title: 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.
abstract: 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)
authors.forename: B O
authors.initials: BO
authors.lastname: Bratton
authors.forename: B
authors.initials: B
authors.lastname: Kramer
mesh_terms: D000349:Africa; D000818:Animals; D004553:Electric Conductivity; D004555:Electric Fish / Q000502:physiology*; D005260:Female; D005618:Fresh Water; D008297:Male; D012737:Sex Factors; D013045:Species Specificity; D014329:Tropical Climate