SHORT-RANGE NAVIGATION OF THE WEAKLY ELECTRIC FISH, GNATHONEMUS-PETERSII L (MORMYRIDAE, TELEOSTEI), IN NOVEL AND FAMILIAR ENVIRONMENTS

We investigated the electrolocation performance of the weakly electric fish, Gnathonemus petersii, in novel and familiar environments. By se lectively interfering with the fish's sensory input, we determined the sensory channels necessary for navigation and orientation. The fish's task was to locate a circular aperture (diameter: 64 mm) in a wall di viding a 200-l aquarium into two equal compartments. To assess the fis h's performance, we measured (1) the time it took the fish to locate t he aperture, (2) the height at which it contacted the divider, (3) its electric organ discharge rate, and (4) the frequency of divider cross ings. In the first experiment (novel environment), 50 naive G. petersi i assigned to five groups of 10 fish each (intact, blind, electrically ''silent,'' blind and ''silent,'' and sham-operated animals) were tes ted with the aperture presented randomly in one of three positions (ap erture center: 7.6, 17.7, 27.8 cm from the bottom). In a novel environ ment, G. petersii depend on active electrolocation. Despite the changi ng aperture position, over the 15 trials, fish with a functioning elec tric organ found the aperture, whereas those without one did not. The electric organ discharge rate was inversely correlated with the amount of time spent searching for the aperture. In a second experiment (fam iliar environment) 20 intact fish learned the position of a fixed aper ture. When we subsequently denervated the electric organ in 10 of thes e animals, their performance did not differ significantly from that of their conspecifics. Thus, once the fish were familiar with the apertu re's position, they no longer depended on active electrolocation. We i nterpret and discuss this behavior as evidence for a ''central expecta tion'' and discuss its possible role in electronavigation.