THE SENSING OF ELECTRICAL CAPACITANCES BY WEAKLY ELECTRIC MORMYRID FISH - EFFECTS OF WATER CONDUCTIVITY

Weakly electric fish can perceive electric properties of objects by mo nitoring the responses of their epidermal electroreceptors (mormyromas ts) to their own electric organ discharges (EOD), a process known as a ctive electrolocation. Mormyrid fish can distinguish capacitative from resistive properties of objects. It is mainly animate objects that po ssess capacitative properties. Water conductivity is a critical enviro nmental 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 properti es of capacitative objects in waters of different ion content and (2) to test a recently formulated hypothesis which states that the detecti on of the features of a capacitative object depends on a comparison of the inputs from the two types of mormyromast primary afferents. Indiv iduals of G. petersii were tested in a conditioned electrolocation pro cedure.With increasing water conductivities from 50 to 1100 mu cm-1, E OD amplitude decreased and the detection threshold for small capacitan ces increased. At 50 muS cm-1, the smallest detectable capacitative va lue was below 0.5 nF; this increased to about 20 nF at 800 muS cm-1. W hen conductivity approached about 1000 muS cm-1, fish were no longer a ble to electrolocate, probably because of the reduction in EOD amplitu de at high conductivities. The fish's ability to discriminate a capaci tative object unequivocally from every resistive object was also teste d at different conductivities. Below about 800 muS cm-1, all fish coul d do so. Above that conductivity, however, fish could no longer discri minate 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 ampl itude is much reduced. With input from B afferents only, an unambiguou s capacitance detection was no longer possible. This supports the hypo thesis that capacitance detection is achieved by comparing inputs of A and B electroreceptor cells.