Evolutionary designs for electric signals and electroreceptors in gymnotoid fishes of Surinam

Abstract

1. Field collections of gymnotoid electric fish in coastal Surinam streams; 23 March–6 April, 1976, yielded 11 species belonging to two families (Rhamphichthyidae and Gymnotidae). Electric organ discharges (EODs) were recorded and power spectra were generated by Fourier analysis. Each species could be classified as either wave or pulse type. Although EODs varied from species to species, individuals had species-typical discharges. EODs appear to function in electrolocation and electric communication. Coexisting pulse fish diverged in either pulse spectrum or pulse repetition rate. Although each species showed ecological preferences for one habitat or another, no general correlation was found between EOD form and habitat type (Fig. 1). 2. Three species of coexisting Hypopomus showed widely differing EOD durations. Peak spectral energies were nonoverlapping (Fig. 4a). 3. Electrophysiologic studies of the electroreceptors in Hypopomus species revealed five types of electroreceptors. Two types of units appear to act as EOD filters, responding maximally to spectral frequencies characteristic of the peak power of the species-specific EOD (Fig. 7). 4. Relationships between EOD rate and spectrum are discussed with reference to their roles in communication and in electrolocation. A theory for the evolution of EOD wave forms is presented (Fig. 9).