factors, including signal type, frequency, duration, and structure. Different combinations of these factors produce signals with different meanings.

Pulse and Wave: Some fish produce short bursts of communicative electric signal called "pulses," and others produce longer signals called "waves" also called "tones" (Hopkins 1972). Pulse discharges are "brief with respect to the period between discharges" and have variable frequency (Hopkins 1972). Wave or tone signals have discharges nearly equal in length to the period between discharges, and relatively stable frequency (Hopkins 1972). Individuals of a single species produce either wave signals or pulse signals, but not both. The Black Ghost Knife fish (Apteronotus albifrons) is a wave producing fish, while the Elephant Nose fish (Gnathonemus petersii) produces pulses (von der Emde 1999).

Depiction of two different types of EOD. Pulse-type are single bursts of electric signal, while wave-types are repeated. Image used with permission of Masashi Kawasaki


EOD Shape: Fish also produce EODs of different structures or shapes. A signal may begin with a strong peak and gradually fade, or it may rise slowly to peak at the end of the signal. EOD shapes vary from species to species, between males and females, or even with age. They also vary from individual to individual.

Other Factors: Even if a fish keeps the structure of its EOD discharge consistent, it can vary the frequency, amplitude, or duration of a signal. There are many ways in which fish electrocommunicative signals differ.


Examples of Electrocommunication

Instead of trying to give a comprehensive account of EOD variation in makeup and meaning, I have listed a number of specific examples below to give an indication of how fish use electrocommunication.

Species: One indication a fish may give of its species is the type of EOD it emits. As note above, some species produce wave signals, and others produce pulses. However, as many wave or pulse generating fish may live in a single area, other indicators are necessary for further distinction. Within wave-type fish, different frequencies can indicate different species. For example, some species in the Moco-moco creek in the Rupununi District of South America are distinguishable by EOD frequency. Sternopygus macrurus EOD frequencies range between 50 and 150 hz, Eigenmannia virescens frequencies average 240 to 580 hz, and Apteronotus albifrons frequencies are even higher- 750-1250 hz (Hopkins 1972).

Gender: Gender information can be conveyed by the frequency of a fish's EOD, though the way in which frequency is related to gender may vary across species. Brown Ghost Knifefish (Apteronotus leptorynchus), males typically have higher EOD frequencies than females: 800-1000 hz for males, 600-800 hz for females(Bastian et al. 2001). However, in Sternopygus macrurus individuals, the females exhibited higher frequency signals (120.1 for females, 66.8 for males) (Hopkins 1972).
Gender information can be indicated by aspects of the EOD other than frequency. Brienomyrus brachyistius triphasic females and males differ in the structure of their EOD pulse. Males emit longer pulses characterized by an initial peak, a gradual decrease, and a shallow dip. Females and juveniles have shorter pulses, no appreciable decrease, and a deeper dip (Hopkins and Bass 1981).

Depiction of a single EOD pulse of Brienomyrus brachyistius triphasic fish. Note the shape difference between female and male pulses. Image adapted from Hopkins and Bass 1981.



Courtship: Males of the Sternopygus macrurus species exhibit specific patterns of variation in their usual wave-type EOD to indicate courtship intent. This pattern is generated by enhancing the normal wave pattern through signal cessation and frequency variation. S. macrurus males interrupt their normal wave pattern or briefly increase the EOD frequency to create a unique pattern, and they exhibit this pattern only in courtship situations (Hopkins 1972).
The males of a species of West African pulse-type electric fish called Brienomyrus brachyistius triphasic also vary EOD frequency in courtship. They drastically increase their normal EOD pulse briefly to produce high frequency bursts of electric signal to attract potential mates (Hopkins and Bass 1981).

Aggression: Studies with the Brown Ghost Knifefish Apteronotus leptorynchus have shown that their aggressive behavior is expressed by varying the duration of their EODs. Short, low frequency “chirps” (~15-30 ms) indicate aggression, while longer “rises” or higher frequency chirps seem to show submission (Trifenbach and Zakon 2002).

Size: Fish size seems to be correlated with the magnitude of the EOD: the larger the fish, the more electrical signal it can produce, and thus the stronger its signal. In male Brown Ghost Knifefish, size and EOD frequency are also positively correlated. The results for females of the same species are less conclusive. (Trifenbach and Zakon 2002).


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