Electric organ discharges of the weakly electric fish Gymnarchus niloticus (Mormyriformes) in its natural habitat

Electrosensory interference in naturally occurring aggregates of a species of weakly electric fish, Eigenmannia virescens

The detection and identification of behaviorally relevant signals in the presence of competing signals in the environment is a major challenge of animal sensory systems. In weakly electric fish such as Eigenmannia virescens, the interactions between the autogenous electric field and the electric fields of nearby conspecifics can have profound effects on the perception of other behaviorally relevant electrosensory information. To better understand the natural signals that the nervous system of Eigenmannia experiences during the processing of electrosensory information, we examined the electrosensory milieu of Eigenmannia in the wild and in the laboratory. Recordings of the electric fields of Eigenmannia were made in 'black' and 'white' waters near the Napo River in eastern Ecuador. Fourier analysis revealed that Eigenmannia typically experience the electric fields of three to five conspecifics during the day and night in each habitat. The median difference in electric organ discharge frequencies between nearby Eigenmannia during the day was 23 Hz in black water habitats, 41 Hz in white water, and 37 Hz at night in both habitats: these signals are known to activate tuberous electroreceptors and downstream CNS circuits. There was no correlation between the number of individual Eigenmannia detected at recording sites and electric organ discharge frequencies. Further, Eigenmannia apparently do not maximize the frequency differences between conspecifics. In laboratory studies fish were preferentially observed in aggregates of two fish or more. Aggregate sizes observed in the laboratory were similar to those in the wild.