Water temperature sensitivity of EOD waveform in Brachyhypopomus pinnicaudatus

The mechanisms that trigger the onset of the breeding season depend on geog raphical latitude. At the edge of Gymnotiform distribution in America, vari ations in day length and water temperature are likely cues to initiate bree ding. In this study we aim to clarify the role of temperature and the inter action between temperature and hormonal state upon electric organ discharge waveform. In breeding ponds, we measured naturally occurring changes of wa ter temperature and of electric organ discharge waveform during two 48-h pe riods in a sample of identified mature males and females of Brachyhypopomus pinnicaudatus. Water temperature, day-night cycle, and sexual maturity eac h modified electric organ discharge waveform. Temperature sensitivity was a lso evaluated in the laboratory in adult sexually-differentiated individual s, adult non-differentiated fish, juveniles, and testosterone-treated fish. Our data strongly suggest an interaction between the effects of temperatur e and steroid hormones upon electric organ discharge waveform. High tempera ture (30 degrees C) induced a significant decay of head negative phase ampl itude in temperature-sensitive fish. This sensitivity was observed in physi ological conditions that coincide with low levels of steroid hormones: juve niles and adult fish kept in captivity at 20-21 degrees C. Conversely, temp erature resistance was observed in mature fish in the breeding habitat and was induced by testosterone treatment and by captivity at 27-28 degrees C.