Harmonic and temporal structure of electric organ discharges of the wave-type in Amazonian knifefishes (Gymnotiformes)

Abstract

Neotropical, nocturnal freshwater knifefishes of the families Sternopygidae and Apteronotidae are electroreceptive, and emit electric organ discharges (EODs) of the wave type for communication and active electrolocation. In a field-collected sample of an estimated 43 gymnotiform species, members of the former family displayed the same type of sinusoidal EOD waveform at frequencies of up to about 800 Hz, with the fundamental frequency, f1, the strongest harmonic in each. Members of the latter (Apteronotidae) displayed f1 frequencies of up to 1800 Hz, and a great diversity in EOD waveform. Apteronotid EODs often differed from those of sternopygids by more harmonics at stronger amplitudes, where f1 was not always the strongest harmonic. The frequency band at −10 dB increased with diminishing f1 amplitude. In contrast to apteronotids, all sternopygids showed the same phase relationship between their respective f1 and f2: a difference of an average 72°, which explains their single type of sinusoidal waveform. In apteronotids a great variety of phase relationships among harmonics was observed, in some their harmonics series cycled through 360° repeatedly. It is argued that the evolutionary driving force for the apteronotids — in contrast to sternopygids — was the greater potential for adaptive radiation.