Water temperature sensitivity of EOD waveform in Brachyhypopomus pinnicaudatus

The mechanisms that trigger the onset of the breeding season depend on geographical latitude. At the edge of Gymnotiform distribution in America, variations in day length and water temperature are likely cues to initiate breeding. In this study we aim to clarify the role of temperature and the interaction between temperature and hormonal state upon electric organ discharge waveform. In breeding ponds, we measured naturally occurring changes of water temperature and of electric organ discharge waveform during two 48-h periods in a sample of identified mature males and females of Brachyhypopomus pinnicaudatus. Water temperature, day-night cycle, and sexual maturity each modified electric organ discharge waveform. Temperature sensitivity was also evaluated in the laboratory in adult sexually-differentiated individuals, adult non-differentiated fish, juveniles, and testosterone-treated fish. Our data strongly suggest an interaction between the effects of temperature and steroid hormones upon electric organ discharge waveform. High temperature (30 °C) induced a significant decay of head negative phase amplitude in temperature-sensitive fish. This sensitivity was observed in physiological conditions that coincide with low levels of steroid hormones: juveniles and adult fish kept in captivity at 20–21 °C. Conversely, temperature resistance was observed in mature fish in the breeding habitat and was induced by testosterone treatment and by captivity at 27–28 °C. Accepted: 23 May 1999