Environmental and hormonal influences upon EOD waveform in gymnotiform pulse fish

Abstract

Temperature is a major variable that affects all biological systems. Environmental temperature determines animal geographical distribution and activity, and influences their reproductive cycle, particularly within the temperate zone. Temperature, as a physical parameter, also strongly affects excitable tissues. The hypothesis of temperature as the most important environmental cue for the onset of breeding in gymnotiform pulse fish of the temperate zone is supported by: (a) a clear temporal correlation that was observed in the wild between water temperature and sexual maturity, and (b) the induction of gonadal maturation and sexual differences after acclimation at high temperature (28 °C) in the laboratory. Temperature sensitivity of EOD waveform (described in Brachyhypopomus pinnicaudatus and Gymnotus carapo) is characterized by the decrease of the EOD's late head-negative phase as temperature increases. This phenomenon depends on electrocyte properties since: (a) experimentally induced changes of discharge rate at constant temperature generate smaller EOD distortion, and (b) the effect of temperature upon EOD also depends on water conductivity. Temperature sensitivity of EOD waveform is negatively correlated with gonadal maturity in Brachyhypopomus pinnicaudatus. High temperature sensitivity was observed during the non-breeding season, whereas low temperature sensitivity was recorded during the breeding season. Temperature sensitivity of EOD waveform in both Brachyhypopomus pinnicaudatus and Gymnotus carapo was modulated by: (a) testosterone treatment (100 μg/g) that decreased temperature sensitivity, and (b) acclimation at high temperature (28 °C, 1 month) that also decreased temperature sensitivity. Temperature is probably acting through the neuroendocrine system, and ultimately interacting with steroid hormones in their effects upon EOD waveform.