1. A spontaneous EOD response to sound is described in two gymnotoids of the pulse Electric Organ Discharge (EOD) type, Hypopomus and Gymnotus, and in one mormyrid, Brienomyrus (Figs. 2-4).
2. In all three species, the EOD response to the sound onset was a transient EOD rate increase. In the low EOD rate Hypopomus (3-6 EODs/s at rest) the first, second, or third EOD interval following sound onset was significantly shorter than the average EOD interval before stimulation. The shortest latency
found was 100 ms, the longest ca. 1.2 s. Gymnotus (around 50 EODs/s at rest) responded similarly, but the third interval after sound onset was the first to be affected even at highest intensities (shortest latencies approx. 60 ms; latencies >0.5 s at low sound intensities). In Brienomyrus (4-8 EODs/s at rest) the response occurred already at the first EOD interval after sound onset.
3. An EOD sound response was recorded in Hypoporous
and in Gymnotus up to 5,000 Hz sound frequency (in one Gymnotus individual: up to 7,000 Hz). Due to technical limitations the low frequency limit of the response could not be exactly determined: the fishes responded well even below 100 Hz. Hypopomus had its maximum sensitivity around 500 Hz (Fig. 5), Gymnotus around 1,000 Hz (Fig. 6).
4. In all three species the EOD sound response was graded with sound intensity (Hypopomus: Fig. 7).
5. No EOD response to sound was found in two gymnotoids of the wave type, Eigenmannia and Apteronotus, and in the gymnotoid pulse fish Rhamphichthys. A criterion is proposed by which it should be possible to predict whether or not a weakly electric fish species will show the EOD sound response.
6. It is concluded that the EOD response to sound is similar to EOD responses to other kinds of stimulation (light, touch, vibration, food, and even electrical). The possible biological function is discussed.