NAVIGATION IN FAMILIAR ENVIRONMENTS BY THE WEAKLY ELECTRIC ELEPHANTNOSE FISH, GNATHONEMUS-PETERSII L (MORMYRIFORMES, TELEOSTEI)

Gnathonemus petersii use electrolocation to navigate in unfamiliar env ironments. The goal of these experiments was to determine whether fish could learn the location of a fixed aperture after interference with selected sensory input. By manipulating environmental cues (aperture h eight and water depth) and comparing the fish's performance, the contr ibutions of the electrosensory system, vision, and hydrostatic pressur e were examined. The fish's task was to find a circular aperture in a wall dividing a 200-litre aquarium into two equal compartments. In exp eriment 1, the position of the aperture was raised by 10.1 cm after th e fish had become familiar with its original location. In experiment 2 , the water level was raised by 10 cm (leaving the aperture unchanged) . When the aperture was raised, intact fish found the new aperture wit h no difficulty, whereas blind, electrically 'silent', and sham-operat ed fish were slow finding the new position. When the water level was r aised, all fish increased the height at which they contacted the wall, increased their electric-organ discharge (EOD) rate, and located the aperture. This increase, in response to the rapid change in water dept h, suggests that all fish used hydrostatic pressure cues to maintain d epth orientation, and that those fish that learned the aperture height had used hydrostatic cues to locate its position. The data suggest th at G. petersii develop an internal representation based on an electros ensory central expectation and hydrostatic cues. The fish develop a se nsory 'image' of their immediate environment and associate a specific image with a specific depth. As the environment becomes more familiar, the fish apparently attend less to electrosensory information and nav igate according to the internal representation, relying primarily on h ydrostatic pressure cues.