Electric Organ Discharge
Unlike the electric eel Electrophorus and the electric catfish Malapterurus, mormyrids cannot produce strong electric discharges for defense or to immobilize prey. Instead, by means of a specialized organ near the tail these fishes generate a relatively weak electric field around their body that they monitor using cells embedded in their skin called electroreceptors. Using active electroreception they are able to calculate the size, position and other characteristics of nearby objects in the water and can be active at night when vision is of little use. Electroreception requires a lot of brain power and these fishes have one of the largest brain mass to body mass ratios among vertebrates, roughly equal to that of Homo sapiens. In mormryids it is the cerebellum that has become massively hypertrophied.
Electric organ discharges, or EODs are also used for communication by mormyrids. Mormyrid EODs are pulses between one-tenth of a millesecond to 20 milleseconds in duration. While the time interval between the pulses is variable, the pulse waveform characteristics are fixed and species-specific. EOD waveforms can differ radically among co-occuring mormyrid species and reproductive males will often develop distinctive waveforms that function in courtship of conspecific females. In this way, EODs serve a function analagous to visual or acoustic signals in many other groups of organisms.
Impressive examples of EOD variation among co-occuring species can be found within the genera Paramormyrops of Lower Guinea and Campylomormyrus of the Congo River. The hypothesis that EODs may in fact accelerate speciation in these "riverine species flocks" and within mormyrids generally is another active research area. EODs are relatively easy to record from living mormyrids and because of their species-specificity and stereotypy, are often useful aides in recognizing species boundaries and working out the taxonomy of this group.