Sex differences in external morphology and electric organ discharges in imported Gnathonemus petersii (Mormyriformes)

Abstract

Abstract. Newly imported, adult weakly discharging electric fish Gnathonemus petersii , were examined for sex differences in the waveform of their electric organ discharge and in external morphology. The discharge waveform of gonadally ripe subjects are quantitatively described at three stages corresponding to the Nigerian pre-rainy, rainy breeding, and post-rainy season in 1988. A sex difference was found in external morphology, with males displaying a distinct indentation at the dorsal margin of the anal fin, regardless of season. Pronounced sex differences in electric organ discharges were found only in fish imported during the Nigerian rainy season, and only in two phase of the tetraphasic discharge, with males exhibiting longer durations of phases 2 and 3. These sex differences were reflected in the Fourier transformation, with male discharges exhibiting lower peak power spectral frequencies than those of females. No sex differences were found in total duration of the electric discharge. Correlations between the electric discharge measures and fish length, as well as between the discharge measures themselves, revealed relationships which varied in both direction and degree as a function of sex and/or season. These results indicate that the duration of individual phases, rather than the entire duration of the electric organ discharge, function in the communication of sex differences in this mormyrid species. Previous work has shown that phase durations of the electric discharge are steroid-dependent and therefore indicative of physiological states of the animal. Thus, the kinds of information conveyed during breeding and non-breeding seasons may be quite different, given great differences in gonadal steroid levels. These findings explain previously unsuccessful attempts to find sex differences as well as a report of a sex difference in a direction opposite to that shown here in G. petersii.