Intraspecific variability of the pulse-type discharges of the African electric fishes, Pollimyrus isidori and Petrocephalus bovei (Mormyridae, Teleostei), and their dependence on water conductivity.

The electric organ discharge (EOD) of the mormyrid Pollimyrus isidori is a short pulse with three phases: (1) weak head positive (P1); (2) strong head negative (N); (3) weak head positive (P2). 1. At a stable water conductivity (100 microS/cm), which is near the upper end of the natural range in tropical Africa, there was a statistically significant difference between the sexes only in one of five EOD parameters, the P-ratio. The P1-amplitude was lower than the P2-amplitude (i.e. P1/P2 less than 1) in males (N = 10), while, on average, the opposite (P1/P2 greater than 1) was true for females (N = 14). Because of wide overlapping we do not consider this sex difference to be a sexual dimorphism. The difference between males and females could be due to well-known biophysical and physiological reasons (discussed later) and need not be the result of intraspecific selection (such as female choice). 2. Water conductivity seriously affected the EOD waveform. The P-ratio decreased in 2/3 of our fish (16 out of 24), as conductivity increased from 5 to 200 microS/cm, causing 6 out of 14 females to change from a P-ratio of greater than 1 to a P-ratio of less than 1, becoming more "male-like". P1 amplitude increased with decreasing conductivity in the EODs of 5 out of 10 males to a more "female-like" shape (P-ratio greater than 1). The P-ratio changed only slightly when above a conductivity of 200 microS/cm. The N-wave duration increased with decreasing conductivity, while the peak amplitude frequency of an EOD amplitude spectrum decreased. 3. Long-term stability was found to be poor in the EOD of 1 female (better in 2 other fish), which changed from a "female-like" waveform (P-ratio greater than 1) to a "male-like" waveform (P-ratio less than 1 over the whole conductivity range) without apparent reason within 120 days. 4. The EOD waveform of Petrocephalus bovei did not show a sex difference. Decreasing conductivity affected the EOD of P. bovei in a similar way to most P. isidori: the P1-wave increased and the P2-wave decreased, while the N-wave broadened strongly. 5. The occurrence of multiple discharges per primary neural command signal at very low conductivities, indicates that P. isidori is adapted to conductivities above 17 microS/cm, and P. bovei to those above 5 microS/cm.(ABSTRACT TRUNCATED AT 400 WORDS)