Principles of electroreception by weakly electric fish.
(A) Electrogenesis in Sternopygus macrurus. Adult fish are on the order of tens of centimeters in length. The electric organ is located along the tail and produces a quasisinusoidal electric field whose frequency varies between 40 and 200 Hz. The electric organ, which is roughly located where the white stripe on the side of the fish appears, is composed of electrocytes with voltage-gated sodium and potassium channels that are concentrated on the caudal aspect of the cell membrane, arranged in series (inset). The currents generated by these channels summed over all electrocytes give rise to a potential difference between the inside and outside of the fish that propagates through the water at the speed of light. The colors surrounding the photograph of the fish correspond to the relative strength of the electric field. In this snapshot of the electric field, the region around the body and head is positive (blue colors) and the region around the tail is negative (red colors). At other times, the positive and negative areas are reversed. This field is detected by electroreceptors that are embedded in the skin. (B) Sternopygus at night or after social interactions. These conditions lead to an increase in the circulating ACTH, which increases the rate of exocytosis of channels in the electrocytes, thereby increasing their density (inset). This, in turn, increases the intensity of the electric field and, therefore, the distance at which the electric field propagates in the water. As a result, salient objects like prey items (purple dot) may be detected at greater distances. (C) Sternopygus during the day or in solitary conditions. Under these circumstances, there is a smaller rate of exocytosis due to lower levels of ACTH and thus fewer channels in the electrocytes (inset). As a result, the fish produces a weaker electric field that will decay over smaller distances. Because electrosensory perception is dependent on the detection of voltage differences in the water, this reduced electric field is less effective for detecting prey and for communicating to nearby conspecifics. Sternopygus photograph courtesy of Scott Shulz.