SHORT-RANGE ORIENTATION IN ELECTRIC FISH - AN EXPERIMENTAL-STUDY OF PASSIVE ELECTROLOCATION

Gymnotiform electric fish are capable of locating and approaching an e lectrically discharging conspecific over a range of 1-2 m in a behavio r called passive electrolocation. This paper investigates the movement s of two species in experiments with approaches to stationary dipoles that are either silenced or jumped to a new direction during an approa ch. Gymnotus carapo fail to find an electrode source in trials in whic h the dipole electrode is switched off in mid-track. They slow their a pproach, become disoriented and drift away from the target within seco nds of the field being switched off. This result suggests that the fis h are unable to construct a cognitive map of a dipole source from brie f exposure to local electrosensory stimuli. The second set of trials s hows that Brachyhypopomus diazi and Gymnotus carapo bend their body to track electric vectors which are suddenly jumped to a new direction. The latency of the bend response is 0.5 s after the jump. Bending init iates a turn that reduces to zero the error between the fish's directi on and the electric field vector and helps keep the fish aligned with the local electric field vector, Together, these experiments suggest t hat passive electrolocation is stimulus-bound and that these fish find the electrical sources simply by tracking instantaneous local electri c current vectors.