Summary

Weakly electric fish can perceive electric properties of objects by monitoring the responses of their epidermal electroreceptors (mormyromasts) to their own electric organ discharges (EOD), a process known as active electrolocation. Mormyrid fish can distinguish capacitative from resistive properties of objects. It is mainly animate objects that possess capacitative properties. Water conductivity is a critical environmental factor that varies widely from season to season and has strong effects on the emitted EOD. The two goals of this study were: (1) to investigate the ability of Gnathonemus petersii to detect the properties of capacitative objects in waters of different ion content and (2) to test a recently formulated hypothesis which states that the detection of the features of a capacitative object depends on a comparison of the inputs from the two types of mormyromast primary afferents. Individuals of G. petersii were tested in a conditioned electrolocation procedure. With increasing water conductivities from 50 to 1100 muS cm-1, EOD amplitude decreased and the detection threshold for small capacitances increased. At 50 muS cm-1, the smallest detectable capacitative value was below 0.5 nF; this increased to about 20 nF at 800 muS cm-1. When conductivity approached about 1000 muS cm-1, fish were no longer able to electrolocate, probably because of the reduction in EOD amplitude at high conductivities. The fish's ability to discriminate a capacitative object unequivocally from every resistive object was also tested at different conductivities. Below about 800 muS cm-1, all fish could do so. Above that conductivity, however, fish could no longer discriminate between capacitative and resistive objects of similar impedance, although they could still discriminate between objects of different impedances. The two types of receptor afferents (from the ‘A’ and ‘B’ cells) of mormyromast electroreceptor organs have different thresholds, with the B afferents being more sensitive. I suggest that only the B receptor cells remain active at about 800 muS cm-1, when the EOD amplitude is much reduced. With input from B afferents only, an unambiguous capacitance detection was no longer possible. This supports the hypothesis that capacitance detection is achieved by comparing inputs of A and B electroreceptor cells.