Electrolocation and jamming avoidance in aHypopygus (Rhamphichthyidae, Gymnotoidei), an electric fish with pulse-type discharges

Abstract

Hypopygus fires its electric organ in short pulses of less than 1 msec duration, at a variable rate between 60 and 100 Hz. While resting, the animal hovers near electrically detectable objects and follows their motions. This “electromotor” response is exploited to measure performance in electrolocation as a function of object size (Figs. 1, 2).Hypopygus pays particular attention to sensory feedback associated with its own discharges. It briefly raises its rate of discharge in response to sudden changes in feedback. Such changes, for example, are caused by extrinsic pulses which briefly precede or coincide with its own discharges (Figs. 3–5). In addition, electrolocation deteriorates when extrinsic pulses of sufficient intensity continually interfere with its own discharges in this manner, whereas non interfering pulses of comparable intensity have no adverse effect (Fig. 6). By modulating its frequency of discharge the animal avoids long sequences of coincidences between its discharges and trains of extrinsic pulses (Figs. 7,9). By only “listening” to sensations associated with its own discharges,Hypopygus appears to be able to electrolocate in the near presence of conspecifics. It remains unknown how the animal distinguishes feedback from its own discharges against sensations caused by extrinsic pulses.