The sensory effects of light on the EOD rate of Gymnotus omarorum.

Gymnotiformes are nocturnal fishes inhabiting the root mats of floating plants. They use their electric organ discharge (EOD) to explore the environment and to communicate. Here we show and describe tonic and phasic sensory-electromotor responses to light distinct from indirect effects depending on the light-induced endogenous circadian rhythm. At dark, principally during the night, inter-EOD interval histograms are bimodal, the main peak corresponds to the basal rate and a secondary peak corresponds to high frequency bouts. Light caused a twofold tonic but opposed effect on the EOD histogram: a) decreased the main mode and b) blocked the high frequency bouts and consequently increased the main peak at the expense of an erasure of the secondary one. Additionally, light evokes phasic responses whose amplitude increases with intensity but whose slow time course and poor adaptation differentiate from the so-called novelty responses evoked by abrupt changes in sensory stimuli of other modalities. We confirmed that Gymnotus omarorum tends to escape from light suggesting that these phasic responses are probably part of a global "light avoidance response". We interpret the data within an ecological context. Fish rest under the shade of aquatic plants during the day and light spots due to the sun's relative movement alerts the fish to hide in shady zones to avoid macroptic predators and facilitate tracking the movement of floating plant islands by wind and/or water currents.