THE ELECTRIC ORGAN DISCHARGE OF BRACHYHYPOPOMUS-PINNICAUDATUS

The electric organ discharge of Brachyhypopomus pinnicaudatus was stud ied by recording (1) the discharge field potentials in water at differ ent conductivities and temperatures and (2) the spatiotemporal pattern of electromotive forces of the equivalent source. An early deflection , head positive (P wave), and a late deflection, head negative (N wave ), are the major components of the discharge, however a striking doubl e positive peak is generated at the abdominal level. Comparisons of th is species with other pulse gymnotids provide evidence for common patt erns of organization of the electrogenic system: (1) There is a head-t o-tail activation wave along the fish; (2) the electromotive force inc reases exponentially from head to tail, but it is differentially atten uated by the passive tissues in male and females; (3) the abdominal re gion generates a complex species-specific waveform, whereas the tail d ischarge is similar across species. In B. pinnicaudatus the electric o rgan discharge waveform is sensitive to endocrine and environmental st imuli. The effect of seasonal sex differences on electrogenic and pass ive tissue, the changes in impedance matching between the fish's body and the environment, and the modulation of membrane properties by temp erature, are able to modify the EOD waveform. Since these factors chan ge during the breeding season, their appropriate combination might be crucial for reproduction.